- 14 Sep, 2020 1 commit
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Tejun Heo authored
adjust_inuse_and_calc_cost() is responsible for reducing the amount of donated weights dynamically in period as the budget runs low. Because we don't want to do full donation calculation in period, we keep latching up inuse by INUSE_ADJ_STEP_PCT of the active weight of the cgroup until the resulting hweight_inuse is satisfactory. Unfortunately, the adj_step calculation was reading the active weight before acquiring ioc->lock. Because the current thread could have lost race to activate the iocg to another thread before entering this function, it may read the active weight as zero before acquiring ioc->lock. When this happens, the adj_step is calculated as zero and the incremental adjustment loop becomes an infinite one. Fix it by fetching the active weight after acquiring ioc->lock. Fixes: b0853ab4 ("blk-iocost: revamp in-period donation snapbacks") Signed-off-by:
Tejun Heo <tj@kernel.org> Signed-off-by:
Jens Axboe <axboe@kernel.dk>
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- 11 Sep, 2020 1 commit
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Tejun Heo authored
Conceptually, root_iocg->hweight_donating must be less than WEIGHT_ONE but all hweight calculations round up and thus it may end up >= WEIGHT_ONE triggering divide-by-zero and other issues. Bound the value to avoid surprises. Fixes: e08d02aa ("blk-iocost: implement Andy's method for donation weight updates") Signed-off-by:
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- 02 Sep, 2020 24 commits
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Tejun Heo authored
These are really cheap to collect and can be useful in debugging iocost behavior. Add them as debug stats for now. Signed-off-by:
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Tejun Heo authored
Update and restore the inuse update tracepoints. Signed-off-by:
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Tejun Heo authored
When an iocg accumulates too much vtime or gets deactivated, we throw away some vtime, which lowers the overall device utilization. As the exact amount which is being thrown away is known, we can compensate by accelerating the vrate accordingly so that the extra vtime generated in the current period matches what got lost. This significantly improves work conservation when involving high weight cgroups with intermittent and bursty IO patterns. Signed-off-by:
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Tejun Heo authored
A low weight iocg can amass a large amount of debt, for example, when anonymous memory gets reclaimed aggressively. If the system has a lot of memory paired with a slow IO device, the debt can span multiple seconds or more. If there are no other subsequent IO issuers, the in-debt iocg may end up blocked paying its debt while the IO device is idle. This patch implements a mechanism to protect against such pathological cases. If the device has been sufficiently idle for a substantial amount of time, the debts are halved. The criteria are on the conservative side as we want to resolve the rare extreme cases without impacting regular operation by forgiving debts too readily. Signed-off-by:
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Tejun Heo authored
Curently, iocost syncs the delay duration to the outstanding debt amount, which seemed enough to protect the system from anon memory hogs. However, that was mostly because the delay calcuation was using hweight_inuse which quickly converges towards zero under debt for delay duration calculation, often pusnishing debtors overly harshly for longer than deserved. The previous patch fixed the delay calcuation and now the protection against anonymous memory hogs isn't enough because the effect of delay is indirect and non-linear and a huge amount of future debt can accumulate abruptly while unthrottled. This patch implements delay hysteresis so that delay is decayed exponentially over time instead of getting cleared immediately as debt is paid off. While the overall behavior is similar to the blk-cgroup implementation used by blk-iolatency, a lot of the details are different and due to the empirical nature of the mechanism, it's challenging to adapt the mechanism for one controller without negatively impacting the other. As the delay is gradually decayed now, there's no point in running it from its own hrtimer. Periodic updates are now performed from ioc_timer_fn() and the dedicated hrtimer is removed. Signed-off-by:
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Tejun Heo authored
Debt handling had several issues. * How much inuse a debtor carries wasn't clearly defined. inuse would be driven down over time from not issuing IOs but it'd be better to clamp it to minimum immediately once in debt. * How much can be paid off was determined by hweight_inuse. As inuse was driven down, the payment amount would fall together regardless of the debtor's active weight. This means that the debtors were punished harshly. * ioc_rqos_merge() wasn't calling blkcg_schedule_throttle() after iocg_kick_delay(). This patch revamps debt handling so that * Debt handling owns inuse for iocgs in debt and keeps them at zero. * Payment amount is determined by hweight_active. This is more deterministic and safer than hweight_inuse but still far from ideal in that it doesn't factor in possible donations from other iocgs for debt payments. This likely needs further improvements in the future. * iocg_rqos_merge() now calls blkcg_schedule_throttle() as necessary. Signed-off-by:
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Tejun Heo authored
When the margin drops below the minimum on a donating iocg, donation is immediately canceled in full. There are a couple shortcomings with the current behavior. * It's abrupt. A small temporary budget deficit can lead to a wide swing in weight allocation and a large surplus. * It's open coded in the issue path but not implemented for the merge path. A series of merges at a low inuse can make the iocg incur debts and stall incorrectly. This patch reimplements in-period donation snapbacks so that * inuse adjustment and cost calculations are factored into adjust_inuse_and_calc_cost() which is called from both the issue and merge paths. * Snapbacks are more gradual. It occurs in quarter steps. * A snapback triggers if the margin goes below the low threshold and is lower than the budget at the time of the last adjustment. * For the above, __propagate_weights() stores the margin in iocg->saved_margin. Move iocg->last_inuse storing together into __propagate_weights() for consistency. * Full snapback is guaranteed when there are waiters. * With precise donation and gradual snapbacks, inuse adjustments are now a lot more effective and the value of scaling inuse on weight changes isn't clear. Removed inuse scaling from weight_update(). Signed-off-by:
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Tejun Heo authored
iocost has various safety nets to combat inuse adjustment calculation inaccuracies. With Andy's method implemented in transfer_surpluses(), inuse adjustment calculations are now accurate and we can make donation amount determinations accurate too. * Stop keeping track of past usage history and using the maximum. Act on the immediate usage information. * Remove donation constraints defined by SURPLUS_* constants. Donate whatever isn't used. * Determine the donation amount so that the iocg will end up with MARGIN_TARGET_PCT budget at the end of the coming period assuming the same usage as the previous period. TARGET is set at 50% of period, which is the previous maximum. This provides smooth convergence for most repetitive IO patterns. * Apply donation logic early at 20% budget. There's no risk in doing so as the calculation is based on the delta between the current budget and the target budget at the end of the coming period. * Remove preemptive iocg activation for zero cost IOs. As donation can reach near zero now, the mere activation doesn't provide any protection anymore. In the unlikely case that this becomes a problem, the right solution is assigning appropriate costs for such IOs. This significantly improves the donation determination logic while also simplifying it. Now all donations are immediate, exact and smooth. Signed-off-by:
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Tejun Heo authored
iocost implements work conservation by reducing iocg->inuse and propagating the adjustment upwards proportionally. However, while I knew the target absolute hierarchical proportion - adjusted hweight_inuse, I couldn't figure out how to determine the iocg->inuse adjustment to achieve that and approximated the adjustment by scaling iocg->inuse using the proportion of the needed hweight_inuse changes. When nested, these scalings aren't accurate even when adjusting a single node as the donating node also receives the benefit of the donated portion. When multiple nodes are donating as they often do, they can be wildly wrong. iocost employed various safety nets to combat the inaccuracies. There are ample buffers in determining how much to donate, the adjustments are conservative and gradual. While it can achieve a reasonable level of work conservation in simple scenarios, the inaccuracies can easily add up leading to significant loss of total work. This in turn makes it difficult to closely cap vrate as vrate adjustment is needed to compensate for the loss of work. The combination of inaccurate donation calculations and vrate adjustments can lead to wide fluctuations and clunky overall behaviors. Andy Newell devised a method to calculate the needed ->inuse updates to achieve the target hweight_inuse's. The method is compatible with the proportional inuse adjustment propagation which allows all hot path operations to be local to each iocg. To roughly summarize, Andy's method divides the tree into donating and non-donating parts, calculates global donation rate which is used to determine the target hweight_inuse for each node, and then derives per-level proportions. There's non-trivial amount of math involved. Please refer to the following pdfs for detailed descriptions. https://drive.google.com/file/d/1PsJwxPFtjUnwOY1QJ5AeICCcsL7BM3bo https://drive.google.com/file/d/1vONz1-fzVO7oY5DXXsLjSxEtYYQbOvsE https://drive.google.com/file/d/1WcrltBOSPN0qXVdBgnKm4mdp9FhuEFQN This patch implements Andy's method in transfer_surpluses(). This makes the donation calculations accurate per cycle and enables further improvements in other parts of the donation logic. Signed-off-by:
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Tejun Heo authored
The way the surplus donation logic is structured isn't great. There are two separate paths for starting/increasing donations and decreasing them making the logic harder to follow and is prone to unnecessary behavior differences. In preparation for improved donation handling, this patch restructures the code so that * All donors - new, increasing and decreasing - are funneled through the same code path. * The target donation calculation is factored into hweight_after_donation() which is called once from the same spot for all possible donors. * Actual inuse adjustment is factored into trasnfer_surpluses(). This change introduces a few behavior differences - e.g. donation amount reduction now uses the max usage of the recent three periods just like new and increasing donations, and inuse now gets adjusted upwards the same way it gets downwards. These differences are unlikely to have severely negative implications and the whole logic will be revamped soon. This patch also removes two tracepoints. The existing TPs don't quite fit the new implementation. A later patch will update and reinstate them. Signed-off-by:
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Tejun Heo authored
Budget donations are inaccurate and could take multiple periods to converge. To prevent triggering vrate adjustments while surplus transfers were catching up, vrate adjustment was suppressed if donations were increasing, which was indicated by non-zero nr_surpluses. This entangling won't be necessary with the scheduled rewrite of donation mechanism which will make it precise and immediate. Let's decouple the two in preparation. Signed-off-by:
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Tejun Heo authored
Instead of marking iocgs with surplus with a flag and filtering for them while walking all active iocgs, build a surpluses list. This doesn't make much difference now but will help implementing improved donation logic which will iterate iocgs with surplus multiple times. Signed-off-by:
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Tejun Heo authored
Currently, iocg->usages[] which are used to guide inuse adjustments are calculated from vtime deltas. This, however, assumes that the hierarchical inuse weight at the time of calculation held for the entire period, which often isn't true and can lead to significant errors. Now that we have absolute usage information collected, we can derive iocg->usages[] from iocg->local_stat.usage_us so that inuse adjustment decisions are made based on actual absolute usage. The calculated usage is clamped between 1 and WEIGHT_ONE and WEIGHT_ONE is also used to signal saturation regardless of the current hierarchical inuse weight. Signed-off-by:
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Tejun Heo authored
Currently, iocost doesn't collect or expose any statistics punting off all monitoring duties to drgn based iocost_monitor.py. While it works for some scenarios, there are some usability and data availability challenges. For example, accurate per-cgroup usage information can't be tracked by vtime progression at all and the number available in iocg->usages[] are really short-term snapshots used for control heuristics with possibly significant errors. This patch implements per-cgroup absolute usage stat counter and exposes it through io.stat along with the current vrate. Usage stat collection and flushing employ the same method as cgroup rstat on the active iocg's and the only hot path overhead is preemption toggling and adding to a percpu counter. Signed-off-by:
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Tejun Heo authored
Currently, debt handling requires only iocg->waitq.lock. In the future, we want to adjust and propagate inuse changes depending on debt status. Let's grab ioc->lock in debt handling paths in preparation. * Because ioc->lock nests outside iocg->waitq.lock, the decision to grab ioc->lock needs to be made before entering the critical sections. * Add and use iocg_[un]lock() which handles the conditional double locking. * Add @pay_debt to iocg_kick_waitq() so that debt payment happens only when the caller grabbed both locks. This patch is prepatory and the comments contain references to future changes. Signed-off-by:
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Tejun Heo authored
The margin handling was pretty inconsistent. * ioc->margin_us and ioc->inuse_margin_vtime were used as vtime margin thresholds. However, the two are in different units with the former requiring conversion to vtime on use. * iocg_kick_waitq() was using a quarter of WAITQ_TIMER_MARGIN_PCT of period_us as the timer slack - ~1.2%. While iocg_kick_delay() was using a quarter of ioc->margin_us - ~12.5%. There aren't strong reasons to use different values for the two. This patch cleans up margin and timer slack handling: * vtime margins are now recorded in ioc->margins.{min, max} on period duration changes and used consistently. * Timer slack is now 1% of period_us and recorded in ioc->timer_slack_ns and used consistently for iocg_kick_waitq() and iocg_kick_delay(). The only functional change is shortening of timer slack. No meaningful visible change is expected. Signed-off-by: -
Tejun Heo authored
They are in microseconds and wrap in around 1.2 hours with u32. While unlikely, confusions from wraparounds are still possible. We aren't saving anything meaningful by keeping these u32. Let's make them u64. Signed-off-by:
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Tejun Heo authored
To improve weight donations, we want to able to scale inuse with a greater accuracy and down below 1. Let's make non-hierarchical weights to use WEIGHT_ONE based fixed point numbers too like hierarchical ones. This doesn't cause any behavior changes yet. Signed-off-by:
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Tejun Heo authored
We're gonna use HWEIGHT_WHOLE for regular weights too. Let's rename it to WEIGHT_ONE. Pure rename. Signed-off-by:
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Tejun Heo authored
iocg_kick_waitq() is the function which pays debt and iocg_kick_delay() updates the actual delay status accordingly. If iocg_kick_delay() is not called after iocg_kick_delay() updated debt, unnecessarily large delays can be applied temporarily. Let's make sure such conditions don't occur by making iocg_kick_waitq() always call iocg_kick_delay() after paying debt. Signed-off-by:
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Tejun Heo authored
We'll make iocg_kick_waitq() call iocg_kick_delay(). Reorder them in preparation. This is pure code reorganization. Signed-off-by:
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Tejun Heo authored
__propagate_weights() currently expects the callers to clamp inuse within [1, active], which is needlessly fragile. The inuse adjustment logic is going to be revamped, in preparation, let's make __propagate_weights() clamp inuse on entry. Also, make it avoid weight updates altogether if neither active or inuse is changed. Signed-off-by:
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Tejun Heo authored
It already propagates two weights - active and inuse - and there will be another soon. Let's drop the confusing misnomers. Rename [__]propagate_active_weights() to [__]propagate_weights() and commit_active_weights() to commit_weights(). This is pure rename. Signed-off-by:
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Tejun Heo authored
blk-iocost has been reading percpu stat counters from remote cpus which on some archs can lead to torn reads in really rare occassions. Use local[64]_t for those counters. Signed-off-by:
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- 01 Sep, 2020 1 commit
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Tejun Heo authored
ioc_pd_free() grabs irq-safe ioc->lock without ensuring that irq is disabled when it can be called with irq disabled or enabled. This has a small chance of causing A-A deadlocks and triggers lockdep splats. Use irqsave operations instead. Signed-off-by:
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- 30 Jul, 2020 1 commit
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Chengming Zhou authored
We shouldn't skip iocg when its abs_vdebt is not zero. Fixes: 0b80f986 ("iocost: protect iocg->abs_vdebt with iocg->waitq.lock") Signed-off-by:
Chengming Zhou <zhouchengming@bytedance.com> Acked-by:
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- 29 Jul, 2020 1 commit
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Ahmed S. Darwish authored
A sequence counter write side critical section must be protected by some form of locking to serialize writers. A plain seqcount_t does not contain the information of which lock must be held when entering a write side critical section. Use the new seqcount_spinlock_t data type, which allows to associate a spinlock with the sequence counter. This enables lockdep to verify that the spinlock used for writer serialization is held when the write side critical section is entered. If lockdep is disabled this lock association is compiled out and has neither storage size nor runtime overhead. Signed-off-by:
Ahmed S. Darwish <a.darwish@linutronix.de> Signed-off-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by:
Daniel Wagner <dwagner@suse.de> Link: https://lkml.kernel.org/r/20200720155530.1173732-21-a.darwish@linutronix.de
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- 24 Jun, 2020 1 commit
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Gustavo A. R. Silva authored
Make use of the struct_size() helper instead of an open-coded version in order to avoid any potential type mistakes. This code was detected with the help of Coccinelle and, audited and fixed manually. Signed-off-by:
Gustavo A. R. Silva <gustavoars@kernel.org> Addresses-KSPP-ID: https://github.com/KSPP/linux/issues/83 Signed-off-by:
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- 14 May, 2020 1 commit
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Tejun Heo authored
When the QoS targets are met and nothing is being throttled, there's no way to tell how saturated the underlying device is - it could be almost entirely idle, at the cusp of saturation or anywhere inbetween. Given that there's no information, it's best to keep vrate as-is in this state. Before 7cd806a9 ("iocost: improve nr_lagging handling"), this was the case - if the device isn't missing QoS targets and nothing is being throttled, busy_level was reset to zero. While fixing nr_lagging handling, 7cd806a9 ("iocost: improve nr_lagging handling") broke this. Now, while the device is hitting QoS targets and nothing is being throttled, vrate keeps getting adjusted according to the existing busy_level. This led to vrate keeping climing till it hits max when there's an IO issuer with limited request concurrency if the vrate started low. vrate starts getting adjusted upwards until the issuer can issue IOs w/o being throttled. From then on, QoS targets keeps getting met and nothing on the system needs throttling and vrate keeps getting increased due to the existing busy_level. This patch makes the following changes to the busy_level logic. * Reset busy_level if nr_shortages is zero to avoid the above scenario. * Make non-zero nr_lagging block lowering nr_level but still clear positive busy_level if there's clear non-saturation signal - QoS targets are met and nr_shortages is non-zero. nr_lagging's role is preventing adjusting vrate upwards while there are long-running commands and it shouldn't keep busy_level positive while there's clear non-saturation signal. * Restructure code for clarity and add comments. Signed-off-by:
Andy Newell <newella@fb.com> Fixes: 7cd806a9 ("iocost: improve nr_lagging handling") Signed-off-by:
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- 05 May, 2020 1 commit
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Tejun Heo authored
abs_vdebt is an atomic_64 which tracks how much over budget a given cgroup is and controls the activation of use_delay mechanism. Once a cgroup goes over budget from forced IOs, it has to pay it back with its future budget. The progress guarantee on debt paying comes from the iocg being active - active iocgs are processed by the periodic timer, which ensures that as time passes the debts dissipate and the iocg returns to normal operation. However, both iocg activation and vdebt handling are asynchronous and a sequence like the following may happen. 1. The iocg is in the process of being deactivated by the periodic timer. 2. A bio enters ioc_rqos_throttle(), calls iocg_activate() which returns without anything because it still sees that the iocg is already active. 3. The iocg is deactivated. 4. The bio from #2 is over budget but needs to be forced. It increases abs_vdebt and goes over the threshold and enables use_delay. 5. IO control is enabled for the iocg's subtree and now IOs are attributed to the descendant cgroups and the iocg itself no longer issues IOs. This leaves the iocg with stuck abs_vdebt - it has debt but inactive and no further IOs which can activate it. This can end up unduly punishing all the descendants cgroups. The usual throttling path has the same issue - the iocg must be active while throttled to ensure that future event will wake it up - and solves the problem by synchronizing the throttling path with a spinlock. abs_vdebt handling is another form of overage handling and shares a lot of characteristics including the fact that it isn't in the hottest path. This patch fixes the above and other possible races by strictly synchronizing abs_vdebt and use_delay handling with iocg->waitq.lock. Signed-off-by:
Vlad Dmitriev <vvd@fb.com> Cc: stable@vger.kernel.org # v5.4+ Fixes: e1518f63 ("blk-iocost: Don't let merges push vtime into the future") Signed-off-by:
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- 30 Apr, 2020 2 commits
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Tejun Heo authored
On each IO completion, iocost decides whether the IO met or missed its latency target. Currently, the targets are fixed numbers per IO type. While this can be good enough for loose latency targets way higher than typical completion latencies, the effect of IO size makes it difficult to tighten the latency target - a target adequate for 4k IOs might be too tight for 512k IOs and vice-versa. iocost already has all the necessary information to account for different IO sizes when testing whether the latency target is met as iocost can calculate the size vtime cost of a given IO. This patch updates the completion path to calculate the size vtime cost of the IO, deduct the nsec equivalent from the observed latency and use the adjusted value to decide whether the target is met. This makes latency targets independent from IO size and enables determining adequate latency targets with fixed size fio runs. Signed-off-by:
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Tejun Heo authored
The use_delay mechanism was introduced by blk-iolatency to hold memory allocators accountable for the reclaim and other shared IOs they cause. The duration of the delay is dynamically balanced between iolatency increasing the value on each target miss and it auto-decaying as time passes and threads get delayed on it. While this works well for iolatency, iocost's control model isn't compatible with it. There is no repeated "violation" events which can be balanced against auto-decaying. iocost instead knows how much a given cgroup is over budget and wants to prevent that cgroup from issuing IOs while over budget. Until now, iocost has been adding the cost of force-issued IOs. However, this doesn't reflect the amount which is already over budget and is simply not enough to counter the auto-decaying allowing anon-memory leaking low priority cgroup to go over its alloted share of IOs. As auto-decaying doesn't make much sense for iocost, this patch introduces a different mode of operation for use_delay - when blkcg_set_delay() are used insted of blkcg_add/use_delay(), the delay duration is not auto-decayed until it is explicitly cleared with blkcg_clear_delay(). iocost is updated to keep the delay duration synchronized to the budget overage amount. With this change, iocost can effectively police cgroups which generate significant amount of force-issued IOs. Signed-off-by:
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- 21 Apr, 2020 1 commit
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Waiman Long authored
Systemtap 4.2 is unable to correctly interpret the "u32 (*missed_ppm)[2]" argument of the iocost_ioc_vrate_adj trace entry defined in include/trace/events/iocost.h leading to the following error: /tmp/stapAcz0G0/stap_c89c58b83cea1724e26395efa9ed4939_6321_aux_6.c:78:8: error: expected ‘;’, ‘,’ or ‘)’ before ‘*’ token , u32[]* __tracepoint_arg_missed_ppm That argument type is indeed rather complex and hard to read. Looking at block/blk-iocost.c. It is just a 2-entry u32 array. By simplifying the argument to a simple "u32 *missed_ppm" and adjusting the trace entry accordingly, the compilation error was gone. Fixes: 7caa4715 ("blkcg: implement blk-iocost") Acked-by:
Steven Rostedt (VMware) <rostedt@goodmis.org> Acked-by:
Waiman Long <longman@redhat.com> Signed-off-by:
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- 12 Mar, 2020 1 commit
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Weiping Zhang authored
Acked-by:
Weiping Zhang <zhangweiping@didiglobal.com> Signed-off-by:
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- 10 Mar, 2020 1 commit
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Tejun Heo authored
vtimes may wrap and time_before/after64() should be used to determine whether a given vtime is before or after another. iocg_is_idle() was incorrectly using plain "<" comparison do determine whether done_vtime is before vtime. Here, the only thing we're interested in is whether done_vtime matches vtime which indicates that there's nothing in flight. Let's test for inequality instead. Signed-off-by:
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- 16 Dec, 2019 1 commit
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Tejun Heo authored
When over-budget IOs are force-issued through root cgroup, iocg_kick_delay() adjusts the async delay accordingly but doesn't actually schedule async throttle for the issuing task. This bug is pretty well masked because sooner or later the offending threads are gonna get directly throttled on regular IOs or have async delay scheduled by mem_cgroup_throttle_swaprate(). However, it can affect control quality on filesystem metadata heavy operations. Let's fix it by invoking blkcg_schedule_throttle() when iocg_kick_delay() says async delay is needed. Signed-off-by:
Josef Bacik <josef@toxicpanda.com> Signed-off-by:
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- 14 Nov, 2019 1 commit
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Jiufei Xue authored
There is a bug that checking the same active_list over and over again in iocg_activate(). The intention of the code was checking whether all the ancestors and self have already been activated. So fix it. Fixes: 7caa4715 ("blkcg: implement blk-iocost") Acked-by:
Jiufei Xue <jiufei.xue@linux.alibaba.com> Signed-off-by:
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- 31 Oct, 2019 1 commit
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Dan Carpenter authored
This code causes a static analysis warning: block/blk-iocost.c:2113 ioc_weight_write() error: double lock 'irq' We disable IRQs in blkg_conf_prep() and re-enable them in blkg_conf_finish(). IRQ disable/enable should not be nested because that means the IRQs will be enabled at the first unlock instead of the second one. Fixes: 7caa4715 ("blkcg: implement blk-iocost") Acked-by:Dan Carpenter <dan.carpenter@oracle.com> Signed-off-by:
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