Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 3 Mar 2023 09:32:50 +0100 | Subject | Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed |
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On Fri, 3 Mar 2023 at 07:51, Zhang Qiao <zhangqiao22@huawei.com> wrote: > > > > 在 2023/3/2 22:55, Vincent Guittot 写道: > > On Thu, 2 Mar 2023 at 15:29, Zhang Qiao <zhangqiao22@huawei.com> wrote: > >> > >> > >> > >> 在 2023/3/2 21:34, Vincent Guittot 写道: > >>> On Thu, 2 Mar 2023 at 10:36, Zhang Qiao <zhangqiao22@huawei.com> wrote: > >>>> > >>>> > >>>> > >>>> 在 2023/2/27 22:37, Vincent Guittot 写道: > >>>>> On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@amazon.de> wrote: > >>>>>> > >>>>>> On Tue, Feb 21, 2023 at 06:26:11PM +0100, Vincent Guittot wrote: > >>>>>>> On Tue, 21 Feb 2023 at 17:57, Roman Kagan <rkagan@amazon.de> wrote: > >>>>>>>> What scares me, though, is that I've got a message from the test robot > >>>>>>>> that this commit drammatically affected hackbench results, see the quote > >>>>>>>> below. I expected the commit not to affect any benchmarks. > >>>>>>>> > >>>>>>>> Any idea what could have caused this change? > >>>>>>> > >>>>>>> Hmm, It's most probably because se->exec_start is reset after a > >>>>>>> migration and the condition becomes true for newly migrated task > >>>>>>> whereas its vruntime should be after min_vruntime. > >>>>>>> > >>>>>>> We have missed this condition > >>>>>> > >>>>>> Makes sense to me. > >>>>>> > >>>>>> But what would then be the reliable way to detect a sched_entity which > >>>>>> has slept for long and risks overflowing in .vruntime comparison? > >>>>> > >>>>> For now I don't have a better idea than adding the same check in > >>>>> migrate_task_rq_fair() > >>>> > >>>> Hi, Vincent, > >>>> I fixed this condition as you said, and the test results are as follows. > >>>> > >>>> testcase: hackbench -g 44 -f 20 --process --pipe -l 60000 -s 100 > >>>> version1: v6.2 > >>>> version2: v6.2 + commit 829c1651e9c4 > >>>> version3: v6.2 + commit 829c1651e9c4 + this patch > >>>> > >>>> ------------------------------------------------- > >>>> version1 version2 version3 > >>>> test1 81.0 118.1 82.1 > >>>> test2 82.1 116.9 80.3 > >>>> test3 83.2 103.9 83.3 > >>>> avg(s) 82.1 113.0 81.9 > >>>> > >>>> ------------------------------------------------- > >>>> After deal with the task migration case, the hackbench result has restored. > >>>> > >>>> The patch as follow, how does this look? > >>>> > >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >>>> index ff4dbbae3b10..3a88d20fd29e 100644 > >>>> --- a/kernel/sched/fair.c > >>>> +++ b/kernel/sched/fair.c > >>>> @@ -4648,6 +4648,26 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se) > >>>> #endif > >>>> } > >>>> > >>>> +static inline u64 sched_sleeper_credit(struct sched_entity *se) > >>>> +{ > >>>> + > >>>> + unsigned long thresh; > >>>> + > >>>> + if (se_is_idle(se)) > >>>> + thresh = sysctl_sched_min_granularity; > >>>> + else > >>>> + thresh = sysctl_sched_latency; > >>>> + > >>>> + /* > >>>> + * Halve their sleep time's effect, to allow > >>>> + * for a gentler effect of sleepers: > >>>> + */ > >>>> + if (sched_feat(GENTLE_FAIR_SLEEPERS)) > >>>> + thresh >>= 1; > >>>> + > >>>> + return thresh; > >>>> +} > >>>> + > >>>> static void > >>>> place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) > >>>> { > >>>> @@ -4664,23 +4684,8 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) > >>>> vruntime += sched_vslice(cfs_rq, se); > >>>> > >>>> /* sleeps up to a single latency don't count. */ > >>>> - if (!initial) { > >>>> - unsigned long thresh; > >>>> - > >>>> - if (se_is_idle(se)) > >>>> - thresh = sysctl_sched_min_granularity; > >>>> - else > >>>> - thresh = sysctl_sched_latency; > >>>> - > >>>> - /* > >>>> - * Halve their sleep time's effect, to allow > >>>> - * for a gentler effect of sleepers: > >>>> - */ > >>>> - if (sched_feat(GENTLE_FAIR_SLEEPERS)) > >>>> - thresh >>= 1; > >>>> - > >>>> - vruntime -= thresh; > >>>> - } > >>>> + if (!initial) > >>>> + vruntime -= sched_sleeper_credit(se); > >>>> > >>>> /* > >>>> * Pull vruntime of the entity being placed to the base level of > >>>> @@ -4690,7 +4695,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial) > >>>> * inversed due to s64 overflow. > >>>> */ > >>>> sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; > >>>> - if ((s64)sleep_time > 60LL * NSEC_PER_SEC) > >>>> + if (se->exec_start != 0 && (s64)sleep_time > 60LL * NSEC_PER_SEC) > >>>> se->vruntime = vruntime; > >>>> else > >>>> se->vruntime = max_vruntime(se->vruntime, vruntime); > >>>> @@ -7634,8 +7639,12 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu) > >>>> */ > >>>> if (READ_ONCE(p->__state) == TASK_WAKING) { > >>>> struct cfs_rq *cfs_rq = cfs_rq_of(se); > >>>> + u64 sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; > >>>> > >>>> - se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > >>>> + if ((s64)sleep_time > 60LL * NSEC_PER_SEC) > >>> > >>> You also need to test (se->exec_start !=0) here because the task might > >> > >> Hi, > >> > >> I don't understand when the another migration happend. Could you tell me in more detail? > > > > se->exec_start is update when the task becomes current. > > > > You can have the sequence: > > > > task TA runs on CPU0 > > TA's se->exec_start = xxxx > > TA is put back into the rb tree waiting for next slice while another > > task is running > > CPU1 pulls TA which migrates on CPU1 > > migrate_task_rq_fair() w/ TA's se->exec_start == xxxx > > TA's se->exec_start = 0 > > TA is put into the rb tree of CPU1 waiting to run on CPU1 > > CPU2 pulls TA which migrates on CPU2 > > migrate_task_rq_fair() w/ TA's se->exec_start == 0 > > TA's se->exec_start = 0 > Hi, Vincent, > > yes, you're right, such sequence does exist. But at this point, p->__state != TASK_WAKING. > > I have a question, Whether there is case that is "p->se.exec_start == 0 && p->__state == TASK_WAKING" ? > I analyzed the code and concluded that this case isn't existed, is it right?
Yes, you're right. Your proposal is enough
Thanks
> > Thanks. > ZhangQiao. > > > > >> > >> I think the next migration will happend after the wakee task enqueued, but at this time > >> the p->__state isn't TASK_WAKING, p->__state already be changed to TASK_RUNNING at ttwu_do_wakeup(). > >> > >> If such a migration exists, Previous code "se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);" maybe > >> perform multiple times,wouldn't it go wrong in this way? > > > > the vruntime have been updated when enqueued but not exec_start > > > >> > >>> migrate another time before being scheduled. You should create a > >>> helper function like below and use it in both place > >> > >> Ok, I will update at next version. > >> > >> > >> Thanks, > >> ZhangQiao. > >> > >>> > >>> static inline bool entity_long_sleep(se) > >>> { > >>> struct cfs_rq *cfs_rq; > >>> u64 sleep_time; > >>> > >>> if (se->exec_start == 0) > >>> return false; > >>> > >>> cfs_rq = cfs_rq_of(se); > >>> sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start; > >>> if ((s64)sleep_time > 60LL * NSEC_PER_SEC) > >>> return true; > >>> > >>> return false; > >>> } > >>> > >>> > >>>> + se->vruntime = -sched_sleeper_credit(se); > >>>> + else > >>>> + se->vruntime -= u64_u32_load(cfs_rq->min_vruntime); > >>>> } > >>>> > >>>> if (!task_on_rq_migrating(p)) { > >>>> > >>>> > >>>> > >>>> Thanks. > >>>> Zhang Qiao. > >>>> > >>>>> > >>>>>> > >>>>>> Thanks, > >>>>>> Roman. > >>>>>> > >>>>>> > >>>>>> > >>>>>> Amazon Development Center Germany GmbH > >>>>>> Krausenstr. 38 > >>>>>> 10117 Berlin > >>>>>> Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss > >>>>>> Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B > >>>>>> Sitz: Berlin > >>>>>> Ust-ID: DE 289 237 879 > >>>>>> > >>>>>> > >>>>>> > >>>>> . > >>>>> > >>> . > >>> > > . > >
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