Messages in this thread | | | From | Vincent Guittot <> | Date | Thu, 2 Mar 2023 15:55:54 +0100 | Subject | Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed |
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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
> > 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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