Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated

From: Vincent Guittot
Date: Tue Mar 14 2023 - 11:33:10 EST

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Le mardi 14 mars 2023 à 14:39:49 (+0100), Vincent Guittot a écrit :
> On Tue, 14 Mar 2023 at 14:38, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
> >
> >
> >
> > 在 2023/3/14 21:26, Vincent Guittot 写道:
> > > On Tue, 14 Mar 2023 at 12:03, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
> > >>
> > >>
> > >>
> > >> 在 2023/3/13 22:23, Vincent Guittot 写道:
> > >>> On Sat, 11 Mar 2023 at 10:57, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
> > >>>>
> > >>>>
> > >>>>
> > >>>> 在 2023/3/10 22:29, Vincent Guittot 写道:
> > >>>>> Le jeudi 09 mars 2023 à 16:14:38 (+0100), Vincent Guittot a écrit :
> > >>>>>> On Thu, 9 Mar 2023 at 15:37, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > >>>>>>>
> > >>>>>>> On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote:
> > >>>>>>>> On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote:
> > >>>>>>>>
> > >>>>>>>>> Then, even if we don't clear exec_start before migrating and keep
> > >>>>>>>>> current value to be used in place_entity on the new cpu, we can't
> > >>>>>>>>> compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT
> > >>>>>>>>
> > >>>>>>>> Blergh -- indeed, irq and steal time can skew them between CPUs :/
> > >>>>>>>> I suppose we can fudge that... wait_start (which is basically what we're
> > >>>>>>>> making it do) also does that IIRC.
> > >>>>>>>>
> > >>>>>>>> I really dislike having this placement muck spreadout like proposed.
> > >>>>>>>
> > >>>>>>> Also, I think we might be over-engineering this, we don't care about
> > >>>>>>> accuracy at all, all we really care about is 'long-time'.
> > >>>>>>
> > >>>>>> you mean taking the patch 1/2 that you mentioned here to add a
> > >>>>>> migrated field:
> > >>>>>> https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@xxxxxxxxxxxxxxx/T/#ma5637eb8010f3f4a4abff778af8db705429d003b
> > >>>>>>
> > >>>>>> And assume that the divergence between the rq_clock_task() can be ignored ?
> > >>>>>>
> > >>>>>> That could probably work but we need to replace the (60LL *
> > >>>>>> NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence
> > >>>>>> would not be unrealistic.
> > >>>>>> and a comment to explain why it's acceptable
> > >>>>>
> > >>>>> Zhang,
> > >>>>>
> > >>>>> Could you try the patch below ?
> > >>>>> This is a rebase/merge/update of:
> > >>>>> -patch 1/2 above and
> > >>>>> -https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@xxxxxxxxx/
> > >>>>
> > >>>>
> > >>>> I applyed and tested this patch, and it make hackbench slower.
> > >>>> According to my previous test results. The good result is 82.1(s).
> > >>>> But the result of this patch is 108.725(s).
> > >>>
> > >>> By "the result of this patch is 108.725(s)", you mean the result of
> > >>> https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@xxxxxxxxx/
> > >>> alone, don't you ?
> > >>
> > >> No, with your patch, the test results is 108.725(s),
> > >
> > > Ok
> > >
> > >>
> > >> git diff:
> > >>
> > >> diff --git a/include/linux/sched.h b/include/linux/sched.h
> > >> index 63d242164b1a..93a3909ae4c4 100644
> > >> --- a/include/linux/sched.h
> > >> +++ b/include/linux/sched.h
> > >> @@ -550,6 +550,7 @@ struct sched_entity {
> > >> struct rb_node run_node;
> > >> struct list_head group_node;
> > >> unsigned int on_rq;
> > >> + unsigned int migrated;
> > >>
> > >> u64 exec_start;
> > >> u64 sum_exec_runtime;
> > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > >> index ff4dbbae3b10..e60defc39f6e 100644
> > >> --- a/kernel/sched/fair.c
> > >> +++ b/kernel/sched/fair.c
> > >> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >> /*
> > >> * We are starting a new run period:
> > >> */
> > >> + se->migrated = 0;
> > >> se->exec_start = rq_clock_task(rq_of(cfs_rq));
> > >> }
> > >>
> > >> @@ -4690,9 +4691,9 @@ 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 ((s64)sleep_time > (1ULL << 63) / scale_load_down(NICE_0_LOAD) / 2) {
> > >> se->vruntime = vruntime;
> > >> - else
> > >> + } else
> > >> se->vruntime = max_vruntime(se->vruntime, vruntime);
> > >> }
> > >>
> > >> @@ -7658,8 +7659,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
> > >> se->avg.last_update_time = 0;
> > >>
> > >> /* We have migrated, no longer consider this task hot */
> > >> - se->exec_start = 0;
> > >> -
> > >> + se->migrated = 1;
> > >> update_scan_period(p, new_cpu);
> > >> }
> > >>
> > >> @@ -8343,6 +8343,8 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
> > >>
> > >> if (sysctl_sched_migration_cost == 0)
> > >> return 0;
> > >> + if (p->se.migrated)
> > >> + return 0;
> > >>
> > >> delta = rq_clock_task(env->src_rq) - p->se.exec_start;
> > >>
> > >>
> > >>
> > >>>
> > >>>>
> > >>>>
> > >>>>> 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
> > >>>
> > >>> Ok, it looks like we are back to normal figures
> > >
> > > What do those results refer to then ?
> >
> > Quote from this email (https://lore.kernel.org/lkml/1cd19d3f-18c4-92f9-257a-378cc18cfbc7@xxxxxxxxxx/).
>
> ok.
>
> Then, there is something wrong in my patch. Let me look at it more deeply

Coudl you try the patc below. It fixes the problem on my system

---
kernel/sched/fair.c | 84 +++++++++++++++++++++++++++++++--------------
1 file changed, 59 insertions(+), 25 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0f499e9a74b5..f8722e47bb0b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4648,23 +4648,36 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
#endif
}

+static inline bool entity_is_long_sleeper(struct sched_entity *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));
+
+ /* Happen while migrating because of clock task divergence */
+ if (sleep_time <= se->exec_start)
+ return false;
+
+ sleep_time -= se->exec_start;
+ if (sleep_time > ((1ULL << 63) / scale_load_down(NICE_0_LOAD)))
+ return true;
+
+ return false;
+}
+
static void
-place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
+place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
{
u64 vruntime = cfs_rq->min_vruntime;
- u64 sleep_time;
-
- /*
- * The 'current' period is already promised to the current tasks,
- * however the extra weight of the new task will slow them down a
- * little, place the new task so that it fits in the slot that
- * stays open at the end.
- */
- if (initial && sched_feat(START_DEBIT))
- vruntime += sched_vslice(cfs_rq, se);

/* sleeps up to a single latency don't count. */
- if (!initial) {
+ if (flags & ENQUEUE_WAKEUP) {
unsigned long thresh;

if (se_is_idle(se))
@@ -4680,20 +4693,43 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
thresh >>= 1;

vruntime -= thresh;
+ } else if sched_feat(START_DEBIT) {
+ /*
+ * The 'current' period is already promised to the current tasks,
+ * however the extra weight of the new task will slow them down a
+ * little, place the new task so that it fits in the slot that
+ * stays open at the end.
+ */
+ vruntime += sched_vslice(cfs_rq, se);
}

/*
* Pull vruntime of the entity being placed to the base level of
- * cfs_rq, to prevent boosting it if placed backwards. If the entity
- * slept for a long time, don't even try to compare its vruntime with
- * the base as it may be too far off and the comparison may get
- * 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)
+ * cfs_rq, to prevent boosting it if placed backwards.
+ * However, min_vruntime can advance much faster than real time, with
+ * the exterme being when an entity with the minimal weight always runs
+ * on the cfs_rq. If the new entity slept for long, its vruntime
+ * difference from min_vruntime may overflow s64 and their comparison
+ * may get inversed, so ignore the entity's original vruntime in that
+ * case.
+ * The maximal vruntime speedup is given by the ratio of normal to
+ * minimal weight: scale_load_down(NICE_0_LOAD) / MIN_SHARES.
+ * When placing a migrated waking entity, its exec_start has been set
+ * from a different rq. In order to take into account a possible
+ * divergence between new and prev rq's clocks task because of irq and
+ * stolen time, we take an additional margin.
+ * So, cutting off on the sleep time of
+ * 2^63 / scale_load_down(NICE_0_LOAD) ~ 104 days
+ * should be safe.
+
+ */
+ if (entity_is_long_sleeper(se))
se->vruntime = vruntime;
else
se->vruntime = max_vruntime(se->vruntime, vruntime);
+
+ if (flags & ENQUEUE_MIGRATED)
+ se->exec_start = 0;
}

static void check_enqueue_throttle(struct cfs_rq *cfs_rq);
@@ -4769,7 +4805,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
account_entity_enqueue(cfs_rq, se);

if (flags & ENQUEUE_WAKEUP)
- place_entity(cfs_rq, se, 0);
+ place_entity(cfs_rq, se, flags);

check_schedstat_required();
update_stats_enqueue_fair(cfs_rq, se, flags);
@@ -7665,9 +7701,6 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
/* Tell new CPU we are migrated */
se->avg.last_update_time = 0;

- /* We have migrated, no longer consider this task hot */
- se->exec_start = 0;
-
update_scan_period(p, new_cpu);
}

@@ -11993,7 +12026,7 @@ static void task_fork_fair(struct task_struct *p)
update_curr(cfs_rq);
se->vruntime = curr->vruntime;
}
- place_entity(cfs_rq, se, 1);
+ place_entity(cfs_rq, se, 0);

if (sysctl_sched_child_runs_first && curr && entity_before(curr, se)) {
/*
@@ -12137,8 +12170,9 @@ static void detach_task_cfs_rq(struct task_struct *p)
/*
* Fix up our vruntime so that the current sleep doesn't
* cause 'unlimited' sleep bonus.
+ * This is the same as placing a waking task.
*/
- place_entity(cfs_rq, se, 0);
+ place_entity(cfs_rq, se, ENQUEUE_WAKEUP);
se->vruntime -= cfs_rq->min_vruntime;
}

--
2.34.1

>
> >
> > >

Next message: Neeraj Sanjay Kale: "[PATCH v11 1/4] serdev: Replace all instances of ENOTSUPP with EOPNOTSUPP"
Previous message: Vladimir Oltean: "[PATCH net] net: phy: mscc: fix deadlock in phy_ethtool_{get,set}_wol()"
In reply to: Vincent Guittot: "Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated"
Next in thread: Zhang Qiao: "Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated"
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