Re: [PATCH v3] sched/fair: sanitize vruntime of entity being placed

[Zhang Qiao]

在 2023/2/27 22:37, Vincent Guittot 写道:
> On Mon, 27 Feb 2023 at 09:43, Roman Kagan <rkagan@xxxxxxxxx> 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@xxxxxxxxx> 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)
+                       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.
>>
>>
>>
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>>
>>
>>
> .
>