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

[Zhang Qiao]

在 2023/3/8 20:55, Vincent Guittot 写道:
> Le mercredi 08 mars 2023 à 09:01:05 (+0100), Vincent Guittot a écrit :
>> On Tue, 7 Mar 2023 at 14:41, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>>
>>>
>>>
>>> 在 2023/3/7 18:26, Vincent Guittot 写道:
>>>> On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>>>>>
>>>>> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> Commit 829c1651e9c4 ("sched/fair: sanitize vruntime of
>>>>>> entity being placed") fix an overflowing bug, but ignore
>>>>>> a case that se->exec_start is reset after a migration.
>>>>>>
>>>>>> For fixing this case, we reset the vruntime of a long
>>>>>> sleeping task in migrate_task_rq_fair().
>>>>>>
>>>>>> Fixes: 829c1651e9c4 ("sched/fair: sanitize vruntime of entity being placed")
>>>>>> Suggested-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>>>>> Signed-off-by: Zhang Qiao <zhangqiao22@xxxxxxxxxx>
>>>>>
>>>>> Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
>>>>>
>>>>>> ---
>>>>>>
>>>>>> v1 -> v2:
>>>>>> - fix some typos and update comments
>>>>>> - reformat the patch
>>>>>>
>>>>>> ---
>>>>>>  kernel/sched/fair.c | 76 ++++++++++++++++++++++++++++++++-------------
>>>>>>  1 file changed, 55 insertions(+), 21 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>>> index 7a1b1f855b96..74c9918ffe76 100644
>>>>>> --- a/kernel/sched/fair.c
>>>>>> +++ b/kernel/sched/fair.c
>>>>>> @@ -4648,11 +4648,45 @@ static void check_spread(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>>>>  #endif
>>>>>>  }
>>>>>>
>>>>>> +static inline bool entity_is_long_sleep(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)) - se->exec_start;
>>>>>> +       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
>>>>>> +               return true;
>>>>>> +
>>>>>> +       return false;
>>>>>> +}
>>>>>> +
>>>>>> +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)
>>>>>>  {
>>>>>>         u64 vruntime = cfs_rq->min_vruntime;
>>>>>> -       u64 sleep_time;
>>>>>>
>>>>>>         /*
>>>>>>          * The 'current' period is already promised to the current tasks,
>>>>>> @@ -4664,23 +4698,8 @@ 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
>>>>>> @@ -4689,8 +4708,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>>>>>>          * 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)
>>>>>> +       if (entity_is_long_sleep(se))
>>>>>>                 se->vruntime = vruntime;
>>>>>>         else
>>>>>>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
>>>>>> @@ -7635,7 +7653,23 @@ 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);
>>>>>>
>>>>>> -               se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
>>>>>> +               /*
>>>>>> +                * We determine whether a task sleeps for long by checking
>>>>>> +                * se->exec_start, and if it is, we sanitize its vruntime at
>>>>>> +                * place_entity(). However, after a migration, this detection
>>>>>> +                * method fails due to se->exec_start being reset.
>>>>>> +                *
>>>>>> +                * For fixing this case, we add the same check here. For a task
>>>>>> +                * which has slept for a long time, its vruntime should be reset
>>>>>> +                * to cfs_rq->min_vruntime with a sleep credit. Because waking
>>>>>> +                * task's vruntime will be added to cfs_rq->min_vruntime when
>>>>>> +                * enqueue, we only need to reset the se->vruntime of waking task
>>>>>> +                * to a credit here.
>>>>>> +                */
>>>>>> +               if (entity_is_long_sleep(se))
>>>>
>>>> I completely overlooked that we can't use rq_clock_task here. Need to
>>>> think a bit more on this
>>>
>>> Hi,Vincent,
>>>
>>> How about using exec_start of the parent sched_entity instant of rq_clock_task()?
>>
>> How do we handle sched_entity without a parent sched_entity ?
> 
> The change below should do the stuff. Not that we can't use it in place entity because
> pelt is not enabled in UP.
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 74c9918ffe76..b8b381b0ff20 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7635,6 +7635,32 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
>         return new_cpu;
>  }
> 
> +static inline bool migrate_long_sleeper(struct sched_entity *se)
> +{
> +       struct cfs_rq *cfs_rq;
> +       u64 sleep_time;
> +
> +       if (se->exec_start == 0)

How about use `se->avg.last_update_time == 0` here?

> +               return false;
> +
> +       cfs_rq = cfs_rq_of(se);
> +       /*
> +        * If the entity slept for a long time, don't even try to normalize its
> +        * vruntime with the base as it may be too far off and might generate
> +        * wrong decision because of s64 overflow.
> +        * We estimate its sleep duration with the last update of se's pelt.
> +        * The last update happened before sleeping. The cfs' pelt is not
> +        * always updated when cfs is idle but this is not a problem because
> +        * its min_vruntime is not updated too, so the situation can't get
> +        * worse.
> +        */
> +       sleep_time = cfs_rq_last_update_time(cfs_rq) - se->avg.last_update_time;
> +       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)

I tested with this patch and found it make hackbench slower.


> +               return true;
> +
> +       return false;
> +}
> +
>  /*
>   * Called immediately before a task is migrated to a new CPU; task_cpu(p) and
>   * cfs_rq_of(p) references at time of call are still valid and identify the
> @@ -7666,7 +7692,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>                  * enqueue, we only need to reset the se->vruntime of waking task
>                  * to a credit here.
>                  */
> -               if (entity_is_long_sleep(se))
> +               if (migrate_long_sleeper(se))
>                         se->vruntime = -sched_sleeper_credit(se);
>                 else
>                         se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
> 
> 
> 
>>
>>
>>
>>>>
>>>>>> +                       se->vruntime = -sched_sleeper_credit(se);
>>>>>> +               else
>>>>>> +                       se->vruntime -= u64_u32_load(cfs_rq->min_vruntime);
>>>>>>         }
>>>>>>
>>>>>>         if (!task_on_rq_migrating(p)) {
>>>>>> --
>>>>>> 2.17.1
>>>>>>
>>>> .
>>>>
> .
>