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

[Zhang Qiao]

在 2023/3/9 18:48, Vincent Guittot 写道:
> On Thu, 9 Mar 2023 at 09:37, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>
>>
>>
>> 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.
> 
> Compared to which version ?
> - v6.2 + revert commit 829c1651e9c4 ?
> - v6.2 ?
> 
> v6.2 has a bug because newly migrated task gets a runtime credit
> whatever its previous vruntime and hackbench generates a lot of
> migration
> 

Sorry, When I was testing, I forgot to add the check 'se->exec_start == 0' in place_entity().
After i re-test, the hackbench result is ok.


> 
> 
>>
>>
>>> +               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
>>>>>>>>
>>>>>> .
>>>>>>
>>> .
>>>
> .
>