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Date: Thu, 9 Mar 2023 22:23:58 +0800
From: Zhang Qiao <zhangqiao22@...wei.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
CC: <linux-kernel@...r.kernel.org>, <mingo@...hat.com>,
<peterz@...radead.org>, <juri.lelli@...hat.com>,
<dietmar.eggemann@....com>, <rostedt@...dmis.org>,
<bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>,
<vschneid@...hat.com>, <rkagan@...zon.de>
Subject: Re: [PATCH v2] sched/fair: sanitize vruntime of entity being migrated
在 2023/3/9 18:48, Vincent Guittot 写道:
> On Thu, 9 Mar 2023 at 09:37, Zhang Qiao <zhangqiao22@...wei.com> 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@...wei.com> wrote:
>>>>>
>>>>>
>>>>>
>>>>> 在 2023/3/7 18:26, Vincent Guittot 写道:
>>>>>> On Mon, 6 Mar 2023 at 14:53, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>>>>>>>
>>>>>>> On Mon, 6 Mar 2023 at 13:57, Zhang Qiao <zhangqiao22@...wei.com> 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@...aro.org>
>>>>>>>> Signed-off-by: Zhang Qiao <zhangqiao22@...wei.com>
>>>>>>>
>>>>>>> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
>>>>>>>
>>>>>>>> ---
>>>>>>>>
>>>>>>>> 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
>>>>>>>>
>>>>>> .
>>>>>>
>>> .
>>>
> .
>
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