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Message-ID: <07c692fd-fe59-1bd4-a6d0-e84bee6dbb3b@huawei.com>
Date: Thu, 9 Mar 2023 16:37:05 +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 2023/3/8 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.
> + 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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