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

[Zhang Qiao]

在 2023/3/14 21:26, Vincent Guittot 写道:
> On Tue, 14 Mar 2023 at 12:03, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>
>>
>>
>> 在 2023/3/13 22:23, Vincent Guittot 写道:
>>> On Sat, 11 Mar 2023 at 10:57, Zhang Qiao <zhangqiao22@xxxxxxxxxx> wrote:
>>>>
>>>>
>>>>
>>>> 在 2023/3/10 22:29, Vincent Guittot 写道:
>>>>> Le jeudi 09 mars 2023 à 16:14:38 (+0100), Vincent Guittot a écrit :
>>>>>> On Thu, 9 Mar 2023 at 15:37, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>>>>>>>
>>>>>>> On Thu, Mar 09, 2023 at 03:28:25PM +0100, Peter Zijlstra wrote:
>>>>>>>> On Thu, Mar 09, 2023 at 02:34:05PM +0100, Vincent Guittot wrote:
>>>>>>>>
>>>>>>>>> Then, even if we don't clear exec_start before migrating and keep
>>>>>>>>> current value to be used in place_entity on the new cpu, we can't
>>>>>>>>> compare the rq_clock_task(rq_of(cfs_rq)) of 2 different rqs AFAICT
>>>>>>>>
>>>>>>>> Blergh -- indeed, irq and steal time can skew them between CPUs :/
>>>>>>>> I suppose we can fudge that... wait_start (which is basically what we're
>>>>>>>> making it do) also does that IIRC.
>>>>>>>>
>>>>>>>> I really dislike having this placement muck spreadout like proposed.
>>>>>>>
>>>>>>> Also, I think we might be over-engineering this, we don't care about
>>>>>>> accuracy at all, all we really care about is 'long-time'.
>>>>>>
>>>>>> you mean taking the patch 1/2 that you mentioned here to add a
>>>>>> migrated field:
>>>>>> https://lore.kernel.org/all/68832dfbb60fda030540b5f4e39c5801942689b1.1648228023.git.tim.c.chen@xxxxxxxxxxxxxxx/T/#ma5637eb8010f3f4a4abff778af8db705429d003b
>>>>>>
>>>>>> And assume that the divergence between the rq_clock_task() can be ignored ?
>>>>>>
>>>>>> That could probably work but we need to replace the (60LL *
>>>>>> NSEC_PER_SEC) by ((1ULL << 63) / NICE_0_LOAD) because 60sec divergence
>>>>>> would not be unrealistic.
>>>>>> and a comment to explain why it's acceptable
>>>>>
>>>>> Zhang,
>>>>>
>>>>> Could you try the patch below ?
>>>>> This is a rebase/merge/update of:
>>>>> -patch 1/2 above and
>>>>> -https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@xxxxxxxxx/
>>>>
>>>>
>>>> I applyed and tested this patch, and it make hackbench slower.
>>>> According to my previous test results. The good result is 82.1(s).
>>>> But the result of this patch is 108.725(s).
>>>
>>> By "the result of this patch is 108.725(s)",  you mean the result of
>>> https://lore.kernel.org/lkml/20230209193107.1432770-1-rkagan@xxxxxxxxx/
>>> alone, don't you ?
>>
>> No, with your patch, the test results is 108.725(s),
> 
> Ok
> 
>>
>> git diff:
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 63d242164b1a..93a3909ae4c4 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -550,6 +550,7 @@ struct sched_entity {
>>         struct rb_node                  run_node;
>>         struct list_head                group_node;
>>         unsigned int                    on_rq;
>> +       unsigned int                    migrated;
>>
>>         u64                             exec_start;
>>         u64                             sum_exec_runtime;
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index ff4dbbae3b10..e60defc39f6e 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>         /*
>>          * We are starting a new run period:
>>          */
>> +       se->migrated = 0;
>>         se->exec_start = rq_clock_task(rq_of(cfs_rq));
>>  }
>>
>> @@ -4690,9 +4691,9 @@ 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 ((s64)sleep_time > (1ULL << 63) / scale_load_down(NICE_0_LOAD) / 2) {
>>                 se->vruntime = vruntime;
>> -       else
>> +       } else
>>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
>>  }
>>
>> @@ -7658,8 +7659,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>>         se->avg.last_update_time = 0;
>>
>>         /* We have migrated, no longer consider this task hot */
>> -       se->exec_start = 0;
>> -
>> +       se->migrated = 1;
>>         update_scan_period(p, new_cpu);
>>  }
>>
>> @@ -8343,6 +8343,8 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
>>
>>         if (sysctl_sched_migration_cost == 0)
>>                 return 0;
>> +       if (p->se.migrated)
>> +               return 0;
>>
>>         delta = rq_clock_task(env->src_rq) - p->se.exec_start;
>>
>>
>>
>>>
>>>>
>>>>
>>>>> 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
>>>
>>> Ok, it looks like we are back to normal figures
> 
> What do those results refer to then ?

Quote from this email (https://lore.kernel.org/lkml/1cd19d3f-18c4-92f9-257a-378cc18cfbc7@xxxxxxxxxx/).

> 
> 
>>>
>>>>>
>>>>> -------------------------------------------------
>>>>>
>>>>> The proposal accepts a divergence of up to 52 days between the 2 rqs.
>>>>>
>>>>> If this work, we will prepare a proper patch
>>>>>
>>>>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>>>>> index 63d242164b1a..cb8af0a137f7 100644
>>>>> --- a/include/linux/sched.h
>>>>> +++ b/include/linux/sched.h
>>>>> @@ -550,6 +550,7 @@ struct sched_entity {
>>>>>         struct rb_node                  run_node;
>>>>>         struct list_head                group_node;
>>>>>         unsigned int                    on_rq;
>>>>> +       unsigned int                    migrated;
>>>>>
>>>>>         u64                             exec_start;
>>>>>         u64                             sum_exec_runtime;
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index 7a1b1f855b96..36acd9598b40 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -1057,6 +1057,7 @@ update_stats_curr_start(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>>>>         /*
>>>>>          * We are starting a new run period:
>>>>>          */
>>>>> +       se->migrated = 0;
>>>>>         se->exec_start = rq_clock_task(rq_of(cfs_rq));
>>>>>  }
>>>>>
>>>>> @@ -4684,13 +4685,23 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
>>>>>
>>>>>         /*
>>>>>          * Pull vruntime of the entity being placed to the base level of
>>>>> -        * cfs_rq, to prevent boosting it if placed backwards.  If the entity
>>>>> -        * slept for a long time, don't even try to compare its vruntime with
>>>>> -        * the base as it may be too far off and the comparison may get
>>>>> -        * inversed due to s64 overflow.
>>>>> +        * cfs_rq, to prevent boosting it if placed backwards.
>>>>> +        * However, min_vruntime can advance much faster than real time, with
>>>>> +        * the exterme being when an entity with the minimal weight always runs
>>>>> +        * on the cfs_rq. If the new entity slept for long, its vruntime
>>>>> +        * difference from min_vruntime may overflow s64 and their comparison
>>>>> +        * may get inversed, so ignore the entity's original vruntime in that
>>>>> +        * case.
>>>>> +        * The maximal vruntime speedup is given by the ratio of normal to
>>>>> +        * minimal weight: NICE_0_LOAD / MIN_SHARES, so cutting off on the
>>>>
>>>> why not is `scale_load_down(NICE_0_LOAD) / MIN_SHARES` here ?
>>>
>>> yes, you are right.
>>>
>>>>
>>>>
>>>>> +        * sleep time of 2^63 / NICE_0_LOAD should be safe.
>>>>> +        * When placing a migrated waking entity, its exec_start has been set
>>>>> +        * from a different rq. In order to take into account a possible
>>>>> +        * divergence between new and prev rq's clocks task because of irq and
>>>>
>>>> This divergence might be larger, it cause `sleep_time` maybe negative.
>>>
>>> AFAICT, we are safe with ((1ULL << 63) / scale_load_down(NICE_0_LOAD)
>>> / 2) as long as the divergence between the 2 rqs clocks task is lower
>>> than 2^52nsec. Do you expect a divergence higher than 2^52 nsec
>>> (around 52 days)?
>>>
>>> We can probably keep using (1ULL << 63) / scale_load_down(NICE_0_LOAD)
>>> which is already half the max value if needed.
>>>
>>> the fact that sleep_time can be negative is not a problem as
>>> s64)sleep_time > will take care of this.
>>
>> In my opinion, when comparing signed with unsigned, the compiler converts the signed value to unsigned.
>> So, if sleep_time < 0, "(s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2" will be true.
>>
>>>
>>>>
>>>>> +        * stolen time, we take an additional margin.
>>>>>          */
>>>>>         sleep_time = rq_clock_task(rq_of(cfs_rq)) - se->exec_start;
>>>>> -       if ((s64)sleep_time > 60LL * NSEC_PER_SEC)
>>>>> +       if ((s64)sleep_time > (1ULL << 63) / NICE_0_LOAD / 2)>                 se->vruntime = vruntime;
>>>>>         else
>>>>>                 se->vruntime = max_vruntime(se->vruntime, vruntime);
>>>>> @@ -7658,7 +7669,7 @@ static void migrate_task_rq_fair(struct task_struct *p, int new_cpu)
>>>>>         se->avg.last_update_time = 0;
>>>>>
>>>>>         /* We have migrated, no longer consider this task hot */
>>>>> -       se->exec_start = 0;
>>>>> +       se->migrated = 1;
>>>>>
>>>>>         update_scan_period(p, new_cpu);
>>>>>  }
>>>>> @@ -8344,6 +8355,9 @@ static int task_hot(struct task_struct *p, struct lb_env *env)
>>>>>         if (sysctl_sched_migration_cost == 0)
>>>>>                 return 0;
>>>>>
>>>>> +       if (p->se.migrated)
>>>>> +               return 0;
>>>>> +
>>>>>         delta = rq_clock_task(env->src_rq) - p->se.exec_start;
>>>>>
>>>>>         return delta < (s64)sysctl_sched_migration_cost;
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>> .
>>>>>
>>> .
>>>
> .
>