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

[Zhang Qiao <zhangqiao22@huawei.com>]

在 2023/3/13 22:23, Vincent Guittot 写道:
> On Sat, 11 Mar 2023 at 10:57, Zhang Qiao <zhangqiao22@huawei.com> 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@infradead.org> 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@linux.intel.com/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@amazon.de/
>>
>>
>> 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@amazon.de/
> alone, don't you ?

No, with your patch, the test results is 108.725(s),

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
> 
>>>
>>> -------------------------------------------------
>>>
>>> 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;
>>>
>>>
>>>
>>>>
>>>>
>>>>>
>>>>>
>>> .
>>>
> .
>