Re: Reporting a performance regression in sched/fair on Unixbench Shell Scripts with commit a53ce18cacb4

From: Saeed Mirzamohammadi
Date: Thu Jul 20 2023 - 19:04:36 EST

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Hi Vincent,

> On Jun 30, 2023, at 1:28 AM, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>
> On Fri, 30 Jun 2023 at 00:20, Saeed Mirzamohammadi
> <saeed.mirzamohammadi@xxxxxxxxxx> wrote:
>>
>>
>>
>>> On Jun 21, 2023, at 9:41 AM, Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx> wrote:
>>>
>>> Hi Chen, Vincent,
>>>
>>>> On Jun 13, 2023, at 11:37 PM, Chen Yu <yu.c.chen@xxxxxxxxx> wrote:
>>>>
>>>> On 2023-06-13 at 19:35:55 +0000, Saeed Mirzamohammadi wrote:
>>>>> Hi Vincent,
>>>>>
>>>>>> On Jun 9, 2023, at 9:52 AM, Vincent Guittot <vincent.guittot@xxxxxxxxxx> wrote:
>>>>>>
>>>>>> Hi Saeed,
>>>>>>
>>>>>> On Fri, 9 Jun 2023 at 00:48, Saeed Mirzamohammadi
>>>>>> <saeed.mirzamohammadi@xxxxxxxxxx> wrote:
>>>>>>>
>>>>>>> Hi all,
>>>>>>>
>>>>>>> I’m reporting a regression of up to 8% with Unixbench Shell Scripts benchmarks after the following commit:
>>>>>>>
>>>>>>> Commit Data:
>>>>>>> commit-id : a53ce18cacb477dd0513c607f187d16f0fa96f71
>>>>>>> subject : sched/fair: Sanitize vruntime of entity being migrated
>>>>>>> author : vincent.guittot@xxxxxxxxxx
>>>>>>> author date : 2023-03-17 16:08:10
>>>>>>>
>>>>>>>
>>>>>>> We have observed this on our v5.4 and v4.14 kernel and not yet tested 5.15 but I expect the same.
>>>>>>
>>>>>> It would be good to confirm that the regression is present on v6.3
>>>>>> where the patch has been merged originally. It can be that there is
>>>>>> hidden dependency with other patches introduced since v5.4
>>>>>
>>>>> Regression is present on v6.3 as well, examples:
>>>>> ub_gcc_224copies_Shell_Scripts_8_concurrent: ~6%
>>>>> ub_gcc_224copies_Shell_Scripts_16_concurrent: ~8%
>>>>> ub_gcc_448copies_Shell_Scripts_1_concurrent: ~2%
>>>
>>> Apologize for the confusion, I should correct the v6.3 upstream result above. v6.3 doesn’t have any regression.
>>> v6.3.y -> no regression
>>> v5.15.y -> no regression
>>> v5.4.y -> 5-8% regression.
>>
>> A gentle reminder if there is any recommendation for v5.4.y and v4.14.y regression. Thanks!
>
> I tried to find why the regression happens only for v5.4.y (or lower)
> and not for v5.15.y (or above) but I haven't been able to find any
> possible reason in the code.
>
> Regarding the 2 commits below, they must come together so we can't
> simply revert 1 and not the other.
> commit 829c1651e9c4 sched/fair: sanitize vruntime of entity being placed
> commit a53ce18cacb4 sched/fair: Sanitize vruntime of entity being migrated
>
Tests were done before and after these 2 commits.

> entity_is_long_sleeper() should never return true in your case. Could
> you try to check that it's the case for you ?
>
Tested this and entity_is_long_sleeper() never returns True.

I actually removed the related part, tested, and the regression is gone with the following change (partial revert):

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3ebd2054996bc..0d70dd6e14844 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -792,9 +792,6 @@ static inline void dequeue_task(struct rq *rq, struct task_struct *p, int flags)

void activate_task(struct rq *rq, struct task_struct *p, int flags)
{
- if (task_on_rq_migrating(p))
- flags |= ENQUEUE_MIGRATED;
-
if (task_contributes_to_load(p))
rq->nr_uninterruptible--;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 83a7cf62c0f53..ef9aca05c7bdf 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3779,9 +3779,6 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)

if (flags & ENQUEUE_WAKEUP)
place_entity(cfs_rq, se, 0);
- /* Entity has migrated, no longer consider this task hot */
- if (flags & ENQUEUE_MIGRATED)
- se->exec_start = 0;

check_schedstat_required();
update_stats_enqueue(cfs_rq, se, flags);
@@ -6182,6 +6179,9 @@ static void migrate_task_rq_fair(struct task_struct *p)

/* Tell new CPU we are migrated */
p->se.avg.last_update_time = 0;
+
+ /* We have migrated, no longer consider this task hot */
+ p->se.exec_start = 0;
}

static void task_dead_fair(struct task_struct *p)

>
>
>
>
>>
>>>
>>>
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> ub_gcc_1copy_Shell_Scripts_1_concurrent : -0.01%
>>>>>>> ub_gcc_1copy_Shell_Scripts_8_concurrent : -0.1%
>>>>>>> ub_gcc_1copy_Shell_Scripts_16_concurrent : -0.12%%
>>>>>>> ub_gcc_56copies_Shell_Scripts_1_concurrent : -2.29%%
>>>>>>> ub_gcc_56copies_Shell_Scripts_8_concurrent : -4.22%
>>>>>>> ub_gcc_56copies_Shell_Scripts_16_concurrent : -4.23%
>>>>>>> ub_gcc_224copies_Shell_Scripts_1_concurrent : -5.54%
>>>>>>> ub_gcc_224copies_Shell_Scripts_8_concurrent : -8%
>>>>>>> ub_gcc_224copies_Shell_Scripts_16_concurrent : -7.05%
>>>>>>> ub_gcc_448copies_Shell_Scripts_1_concurrent : -6.4%
>>>>>>> ub_gcc_448copies_Shell_Scripts_8_concurrent : -8.35%
>>>>>>> ub_gcc_448copies_Shell_Scripts_16_concurrent : -7.09%
>>>>>>>
>>>>>>> Link to unixbench:
>>>>>>> github.com/kdlucas/byte-unixbench
>>>>>>
>>>>>> I tried to reproduce the problem with v6.3 on my system but I don't
>>>>>> see any difference with or without the patch
>>>>>>
>>>>>> Do you have more details on your setup ? number of cpu and topology ?
>>>>>>
>>>>> model name : Intel(R) Xeon(R) CPU E5-2690 v4 @ 2.60GHz
>>>>>
>>>>> Topology:
>>>>> node 0 1
>>>>> 0: 10 21
>>>>> 1: 21 10
>>>>>
>>>>> Architecture: x86_64
>>>>> CPU op-mode(s): 32-bit, 64-bit
>>>>> CPU(s): 56
>>>>> On-line CPU(s) list: 0-55
>>>>> Thread(s) per core: 2
>>>>> Core(s) per socket: 14
>>>>> Socket(s): 2
>>>>> NUMA node(s): 2
>>>>>
>>>> Tested on a similar platform E5-2697 v2 @ 2.70GHz which has 2 nodes,
>>>> 24 cores/48 CPUs in total, however I could not reproduce the issue.
>>>> Since the regression was reported mainly against 224 and 448 copies case
>>>> on your platform, I tested unixbench shell1 with 4 x 48 = 192 copies.
>>>>
>>>>
>>>> a53ce18cacb477dd 213acadd21a080fc8cda8eebe6d
>>>> ---------------- ---------------------------
>>>> %stddev %change %stddev
>>>> \ | \
>>>> 21304 +0.5% 21420 unixbench.score
>>>> 632.43 +0.0% 632.44 unixbench.time.elapsed_time
>>>> 632.43 +0.0% 632.44 unixbench.time.elapsed_time.max
>>>> 11837046 -4.7% 11277727 unixbench.time.involuntary_context_switches
>>>> 864713 +0.1% 865914 unixbench.time.major_page_faults
>>>> 9600 +4.0% 9984 unixbench.time.maximum_resident_set_size
>>>> 8.433e+08 +0.6% 8.48e+08 unixbench.time.minor_page_faults
>>>> 4096 +0.0% 4096 unixbench.time.page_size
>>>> 3741 +1.1% 3783 unixbench.time.percent_of_cpu_this_job_got
>>>> 18341 +1.3% 18572 unixbench.time.system_time
>>>> 5323 +0.6% 5353 unixbench.time.user_time
>>>> 78197044 -3.1% 75791701 unixbench.time.voluntary_context_switches
>>>> 57178573 +0.4% 57399061 unixbench.workload
>>>>
>>>> There is no much difference with a53ce18cacb477dd applied or not.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> a2e90611b9f425ad 829c1651e9c4a6f78398d3e6765
>>>> ---------------- ---------------------------
>>>> %stddev %change %stddev
>>>> \ | \
>>>> 19985 +8.6% 21697 unixbench.score
>>>> 632.64 -0.0% 632.53 unixbench.time.elapsed_time
>>>> 632.64 -0.0% 632.53 unixbench.time.elapsed_time.max
>>>> 11453985 +3.7% 11880259 unixbench.time.involuntary_context_switches
>>>> 818996 +3.1% 844681 unixbench.time.major_page_faults
>>>> 9600 +0.0% 9600 unixbench.time.maximum_resident_set_size
>>>> 7.911e+08 +8.4% 8.575e+08 unixbench.time.minor_page_faults
>>>> 4096 +0.0% 4096 unixbench.time.page_size
>>>> 3767 -0.4% 3752 unixbench.time.percent_of_cpu_this_job_got
>>>> 18873 -2.4% 18423 unixbench.time.system_time
>>>> 4960 +7.1% 5313 unixbench.time.user_time
>>>> 75436000 +10.8% 83581483 unixbench.time.voluntary_context_switches
>>>> 53553404 +8.7% 58235303 unixbench.workload
>>>>
>>>> Previously with 829c1651e9c4a6f introduced, there is 8.6% improvement. And this improvement
>>>> remains with a53ce18cacb477dd applied.
>>>>
>>>> Can you send the full test script so I can have a try locally?
>>>
>>> Thanks for testing this. For v5.4.y kernel (not for v6.3.y or v5.15.y), there is an 8% regression with the following test: ub_gcc_448copies_Shell_Scripts_8_concurrent
>>> And that’s ’shell8’ with ‘-c 448’ copies passed as argument.
>>>
>>> Thanks,
>>> Saeed
>>>
>>>>
>>>> thanks,
>>>> Chenyu

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