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Message-ID: <69e18715-f868-132b-8898-0787a60e6840@linux.vnet.ibm.com>
Date: Tue, 7 Mar 2023 16:20:25 +0530
From: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: qyousef@...alina.io, chris.hyser@...cle.com,
patrick.bellasi@...bug.net, David.Laight@...lab.com,
pjt@...gle.com, pavel@....cz, qperret@...gle.com,
tim.c.chen@...ux.intel.com, joshdon@...gle.com, timj@....org,
kprateek.nayak@....com, yu.c.chen@...el.com,
youssefesmat@...omium.org, joel@...lfernandes.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,
linux-kernel@...r.kernel.org, parth@...ux.ibm.com, tj@...nel.org,
lizefan.x@...edance.com, hannes@...xchg.org,
cgroups@...r.kernel.org, corbet@....net, linux-doc@...r.kernel.org,
Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Subject: Re: [PATCH v12 8/8] sched/fair: Add latency list
On 3/7/23 3:49 PM, Vincent Guittot wrote:
> Le mardi 07 mars 2023 à 00:34:49 (+0530), Shrikanth Hegde a écrit :
>>> Le lundi 06 mars 2023 à 17:03:30 (+0530), Shrikanth Hegde a écrit :
>>>> On 3/5/23 6:33 PM, Vincent Guittot wrote:
>>>>> On Sat, 4 Mar 2023 at 16:13, Shrikanth Hegde <sshegde@...ux.vnet.ibm.com> wrote:
>>>>>> On 3/3/23 10:01 PM, Vincent Guittot wrote:
>>>>>>> Le jeudi 02 mars 2023 à 23:37:52 (+0530), Shrikanth Hegde a écrit :
>>>>>>>> On 3/2/23 8:30 PM, Shrikanth Hegde wrote:
>>>>>>>>> On 3/2/23 6:47 PM, Vincent Guittot wrote:
>>>>>>>>>> On Thu, 2 Mar 2023 at 12:00, Shrikanth Hegde <sshegde@...ux.vnet.ibm.com> wrote:
>>>>>>>>>>> On 3/2/23 1:20 PM, Vincent Guittot wrote:
>>>>>>>>>>>> On Wed, 1 Mar 2023 at 19:48, shrikanth hegde <sshegde@...ux.vnet.ibm.com> wrote:
>>>>>>>>>>>>> On 2/24/23 3:04 PM, Vincent Guittot wrote:
>>>>>>> [...]
>>>>>>>
>>>>>>>>>>>>> Ran the schbench and hackbench with this patch series. Here comparison is
>>>>>>>>>>>>> between 6.2 stable tree, 6.2 + Patch and 6.2 + patch + above re-arrange of
>>>>>>>>>>>>> latency_node. Ran two cgroups, in one cgroup running stress-ng at 50%(group1)
>>>>>>>>>>>>> and other is running these benchmarks (group2). Set the latency nice
>>>>>>>>>>>>> of group2 to -20. These are run on Power system with 12 cores with SMT=8.
>>>>>>>>>>>>> Total of 96 CPU.
>>>>>>>>>>>>>
>>>>>>>>>>>>> schbench gets lower latency compared to stabletree. Whereas hackbench seems
>>>>>>>>>>>>> to regress under this case. Maybe i am doing something wrong. I will re-run
>>>>>>>>>>>>> and attach the numbers to series.
>>>>>>>>>>>>> Please suggest if any variation in the test i need to try.
>>>>>>>>>>>> hackbench takes advanatge of a latency nice 19 as it mainly wants to
>>>>>>>>>>>> run longer slice to move forward rather than preempting others all the
>>>>>>>>>>>> time
>>>>>>>>>>> hackbench still seems to regress in different latency nice values compared to
>>>>>>>>>>> baseline of 6.2 in this case. up to 50% in some cases.
>>>>>>>>>>>
>>>>>>>>>>> 12 core powerpc system with SMT=8 i.e 96 CPU
>>>>>>>>>>> running 2 CPU cgroups. No quota assigned.
>>>>>>>>>>> 1st cgroup is running stress-ng with 48 threads. Consuming 50% of CPU.
>>>>>>>>>>> latency is not changed for this cgroup.
>>>>>>>>>>> 2nd cgroup is running hackbench. This cgroup is assigned the different latency
>>>>>>>>>>> nice values of 0, -20 and 19.
>>>>>>>>>> According to your other emails, you are using the cgroup interface and
>>>>>>>>>> not the task's one. Do I get it right ?
>>>>>>>>> right. I create cgroup, attach bash command with echo $$,
>>>>>>>>> assign the latency nice to cgroup, and run hackbench from that bash prompt.
>>>>>>>>>
>>>>>>>>>> I haven't run test such tests in a cgroup but at least the test with
>>>>>>>>>> latency_nice == 0 should not make any noticeable difference. Does this
>>>>>>>>>> include the re-arrange patch that you have proposed previously ?
>> Ran the test on a different system altogether. I don't see similar regression there.
>> In fact latency nice is helping in reducing the latency as expected.
>> It is much bigger system with 60 cores. i.e total of 480 cpu.
>>
>> Tested in the same way. created two cgroups. one is running the micro benchmarks
>> and other is running stress-ng at different utilization point.
>> This data is at 50% utilization point. Similar observations w.r.t latency
>> is seen at 0%, 25%, 75% and 100% utilization as well.
>>
> Thanks for testing on a different system which seems to get results aligned with what
> Prateek and I have seen on our system.
>
>
>> ==========
>> schbench
>> ==========
>> 6.2 6.2 + V12 + LN=0
>> Groups: 1
>> 50.0th: 14.0 12.5
>> 75.0th: 16.5 14.0
>> 90.0th: 18.5 15.5
>> 95.0th: 20.5 17.0
>> 99.0th: 27.5 21.0
>> 99.5th: 36.0 23.5
>> Groups: 2
>> 50.0th: 14.0 16.0
>> 75.0th: 17.0 18.0
>> 90.0th: 20.0 21.0
>> 95.0th: 23.0 23.0
>> 99.0th: 71.0 34.0
>> 99.5th: 1170.0 96.0
>> 99.9th: 5088.0 3212.0
>> Groups: 4
>> 50.0th: 20.5 19.5
>> 75.0th: 24.5 22.5
>> 90.0th: 31.0 26.0
>> 95.0th: 260.5 28.0
>> 99.0th: 3644.0 35.0
>> 99.5th: 5152.0 44.5
>> 99.9th: 8076.0 168.5
>> Groups: 8
>> 50.0th: 26.0 25.5
>> 75.0th: 32.5 31.5
>> 90.0th: 41.5 36.5
>> 95.0th: 794.0 39.5
>> 99.0th: 5992.0 66.0
>> 99.5th: 7208.0 159.0
>> 99.9th: 9392.0 1604.0
>> Groups: 16
>> 50.0th: 37.5 34.0
>> 75.0th: 49.5 44.5
>> 90.0th: 70.0 53.5
>> 95.0th: 1284.0 58.5
>> 99.0th: 5600.0 102.5
>> 99.5th: 7216.0 368.5
>> 99.9th: 9328.0 5192.0
>> Groups: 32
>> 50.0th: 59.0 54.5
>> 75.0th: 83.0 74.5
>> 90.0th: 118.5 91.0
>> 95.0th: 1921.0 100.5
>> 99.0th: 6672.0 317.0
>> 99.5th: 8252.0 2264.0
>> 99.9th: 10448.0 8388.0
>>
>>
>> ===========
>> hackbench
>> ==========
>>
>> type Groups 6.2 | 6.2 + V12 + LN=0
>> Process 10 0.19 | 0.19
>> Process 20 0.34 | 0.34
>> Process 30 0.45 | 0.44
>> Process 40 0.58 | 0.57
>> Process 50 0.70 | 0.69
>> Process 60 0.82 | 0.80
>> thread 10 0.20 | 0.20
>> thread 20 0.36 | 0.36
>> Process(Pipe) 10 0.24 | 0.21
>> Process(Pipe) 20 0.46 | 0.40
>> Process(Pipe) 30 0.65 | 0.58
>> Process(Pipe) 40 0.90 | 0.68
>> Process(Pipe) 50 1.04 | 0.83
>> Process(Pipe) 60 1.16 | 0.86
>> thread(Pipe) 10 0.19 | 0.18
>> thread(Pipe) 20 0.46 | 0.37
>>
>>
> [...]
>
>>>>>>
>>>>>> Do you want me to try any other experiment on this further?
>>>>> Yes, would be good to know which of the 3 changes in the patch create
>>>>> the regression
>>>>>
>>>>> I suspect the 1st change to be the root cause of your problem but It
>>>>> would be good if you can confirm my assumption with some tests
>>>>>
>>>>> Thanks
>>>> Applied each change individually. 3rd change seems to cause the regression.
>>>> Kept only the 3rd change and numbers are same as stable 6.2 for latency nice
>>>> value of 0.
>>> Ok, it's the patch 1 that aims to prevent some unfairness with low weight
>>> waking task. And your platform probably falls in the last part of the commit:
>>>
>>> " Strictly speaking, we should use cfs->min_vruntime instead of
>>> curr->vruntime but it doesn't worth the additional overhead and complexity
>>> as the vruntime of current should be close to min_vruntime if not equal."
>>>
>>> Could you try the patch below on top of v12 ?
>>>
>>> ---
>>> kernel/sched/fair.c | 21 +++++++++++----------
>>> 1 file changed, 11 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index e2aeb4511686..77b03a280912 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -5049,7 +5049,7 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>> }
>>>
>>> static int
>>> -wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
>>> +wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se, struct cfs_rq *cfs_rq);
>>>
>>> /*
>>> * Pick the next process, keeping these things in mind, in this order:
>>> @@ -5088,16 +5088,16 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>> second = curr;
>>> }
>>>
>>> - if (second && wakeup_preempt_entity(second, left) < 1)
>>> + if (second && wakeup_preempt_entity(second, left, cfs_rq) < 1)
>>> se = second;
>>> }
>>>
>>> - if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
>>> + if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left, cfs_rq) < 1) {
>>> /*
>>> * Someone really wants this to run. If it's not unfair, run it.
>>> */
>>> se = cfs_rq->next;
>>> - } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
>>> + } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left, cfs_rq) < 1) {
>>> /*
>>> * Prefer last buddy, try to return the CPU to a preempted task.
>>> */
>>> @@ -5107,7 +5107,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>> /* Check for latency sensitive entity waiting for running */
>>> latency = __pick_first_latency(cfs_rq);
>>> if (latency && (latency != se) &&
>>> - wakeup_preempt_entity(latency, se) < 1)
>>> + wakeup_preempt_entity(latency, se, cfs_rq) < 1)
>>> se = latency;
>>>
>>> return se;
>>> @@ -7808,7 +7808,7 @@ static unsigned long wakeup_gran(struct sched_entity *se)
>>> *
>>> */
>>> static int
>>> -wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>>> +wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se, struct cfs_rq *cfs_rq)
>>> {
>>> s64 gran, vdiff = curr->vruntime - se->vruntime;
>>> s64 offset = wakeup_latency_gran(curr, se);
>>> @@ -7818,6 +7818,8 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>>>
>>> gran = offset + wakeup_gran(se);
>>>
>>> + if (vdiff > gran)
>>> + return 1;
>>> /*
>>> * At wake up, the vruntime of a task is capped to not be older than
>>> * a sched_latency period compared to min_vruntime. This prevents long
>>> @@ -7827,9 +7829,8 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>>> * for low priority task. Make sure that long sleeping task will get a
>>> * chance to preempt current.
>>> */
>>> - gran = min_t(s64, gran, get_latency_max());
>>> -
>>> - if (vdiff > gran)
>>> + vdiff = cfs_rq->min_vruntime - se->vruntime;
>>> + if (vdiff > get_latency_max())
>>> return 1;
>>>
>>> return 0;
>>> @@ -7933,7 +7934,7 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>>> return;
>>>
>>> update_curr(cfs_rq_of(se));
>>> - if (wakeup_preempt_entity(se, pse) == 1) {
>>> + if (wakeup_preempt_entity(se, pse, cfs_rq_of(se)) == 1) {
>>> /*
>>> * Bias pick_next to pick the sched entity that is
>>> * triggering this preemption.
>>> --
>>> 2.34.1
>> Tried above patch on top of V12. Numbers are worse than V12. We maybe running into
>> a corner case on this system.
> Yes it can be a corner case.
>
> Nevertheless, the patch above has a problem and does an unsigned comparison instead of a signed
> one. I have forced the signed comparison in the patch below to be applied on top of
> previous one:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 77b03a280912..22a497f92dbb 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7830,7 +7830,7 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se, struct
> * chance to preempt current.
> */
> vdiff = cfs_rq->min_vruntime - se->vruntime;
> - if (vdiff > get_latency_max())
> + if (vdiff > (s64)get_latency_max())
> return 1;
>
> return 0;
Tested the above patch on top of previous patch + V12.
Numbers are still worse than V12. Same as V12+previous patch.
>
>
>> Type Groups 6.2 | 6.2+V12
>>
>> Process 10 0.33 | 0.44
>> Process 20 0.61 | 0.90
>> Process 30 0.87 | 1.29
>> Process 40 1.10 | 1.69
>> Process 50 1.34 | 2.08
>> Process 60 1.58 | 2.39
>> thread 10 0.36 | 0.53
>> thread 20 0.64 | 0.94
>> Process(Pipe) 10 0.18 | 0.46
>> Process(Pipe) 20 0.32 | 0.75
>> Process(Pipe) 30 0.42 | 1.01
>> Process(Pipe) 40 0.56 | 1.15
>> Process(Pipe) 50 0.68 | 1.38
>> Process(Pipe) 60 0.80 | 1.56
>>
>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index cdcd991bbcf1..c89c201dd164 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -7827,7 +7827,6 @@ wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se)
>>>> * for low priority task. Make sure that long sleeping task will get a
>>>> * chance to preempt current.
>>>> */
>>>> - gran = min_t(s64, gran, get_latency_max());
>>>>
>>>> if (vdiff > gran)
>>>> return 1;
>>>>
>>>>
>>>>
> [...]
>
>>
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