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Message-ID: <CAKfTPtDLptU9j9iCRtOokYzRE0SMqXZHBH0xPjqhaB=NPOes4w@mail.gmail.com>
Date: Wed, 8 Mar 2023 09:00:37 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
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
Subject: Re: [PATCH v12 8/8] sched/fair: Add latency list
On Tue, 7 Mar 2023 at 11:52, Shrikanth Hegde <sshegde@...ux.vnet.ibm.com> wrote:
>
>
> 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.
So It really looks like a corner case for this system and I'm not sure
we can do anything as others don't face he problem
>
>
> >
> >
> >> 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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