Messages in this thread Patch in this message | | | Date | Tue, 3 Jan 2017 12:37:59 +0100 | From | Vincent Guittot <> | Subject | Re: [LKP] [lkp-developer] [sched/fair] 4e5160766f: +149% ftq.noise.50% regression |
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Hi Dietmar and Ying,
Le Tuesday 03 Jan 2017 à 11:38:39 (+0100), Dietmar Eggemann a écrit : > Hi Vincent and Ying, > > On 01/02/2017 04:42 PM, Vincent Guittot wrote: > >Hi Ying, > > > >On 28 December 2016 at 09:17, Huang, Ying <ying.huang@intel.com> wrote: > >>Vincent Guittot <vincent.guittot@linaro.org> writes: > >> > >>>Le Tuesday 13 Dec 2016 . 09:47:30 (+0800), Huang, Ying a .crit : > >>>>Hi, Vincent, > >>>> > >>>>Vincent Guittot <vincent.guittot@linaro.org> writes: > > [...] >
[snip]
> >> > >>The test result is as follow, > >> > >>========================================================================================= > >>compiler/cpufreq_governor/freq/kconfig/nr_task/rootfs/samples/tbox_group/test/testcase: > >> gcc-6/powersave/20/x86_64-rhel-7.2/100%/debian-x86_64-2016-08-31.cgz/6000ss/lkp-hsw-d01/cache/ftq > >> > >>commit: > >> 4e5160766fcc9f41bbd38bac11f92dce993644aa: first bad commit > >> 09a43ace1f986b003c118fdf6ddf1fd685692d49: parent of first bad commit > >> 0613870ea53a7a279d8d37f2a3ce40aafc155fc8: debug commit with above patch > >> > >>4e5160766fcc9f41 09a43ace1f986b003c118fdf6d 0613870ea53a7a279d8d37f2a3 > >>---------------- -------------------------- -------------------------- > >> %stddev %change %stddev %change %stddev > >> \ | \ | \ > >> 61670 ±228% -96.5% 2148 ± 11% -94.7% 3281 ± 58% ftq.noise.25% > >> 3463 ± 10% -60.0% 1386 ± 19% -26.3% 2552 ± 58% ftq.noise.50% > >> 1116 ± 23% -72.6% 305.99 ± 30% -35.8% 716.15 ± 64% ftq.noise.75% > >> 3843815 ± 3% +3.1% 3963589 ± 1% -49.6% 1938221 ±100% ftq.time.involuntary_context_switches > >> 5.33 ± 30% +21.4% 6.46 ± 14% -71.7% 1.50 ±108% time.system_time > >> > >> > >>It appears that the system_time and involuntary_context_switches reduced > >>much after applied the debug patch, which is good from noise point of > >>view. ftq.noise.50% reduced compared with the first bad commit, but > >>have not restored to that of the parent of the first bad commit. > > > >Thanks for testing. I will try to improve it a bit but not sure that I > >can reduce more. > > Is this a desktop system where this regression comes from autogroups (1 > level taskgroups) or a server system with systemd (2 level taskgroups)? > > Since the PELT rewrite (v4.2) I have ~60 autogroups per cpu > (&rq->leaf_cfs_rq_list) on my Ubuntu desktop system permanently (Intel > i7-4750HQ) whereas in v4.1 there were 0 - 10. > > $ for i in `seq 0 7`; do cat /proc/sched_debug | grep > "cfs_rq\[$i\]:/autogroup-" | wc -l; done > 58 > 61 > 63 > 65 > 60 > 59 > 62 > 56 > > Couldn't we still remove these autogroups by if (!cfs_rq->nr_running && > !se->avg.load_avg && !se->avg.util_avg) in update_blocked_averages()? > > Vincent, like we discussed in September last year, the proper fix would > probably be a cfs-rq->nr_attached which IMHO is not doable w/o being an > atomic because of migrate_task_rq_fair()->remove_entity_load_avg() not > holding the rq lock.
I remember the discussion and even if I agree that a large number of taskgroup increases the number of loop in update_blocked_averages() and as a result the time spent in the update, I don't think that this is the root cause of this regression because the patch "sched/fair: Propagate asynchrous detach" doesn't add more loops to update_blocked_averages but it adds more thing to do per loop.
Then, I think I'm still too conservative in the condition for calling update_load_avg(cfs_rq->tg->se[cpu], 0). This call has been added to propagate gcfs_rq->propagate_avg flag to parent so we don't need to call it even if load_avg is not null but only when propagate_avg flag is set. The patch below should improve thing compare to the previous version because it will call update_load_avg(cfs_rq->tg->se[cpu], 0) only if an asynchrounous detach happened (propagate_avg is set).
Ying, could you test the patch below instead of the previous one ?
--- kernel/sched/fair.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 6559d19..a4f5c35 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6915,6 +6915,7 @@ static void update_blocked_averages(int cpu) { struct rq *rq = cpu_rq(cpu); struct cfs_rq *cfs_rq; + struct sched_entity *se; unsigned long flags; raw_spin_lock_irqsave(&rq->lock, flags); @@ -6932,9 +6933,10 @@ static void update_blocked_averages(int cpu) if (update_cfs_rq_load_avg(cfs_rq_clock_task(cfs_rq), cfs_rq, true)) update_tg_load_avg(cfs_rq, 0); - /* Propagate pending load changes to the parent */ - if (cfs_rq->tg->se[cpu]) - update_load_avg(cfs_rq->tg->se[cpu], 0); + /* Propagate pending load changes to the parent if any */ + se = cfs_rq->tg->se[cpu]; + if (se && cfs_rq->propagate_avg) + update_load_avg(se, 0); } raw_spin_unlock_irqrestore(&rq->lock, flags); } -- 2.7.4
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