Messages in this thread | | | Subject | Re: [LKP] [lkp-developer] [sched/fair] 4e5160766f: +149% ftq.noise.50% regression | From | Dietmar Eggemann <> | Date | Tue, 3 Jan 2017 11:38:39 +0100 |
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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:
[...]
>>> --- >>> kernel/sched/fair.c | 29 ++++++++++++++++++++++++++++- >>> 1 file changed, 28 insertions(+), 1 deletion(-) >>> >>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >>> index 090a9bb..8efa113 100644 >>> --- a/kernel/sched/fair.c >>> +++ b/kernel/sched/fair.c >>> @@ -3138,6 +3138,31 @@ static inline int propagate_entity_load_avg(struct sched_entity *se) >>> return 1; >>> } >>> >>> +/* Check if we need to update the load and the utilization of a group_entity */ >>> +static inline bool skip_blocked_update(struct sched_entity *se) >>> +{ >>> + struct cfs_rq *gcfs_rq = group_cfs_rq(se); >>> + >>> + /* >>> + * If sched_entity still have not null load or utilization, we have to >>> + * decay it. >>> + */ >>> + if (se->avg.load_avg || se->avg.util_avg) >>> + return false; >>> + >>> + /* >>> + * If there is a pending propagation, we have to update the load and >>> + * the utilizaion of the sched_entity >>> + */ >>> + if (gcfs_rq->propagate_avg) >>> + return false; >>> + >>> + /* >>> + * Other wise, the load and the utilizaiton of the sched_entity is >>> + * already null so it will be a waste of time to try to decay it >>> + */ >>> + return true; >>> +} >>> #else /* CONFIG_FAIR_GROUP_SCHED */ >>> >>> static inline void update_tg_load_avg(struct cfs_rq *cfs_rq, int force) {} >>> @@ -6858,6 +6883,7 @@ 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); >>> @@ -6876,7 +6902,8 @@ static void update_blocked_averages(int cpu) >>> update_tg_load_avg(cfs_rq, 0); >>> >>> /* Propagate pending load changes to the parent */ >>> - if (cfs_rq->tg->se[cpu]) >>> + se = cfs_rq->tg->se[cpu]; >>> + if (se && !skip_blocked_update(se)) >>> update_load_avg(cfs_rq->tg->se[cpu], 0); >>> } >>> raw_spin_unlock_irqrestore(&rq->lock, flags); >> >> 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.
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