From: Chen Yu <yu.c.chen@intel.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Abel Wu <wuyun.abel@bytedance.com>,
Peter Zijlstra <peterz@infradead.org>,
Mel Gorman <mgorman@suse.de>,
Vincent Guittot <vincent.guittot@linaro.org>,
Josh Don <joshdon@google.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core
Date: Wed, 7 Sep 2022 15:27:41 +0800 [thread overview]
Message-ID: <YxhH7cB+OIMAB0dM@chenyu5-mobl1> (raw)
In-Reply-To: <20220906095717.maao4qtel4fhbmfq@techsingularity.net>
Hi Mel,
On 2022-09-06 at 10:57:17 +0100, Mel Gorman wrote:
> On Mon, Sep 05, 2022 at 10:40:00PM +0800, Abel Wu wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 6089251a4720..59b27a2ef465 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6427,21 +6427,36 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > if (sd_share) {
> > > /* because !--nr is the condition to stop scan */
> > > nr = READ_ONCE(sd_share->nr_idle_scan) + 1;
> > > - /* overloaded LLC is unlikely to have idle cpu/core */
> > > - if (nr == 1)
> > > - return -1;
> > > +
> > > + /*
> > > + * Non-overloaded case: Scan full domain if there is
> > > + * an idle core. Otherwise, scan for an idle
> > > + * CPU based on nr_idle_scan
> > > + * Overloaded case: Unlikely to have an idle CPU but
> > > + * conduct a limited scan if there is potentially
> > > + * an idle core.
> > > + */
> > > + if (nr > 1) {
> > > + if (has_idle_core)
> > > + nr = sd->span_weight;
> > > + } else {
> > > + if (!has_idle_core)
> > > + return -1;
> > > + nr = 2;
> > > + }
> > > }
> > > }
> > > for_each_cpu_wrap(cpu, cpus, target + 1) {
> > > + if (!--nr)
> > > + break;
> > > +
> > > if (has_idle_core) {
> > > i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > > if ((unsigned int)i < nr_cpumask_bits)
> > > return i;
> > > } else {
> > > - if (!--nr)
> > > - return -1;
> > > idle_cpu = __select_idle_cpu(cpu, p);
> > > if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > > break;
> >
> > I spent last few days testing this, with 3 variations (assume
> > has_idle_core):
> >
> > a) full or limited (2cores) scan when !nr_idle_scan
> > b) whether clear sds->has_idle_core when partial scan failed
> > c) scale scan depth with load or not
> >
> > some observations:
> >
> > 1) It seems always bad if not clear sds->has_idle_core when
> > partial scan fails. It is due to over partially scanned
> > but still can not find an idle core. (Following ones are
> > based on clearing has_idle_core even in partial scans.)
> >
>
> Ok, that's rational. There will be corner cases where there was no idle
> CPU near the target when there is an idle core far away but it would be
> counter to the purpose of SIS_UTIL to care about that corner case.
>
> > 2) Unconditionally full scan when has_idle_core is not good
> > for netperf_{udp,tcp} and tbench4. It is probably because
> > the SIS success rate of these workloads is already high
> > enough (netperf ~= 100%, tbench4 ~= 50%, compared to that
> > hackbench ~= 3.5%) which negate a lot of the benefit full
> > scan brings.
> >
>
> That's also rational. For a single client/server on netperf, it's expected
> that the SIS success rate is high and scanning is minimal. As the client
> and server are sharing data on localhost and somewhat synchronous, it may
> even partially benefit from SMT sharing.
>
Maybe off-topic, since we monitor the success rate and also other metrics
for each optimization in SIS path, is it possible to merge your statistics
patch [1] into upstream so we don't need to rebase in the future(although
it is targeting kernel development)?
Link: https://lore.kernel.org/lkml/20210726102247.21437-2-mgorman@techsingularity.net/
[cut]
thanks,
Chenyu
next prev parent reply other threads:[~2022-09-07 7:28 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-12 8:20 [PATCH 0/5] sched/fair: SIS improvements and cleanups Abel Wu
2022-07-12 8:20 ` [PATCH 1/5] sched/fair: ignore SIS_UTIL when has idle core Abel Wu
2022-07-13 3:47 ` Chen Yu
2022-07-13 16:14 ` Abel Wu
2022-07-14 6:19 ` Yicong Yang
2022-07-14 6:58 ` Abel Wu
2022-07-14 7:15 ` Yicong Yang
2022-07-14 8:00 ` Abel Wu
2022-07-14 8:16 ` Yicong Yang
2022-07-14 8:34 ` Yicong Yang
2022-08-04 9:59 ` Chen Yu
2022-08-15 2:54 ` Abel Wu
2022-08-10 13:50 ` Chen Yu
2022-08-15 2:44 ` Abel Wu
2022-08-29 13:08 ` Mel Gorman
2022-08-29 14:11 ` Abel Wu
2022-08-29 14:56 ` Mel Gorman
2022-09-01 13:08 ` Abel Wu
2022-09-02 4:12 ` Abel Wu
2022-09-02 10:25 ` Mel Gorman
2022-09-05 14:40 ` Abel Wu
2022-09-06 9:57 ` Mel Gorman
2022-09-07 7:27 ` Chen Yu [this message]
2022-09-07 8:41 ` Mel Gorman
2022-09-07 7:52 ` Abel Wu
2022-07-12 8:20 ` [PATCH 2/5] sched/fair: default to false in test_idle_cores Abel Wu
2022-08-29 12:36 ` Mel Gorman
2022-07-12 8:20 ` [PATCH 3/5] sched/fair: remove redundant check in select_idle_smt Abel Wu
2022-08-29 12:36 ` Mel Gorman
2022-07-12 8:20 ` [PATCH 4/5] sched/fair: avoid double search on same cpu Abel Wu
2022-08-29 12:36 ` Mel Gorman
2022-07-12 8:20 ` [PATCH 5/5] sched/fair: remove useless check in select_idle_core Abel Wu
2022-08-29 12:37 ` Mel Gorman
2022-08-15 13:31 ` [PATCH 0/5] sched/fair: SIS improvements and cleanups Abel Wu
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