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Date:   Fri, 26 Mar 2021 09:58:44 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Don <joshdon@google.com>
Cc:     Mel Gorman <mgorman@suse.de>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org,
        David Rientjes <rientjes@google.com>,
        Oleg Rombakh <olegrom@google.com>, linux-doc@vger.kernel.org,
        Paul Turner <pjt@google.com>
Subject: Re: [PATCH v2] sched: Warn on long periods of pending need_resched
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On Thu, Mar 25, 2021 at 02:58:52PM -0700, Josh Don wrote:
> > On Wed, Mar 24, 2021 at 01:39:16PM +0000, Mel Gorman wrote:
> > I'm not going to NAK because I do not have hard data that shows they must
> > exist. However, I won't ACK either because I bet a lot of tasty beverages
> > the next time we meet that the following parameters will generate reports
> > if removed.
> >
> > kernel.sched_latency_ns
> > kernel.sched_migration_cost_ns
> > kernel.sched_min_granularity_ns
> > kernel.sched_wakeup_granularity_ns
> >
> > I know they are altered by tuned for different profiles and some people do
> > go the effort to create custom profiles for specific applications. They
> > also show up in "Official Benchmarking" such as SPEC CPU 2017 and
> > some vendors put a *lot* of effort into SPEC CPU results for bragging
> > rights. They show up in technical books and best practice guids for
> > applications.  Finally they show up in Google when searching for "tuning
> > sched_foo". I'm not saying that any of these are even accurate or a good
> > idea, just that they show up near the top of the results and they are
> > sufficiently popular that they might as well be an ABI.
> 
> +1, these seem like sufficiently well-known scheduler tunables, and
> not really SCHED_DEBUG.

So we've never made any guarantees on their behaviour, nor am I willing
to make any.

In fact, I propose we merge the below along with the debugfs move. Just
to make absolutely sure any 'tuning' is broken.


---
Subject: sched,fair: Alternative sched_slice()
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu Mar 25 13:44:46 CET 2021

The current sched_slice() seems to have issues; there's two possible
things that could be improved:

 - the 'nr_running' used for __sched_period() is daft when cgroups are
   considered. Using the RQ wide h_nr_running seems like a much more
   consistent number.

 - (esp) cgroups can slice it real fine (pun intendend), which makes for
   easy over-scheduling, ensure min_gran is what the name says.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/fair.c     |   15 ++++++++++++++-
 kernel/sched/features.h |    3 +++
 2 files changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
  */
 static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
+	unsigned int nr_running = cfs_rq->nr_running;
+	u64 slice;
+
+	if (sched_feat(ALT_PERIOD))
+		nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
+
+	slice = __sched_period(nr_running + !se->on_rq);
+
+	if (sched_feat(BASE_SLICE))
+		slice -= sysctl_sched_min_granularity;
 
 	for_each_sched_entity(se) {
 		struct load_weight *load;
@@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
 		}
 		slice = __calc_delta(slice, se->load.weight, load);
 	}
+
+	if (sched_feat(BASE_SLICE))
+		slice += sysctl_sched_min_granularity;
+
 	return slice;
 }
 
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,3 +90,6 @@ SCHED_FEAT(WA_BIAS, true)
  */
 SCHED_FEAT(UTIL_EST, true)
 SCHED_FEAT(UTIL_EST_FASTUP, true)
+
+SCHED_FEAT(ALT_PERIOD, true)
+SCHED_FEAT(BASE_SLICE, true)