linux-kernel - Re: [PATCH v2] sched: Warn on long periods of pending need

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Date:   Thu, 25 Mar 2021 14:58:52 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        David Rientjes <rientjes@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>, linux-doc@...r.kernel.org,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v2] sched: Warn on long periods of pending need_resched

> 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.