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Date:   Thu, 28 May 2020 20:29:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Jonathan Corbet <corbet@lwn.net>,
        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>, Mel Gorman <mgorman@suse.de>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Quentin Perret <qperret@google.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Patrick Bellasi <patrick.bellasi@matbug.net>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default
 boost value
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On Thu, May 28, 2020 at 05:51:31PM +0100, Qais Yousef wrote:

> In my head, the simpler version of
> 
> 	if (rt_task(p) && !uc->user_defined)
> 		// update_uclamp_min
> 
> Is a single branch and write to cache, so should be fast. I'm failing to see
> how this could generate an overhead tbh, but will not argue about it :-)

Mostly true; but you also had a load of that sysctl in there, which is
likely to be a miss, and those are expensive.

Also; if we're going to have to optimize this, less logic is in there,
the less we need to take out. Esp. for stuff that 'never' changes, like
this.

> > It's more code, but it is all outside of the normal paths where we care
> > about performance.
> 
> I am happy to take that direction if you think it's worth it. I'm thinking
> task_woken_rt() is good. But again, maybe I am missing something.

Basic rule, if the state 'never' changes, don't touch fast paths.

Such little things can be very difficult to measure, but at some point
they cause death-by-a-thousnd-cuts.

> > Indeed, that one. The fact that regular distros cannot enable this
> > feature due to performance overhead is unfortunate. It means there is a
> > lot less potential for this stuff.
> 
> I had a humble try to catch the overhead but wasn't successful. The observation
> wasn't missed by us too then.

Right, I remember us doing benchmarks when we introduced all this and
clearly we missed something. I would be good if Mel can share which
benchmark hurt most so we can go have a look.