From: Dietmar Eggemann <dietmar.eggemann@arm.com>
To: Peter Zijlstra <peterz@infradead.org>, 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>,
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
Date: Thu, 28 May 2020 21:20:34 +0200 [thread overview]
Message-ID: <f3081cf2-24f1-d0f9-e76b-d868538f3245@arm.com> (raw)
In-Reply-To: <20200528182913.GQ2483@worktop.programming.kicks-ass.net>
On 28/05/2020 20:29, Peter Zijlstra wrote:
> 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.
IIRC, it was a local mmtests netperf-udp with various buffer sizes?
At least that's what we're trying to run right now on a '2 Sockets Xeon
E5 2x10-Cores (40 CPUs)' w/ 3 different kernel ((1) wo_clamp (2)
tsk_uclamp (3) tskgrp_uclamp).
We have currently Ubuntu Desktop on it. I think that systemd uses
cgroups (especially cpu controller) differently on a (Ubuntu) Server.
Maybe this has an influence here as well?
next prev parent reply other threads:[~2020-05-28 19:20 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-11 15:40 [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Qais Yousef
2020-05-11 15:40 ` [PATCH 2/2] Documentation/sysctl: Document uclamp sysctl knobs Qais Yousef
2020-05-11 17:18 ` [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value Qais Yousef
2020-05-12 2:10 ` Pavan Kondeti
2020-05-12 11:46 ` Qais Yousef
2020-05-15 11:08 ` Patrick Bellasi
2020-05-18 8:31 ` Dietmar Eggemann
2020-05-18 16:49 ` Qais Yousef
2020-05-28 13:23 ` Peter Zijlstra
2020-05-28 15:58 ` Qais Yousef
2020-05-28 16:11 ` Peter Zijlstra
2020-05-28 16:51 ` Qais Yousef
2020-05-28 18:29 ` Peter Zijlstra
2020-05-28 19:08 ` Patrick Bellasi
2020-05-28 19:20 ` Dietmar Eggemann [this message]
2020-05-29 9:11 ` Qais Yousef
2020-05-29 10:21 ` Mel Gorman
2020-05-29 15:11 ` Qais Yousef
2020-05-29 16:02 ` Mel Gorman
2020-05-29 16:05 ` Qais Yousef
2020-05-29 10:08 ` Mel Gorman
2020-05-29 16:04 ` Qais Yousef
2020-05-29 16:57 ` Mel Gorman
2020-06-02 16:46 ` Dietmar Eggemann
2020-06-03 8:29 ` Patrick Bellasi
2020-06-03 10:10 ` Mel Gorman
2020-06-03 14:59 ` Vincent Guittot
2020-06-03 16:52 ` Qais Yousef
2020-06-04 12:14 ` Vincent Guittot
2020-06-05 10:45 ` Qais Yousef
2020-06-09 15:29 ` Vincent Guittot
2020-06-08 12:31 ` Qais Yousef
2020-06-08 13:06 ` Valentin Schneider
2020-06-08 14:44 ` Steven Rostedt
2020-06-11 10:13 ` Qais Yousef
2020-06-09 17:10 ` Vincent Guittot
2020-06-11 10:24 ` Qais Yousef
2020-06-11 12:01 ` Vincent Guittot
2020-06-23 15:44 ` Qais Yousef
2020-06-24 8:45 ` Vincent Guittot
2020-06-05 7:55 ` Patrick Bellasi
2020-06-05 11:32 ` Qais Yousef
2020-06-05 13:27 ` Patrick Bellasi
2020-06-03 9:40 ` Mel Gorman
2020-06-03 12:41 ` Qais Yousef
2020-06-04 13:40 ` Mel Gorman
2020-06-05 10:58 ` Qais Yousef
2020-06-11 10:58 ` Qais Yousef
2020-06-16 11:08 ` Qais Yousef
2020-06-16 13:56 ` Lukasz Luba
-- strict thread matches above, loose matches on Subject: below --
2020-04-03 12:30 Qais Yousef
2020-04-14 18:21 ` Patrick Bellasi
2020-04-15 7:46 ` Patrick Bellasi
2020-04-20 15:04 ` Qais Yousef
2020-04-20 8:24 ` Dietmar Eggemann
2020-04-20 15:19 ` Qais Yousef
2020-04-21 0:52 ` Steven Rostedt
2020-04-21 11:16 ` Dietmar Eggemann
2020-04-21 11:23 ` Qais Yousef
2020-04-20 14:50 ` Qais Yousef
2020-04-15 10:11 ` Quentin Perret
2020-04-20 15:08 ` Qais Yousef
2020-04-20 8:29 ` Dietmar Eggemann
2020-04-20 15:13 ` Qais Yousef
2020-04-21 11:18 ` Dietmar Eggemann
2020-04-21 11:27 ` Qais Yousef
2020-04-22 10:59 ` Dietmar Eggemann
2020-04-22 13:13 ` Qais Yousef
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