linux-kernel - Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20180130131531.GD2269@hirez.programming.kicks-ass.net>
Date:   Tue, 30 Jan 2018 14:15:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>, rjw@...ysocki.net,
        srinivas.pandruvada@...ux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle
 candidate and the basis for SIS

On Tue, Jan 30, 2018 at 12:57:18PM +0000, Mel Gorman wrote:
> On Tue, Jan 30, 2018 at 12:50:54PM +0100, Peter Zijlstra wrote:

> > Not saying this patch is bad; but Rafael / Srinivas we really should do
> > better. Why isn't cpufreq (esp. sugov) fixing this? HWP or not, we can
> > still give it hints, and it looks like we're not doing that.
> > 
> 
> I'm not sure if HWP can fix it because of the per-cpu nature of its
> decisions. I believe it can only give the most basic of hints to hardware
> like an energy performance profile or bias (EPP and EPB respectively).
> Of course HWP can be turned off but not many people can detect that it's
> an appropriate decision, or even desirable, and there is always the caveat
> that disabling it increases the system CPU footprint.

IA32_HWP_REQUEST has "Minimum_Performance", "Maximum_Performance" and
"Desired_Performance" fields which can be used to give explicit
frequency hints. And we really _should_ be doing that.

Because, esp. in this scenario; a task migrating; the hardware really
can't do anything sensible, whereas the OS _knows_.