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

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Message-ID: <20180131101710.GM2269@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Jan 2018 11:17:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Mike Galbraith <efault@....de>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        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 Wed, Jan 31, 2018 at 10:22:49AM +0100, Rafael J. Wysocki wrote:
> On Tuesday, January 30, 2018 2:15:31 PM CET Peter Zijlstra wrote:

> > 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_.
> 
> But IA32_HWP_REQUEST is not a cheap MSR to write to.

That just means we might need to throttle writing to it, like it already
does for the regular pstate (PERF_CTRL) msr in any case (also, is that a
cheap msr?)

Not touching it at all seems silly.

But now that you made me look, intel_pstate_hwp_set() is horrible crap.
You should _never_ do things like:

  rdmsr_on_cpu()
  /* frob value */
  wrmsr_on_cpu()

That's insane.