From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751786AbeBAHwH (ORCPT <rfc822;w@1wt.eu>);
        Thu, 1 Feb 2018 02:52:07 -0500
Received: from cloudserver094114.home.pl ([79.96.170.134]:58952 "EHLO
        cloudserver094114.home.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751172AbeBAHwE (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 1 Feb 2018 02:52:04 -0500
From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>,
        Mike Galbraith <efault@gmx.de>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        srinivas.pandruvada@linux.intel.com
Subject: Re: [PATCH 4/4] sched/fair: Use a recently used CPU as an idle candidate and the basis for SIS
Date: Thu, 01 Feb 2018 08:50:28 +0100
Message-ID: <3838689.GpfBVtfPr0@aspire.rjw.lan>
In-Reply-To: <20180131101710.GM2269@hirez.programming.kicks-ass.net>
References: <20180130104555.4125-1-mgorman@techsingularity.net> <4353713.vEOq6OHvJN@aspire.rjw.lan> <20180131101710.GM2269@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Transfer-Encoding: 7Bit
Content-Type: text/plain; charset="us-ascii"
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wednesday, January 31, 2018 11:17:10 AM CET Peter Zijlstra wrote:
> 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.

OK

So what field precisely would you touch?  "desired"?  If so, does that actually
guarantee anything to happen?

> 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.

I guess you mean it does too many IPIs?  Or that it shouldn't do any IPIs
at all?