linux-kernel - Re: [PATCH 7/7] x86,idle: do not leave mm in idle state

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Date:   Fri, 22 Jun 2018 11:53:33 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Mike Galbraith <efault@....de>, songliubraving@...com,
        kernel-team <kernel-team@...com>
Subject: Re: [PATCH 7/7] x86,idle: do not leave mm in idle state

On Fri, 2018-06-22 at 08:36 -0700, Andy Lutomirski wrote:
> On Wed, Jun 20, 2018 at 12:57 PM Rik van Riel <riel@...riel.com>
> wrote:
> > 
> > Do not call leave_mm when going into a cstate. Now that mprotect
> > and
> > madvise no longer send IPIs for TLB shootdowns to idle CPUs, there
> > is
> > no real reason to disable lazy TLB mode in idle states.
> > 
> > This seems to help performance on Broadwell systems. Haswell
> > performance
> > numbers are inconclusive.
> 
> I'm skeptical.  The code you're removing is more about power
> consumption than about performance.  If a task migrates from one cpu
> to another, runs for awhile, and exits, we don't want to IPI the old
> CPU if the old CPU is deeply asleep.  The logic you're removing is a
> bit awkwardly written, but the intent is to only do the leave_mm() in
> deep sleep.  We should arguably change the condition to check the
> expected sleep duration instead of the microarchitectural properties
> of the target state, though.

That is fair.

A quick hack could be to just remove the tlb flush
from C3 state (which gets called a lot), but keep
it for C6 state and deeper (which get called more
when a CPU is really idle).

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