linux-kernel - Re: [PATCH] sched,x86: optimize switch

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Message-ID: <51F9C0B8.9000707@redhat.com>
Date:	Wed, 31 Jul 2013 21:58:16 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Paul Turner <pjt@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	jmario@...hat.com, Peter Anvin <hpa@...or.com>, dzickus@...hat.com,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched,x86: optimize switch_mm for multi-threaded workloads

On 07/31/2013 08:41 PM, Linus Torvalds wrote:
> On Wed, Jul 31, 2013 at 4:14 PM, Paul Turner <pjt@...gle.com> wrote:
>> We attached the following explanatory comment to our version of the patch:
>>
>> /*
>> * In the common case (two user threads sharing mm
>> * switching) the bit will be set; avoid doing a write
>> * (via atomic test & set) unless we have to.  This is
>> * safe, because no other CPU ever writes to our bit
>> * in the mask, and interrupts are off (so we can't
>> * take a TLB IPI here.)  If we don't do this, then
>> * switching threads will pingpong the cpumask
>> * cacheline.
>> */
>
> So as mentioned, the "interrupts will be off" is actually dubious.
> It's true for the context switch case, but not for the activate_mm().
>
> However, as Rik points out, activate_mm() is different in that we
> shouldn't have any preexisting MMU state anyway. And besides, that
> should never trigger the "prev == next" case.
>
> But it does look a bit messy, and even your comment is a bit
> misleading (it might make somebody think that all of switch_mm() is
> protected from interrupts)
> .
> Anyway, I'm perfectly ok with the patch itself, but I just wanted to
> make sure people had thought about these things.

Would you like me to document the things we found in the comment,
and resend a patch, or is the patch good as-is?

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