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

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Message-ID: <CA+55aFwTNQFFJbeP4bYuMBREq=n6GU2vV71ftr_7OnE6SK5OSw@mail.gmail.com>
Date:	Wed, 31 Jul 2013 17:41:39 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	Rik van Riel <riel@...hat.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 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.

                Linus
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