linux-kernel - Re: Random panic in load

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Message-ID: <CA+55aFxH=zWYmyfudWD4HWQFrGD9N6+ELbm130_tqEfzjMavAg@mail.gmail.com>
Date:	Wed, 23 Jul 2014 10:15:23 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Michel Dänzer <michel@...nzer.net>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Random panic in load_balance() with 3.16-rc

On Wed, Jul 23, 2014 at 10:04 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Jul 23, 2014 at 09:54:23AM -0700, Linus Torvalds wrote:
>
>> So the length is fine, and the disassembly shows that it is fixed (16
>> 32-bit words - why the heck does it use "movsl" rather than "movsq",
>> whatever).
>
> Which is exactly right btw, he's got CONFIG_NR_CPUS=512 and 8*4*16=512.

That's not my point. Why the f*ck does it use "movsl", when "movsq"
should work as well or better.

Then it should use a count of 8. Because 8*8*8 is also 512 bits.

Of course, with the enhanced string instructions, it's quite possible
that "movsb" with a count of 64 (64*8) is the best option.

Anyway, my gcc version creates a series of 8 "movq" pairs instead,
which will beat all other cases, at the cost of much bigger code
footprint.

              Linus
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