linux-kernel - Re: [patch] x86, mm: pass in 'total' to __copy_from_user

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Message-Id: <200903020119.34455.nickpiggin@yahoo.com.au>
Date:	Mon, 2 Mar 2009 01:19:33 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	Salman Qazi <sqazi@...gle.com>, davem@...emloft.net,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>
Subject: Re: [patch] x86, mm: pass in 'total' to __copy_from_user_*nocache()

On Sunday 01 March 2009 05:52:19 Linus Torvalds wrote:
> On Sat, 28 Feb 2009, Ingo Molnar wrote:
> > OTOH, given how draconian non-temporal stores are, i'm leaning
> > towards removing them from the x86 code altogether. If it matter
> > to performance somewhere it can be reintroduced, based on really
> > well backed up numbers.
>
> It would be interesting to see if we could instead base the decision on
> what we really do care about, namely going to do IO.
>
> And the thing is, in this path we _do_ kind of know that. The caller
> (normally generic_perform_write) already does that whole
> balance_dirty_pages_ratelimited() thing.

>
> So rather than passing in the "total_size" thing, we _could_ pass in
> something that is based on
>
>  - are we O_DIRECT? If so: use uncached

Zero copies in that case :) O_SYNC, you mean.


>  - perhaps: are we really _really_ large? If so: use uncached, we know the
>    caches aren't going to capture it.

But how large, and which caches? I wouldn't expect very many apps at all
to pass buffers larger than even quite small LLC sizes.


>  - are we starting writeout due to dirty page balancing: if so, use
>    uncached.

Although that should tend to write out oldest written data, wheras
the newly written data might still benefit from being warm in cache
(eg. in the cpp|cc|as|ld case).


> But on the other hand, I could personally certainly also imagine just not
> doing that whole uncached thing at all. Myself, I tend to care about the
> peformance of the cached case much more than some odd iozone thing. But
> others will have different priorities..

FWIW I agree with you. rep mov I think gives the CPU a good window into
proceedings and usually should do a good job.

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