linux-kernel - Re: [PATCH 10/13] x86: Move cond resched for copy_{from,to}

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Message-ID: <20130815050415.GA2025@redhat.com>
Date:	Thu, 15 Aug 2013 08:04:15 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 10/13] x86: Move cond resched for copy_{from,to}_user
 into low level code 64bit

On Sat, Aug 10, 2013 at 08:42:58AM -0700, Linus Torvalds wrote:
> On Fri, Aug 9, 2013 at 4:04 PM, Andi Kleen <andi@...stfloor.org> wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> >
> > Move the cond_resched() check for CONFIG_PREEMPT_VOLUNTARY into
> > the low level copy_*_user code. This avoids some code bloat and
> > makes check much more efficient by avoiding unnecessary function calls.
> 
> May I suggest going one step further, and just removing the
> cond_resched() _entirely_, leaving just the debug test?
> 
> There really is zero reason for doing a cond_resched() for user
> accesses. If they take a page fault, then yes, by all means do that
> (and maybe we should add one to the page fault trap if we don't have
> it already), but without a page fault they really aren't that
> expensive.
> 
> We do many more expensive things without any cond_resched(), and doing
> that cond_resched() really doesn't make much sense *unless* there's a
> big expensive loop involved.
> 
> Most of this series looks fine, but I really think that we
> could/should just take that extra step, and say "no, user accesses
> don't imply that we need to check for scheduling".
> 
>                     Linus

In fact we are doing exactly this since 3.11-rc1.

-- 
MST
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