Re: [PATCH 10/13] x86: Move cond resched for copy_{from,to}_userinto low level code 64bit

From: Michael S. Tsirkin
Date: Thu Aug 15 2013 - 01:02:51 EST

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@xxxxxxxxxxxxxx> wrote:
> > From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> >
> > 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.

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