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

[Linus Torvalds]

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
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