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SubjectRe: [PATCH] mm: call cond_resched() per MAX_ORDER_NR_PAGES pages copy
On Mon, Nov 18, 2013 at 12:48:54PM -0800, Dave Hansen wrote:
> On 11/18/2013 12:20 PM, Naoya Horiguchi wrote:
> >> > Really, though, a lot of things seem to have MAX_ORDER set up so that
> >> > it's at 256MB or 512MB. That's an awful lot to do between rescheds.
> > Yes.
> >
> > BTW, I found that we have the same problem for other functions like
> > copy_user_gigantic_page, copy_user_huge_page, and maybe clear_gigantic_page.
> > So we had better handle them too.
>
> Is there a problem you're trying to solve here? The common case of the
> cond_resched() call boils down to a read of a percpu variable which will
> surely be in the L1 cache after the first run around the loop. In other
> words, it's about as cheap of an operation as we're going to get.

Yes, cond_resched() is cheap if should_resched() is false (and it is in
common case).

> Why bother trying to "optimize" it?

I thought that if we call cond_resched() too often, the copying thread can
take too long in a heavy load system, because the copying thread always
yields the CPU in every loop.

But it seems to be an extreme case, so I can't push it strongly.

Thanks,
Naoya


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