Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR_PAGES pages copy

[Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>]

On Mon, Nov 18, 2013 at 11:02:32AM -0800, Dave Hansen wrote:
> On 11/18/2013 10:54 AM, Naoya Horiguchi wrote:
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index cb5d152b58bc..661ff5f66591 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -454,7 +454,8 @@ static void __copy_gigantic_page(struct page *dst, struct page *src,
> >  	struct page *src_base = src;
> >  
> >  	for (i = 0; i < nr_pages; ) {
> > -		cond_resched();
> > +		if (i % MAX_ORDER_NR_PAGES == 0)
> > +			cond_resched();
> >  		copy_highpage(dst, src);
> 
> This is certainly OK on x86, but remember that MAX_ORDER can be
> overridden by a config variable.  Just picking one at random:
> 
> config FORCE_MAX_ZONEORDER
>         int "Maximum zone order"
>         range 9 64 if PPC64 && PPC_64K_PAGES
> ...
> 
> Would it be OK to only resched once every 2^63 pages? ;)

You're right. We need use more reliable value here.
HPAGE_SIZE/PAGE_SIZE looks better to me.

> 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.

Thanks,
Naoya

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