linux-kernel - Re: [PATCH] mm: call cond_resched() per MAX_ORDER_NR

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Date:	Mon, 18 Nov 2013 16:56:18 -0500
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	dave.jiang@...el.com, akpm@...ux-foundation.org, dhillf@...il.com,
	Mel Gorman <mgorman@...e.de>
Subject: Re: [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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