linux-kernel - Re: [regression v4.0-rc1] mm: IPIs from TLB flushes causing significant performance degradation.

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Date:	Mon, 2 Mar 2015 18:37:47 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>, Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Matt B <jackdachef@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, xfs@....sgi.com
Subject: Re: [regression v4.0-rc1] mm: IPIs from TLB flushes causing
 significant performance degradation.

On Mon, Mar 2, 2015 at 6:22 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> There might be some other case where the new "just change the
> protection" doesn't do the "oh, but it the protection didn't change,
> don't bother flushing". I don't see it.

Hmm. I wonder.. In change_pte_range(), we just unconditionally change
the protection bits.

But the old numa code used to do

    if (!pte_numa(oldpte)) {
        ptep_set_numa(mm, addr, pte);

so it would actually avoid the pte update if a numa-prot page was
marked numa-prot again.

But are those migrate-page calls really common enough to make these
things happen often enough on the same pages for this all to matter?

Odd.

So it would be good if your profiles just show "there's suddenly a
*lot* more calls to flush_tlb_page() from XYZ" and the culprit is
obvious that way..

                       Linus
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