From: Mel Gorman <mgorman@suse.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Chinner <david@fromorbit.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
Aneesh Kumar <aneesh.kumar@linux.vnet.ibm.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
xfs@oss.sgi.com, ppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH 1/4] mm: thp: Return the correct value for change_huge_pmd
Date: Sat, 7 Mar 2015 20:56:16 +0000 [thread overview]
Message-ID: <20150307205616.GZ3087@suse.de> (raw)
In-Reply-To: <CA+55aFyCgzNGU-VAaKvwTYFhtJc_ugLK6hRzZBCxMYdAt5TVuA@mail.gmail.com>
On Sat, Mar 07, 2015 at 12:31:03PM -0800, Linus Torvalds wrote:
> On Sat, Mar 7, 2015 at 7:20 AM, Mel Gorman <mgorman@suse.de> wrote:
> >
> > if (!prot_numa || !pmd_protnone(*pmd)) {
> > - ret = 1;
> > entry = pmdp_get_and_clear_notify(mm, addr, pmd);
> > entry = pmd_modify(entry, newprot);
> > ret = HPAGE_PMD_NR;
>
> Hmm. I know I acked this already, but the return value - which correct
> - is still potentially something we could improve upon.
>
> In particular, we don't need to flush the TLB's if the old entry was
> not present. Sadly, we don't have a helper function for that.
>
> But the code *could* do something like
>
> entry = pmdp_get_and_clear_notify(mm, addr, pmd);
> ret = pmd_tlb_cacheable(entry) ? HPAGE_PMD_NR : 1;
> entry = pmd_modify(entry, newprot);
>
> where pmd_tlb_cacheable() on x86 would test if _PAGE_PRESENT (bit #0) is set.
>
I agree with you in principle. pmd_tlb_cacheable looks and sounds very
similar to pte_accessible().
> In particular, that would mean that as we change *from* a protnone
> (whether NUMA or really protnone) we wouldn't need to flush the TLB.
>
> In fact, we could make it even more aggressive: it's not just an old
> non-present TLB entry that doesn't need flushing - we can avoid the
> flushing whenever we strictly increase the access rigths. So we could
> have something that takes the old entry _and_ the new protections into
> account, and avoids the TLB flush if the new entry is strictly more
> permissive.
>
> This doesn't explain the extra TLB flushes Dave sees, though, because
> the old code didn't make those kinds of optimizations either. But
> maybe something like this is worth doing.
>
I think it is worth doing although it'll be after LSF/MM before I do it. I
severely doubt this is what Dave is seeing because the vmstats indicated
there was no THP activity but it's still a good idea.
--
Mel Gorman
SUSE Labs
next prev parent reply other threads:[~2015-03-07 20:56 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-03-07 15:20 [RFC PATCH 0/4] Automatic NUMA balancing and PROT_NONE handling followup v2r8 Mel Gorman
2015-03-07 15:20 ` [PATCH 1/4] mm: thp: Return the correct value for change_huge_pmd Mel Gorman
2015-03-07 20:13 ` Linus Torvalds
2015-03-07 20:31 ` Linus Torvalds
2015-03-07 20:56 ` Mel Gorman [this message]
2015-03-07 15:20 ` [PATCH 2/4] mm: numa: Remove migrate_ratelimited Mel Gorman
2015-03-07 15:20 ` [PATCH 3/4] mm: numa: Mark huge PTEs young when clearing NUMA hinting faults Mel Gorman
2015-03-07 18:33 ` Linus Torvalds
2015-03-07 18:42 ` Linus Torvalds
2015-03-07 15:20 ` [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur Mel Gorman
2015-03-07 16:36 ` Ingo Molnar
2015-03-07 17:37 ` Mel Gorman
2015-03-08 9:54 ` Ingo Molnar
2015-03-07 19:12 ` Linus Torvalds
2015-03-08 10:02 ` Ingo Molnar
2015-03-08 18:35 ` Linus Torvalds
2015-03-08 18:46 ` Linus Torvalds
2015-03-09 11:29 ` Dave Chinner
2015-03-09 16:52 ` Linus Torvalds
2015-03-09 19:19 ` Dave Chinner
2015-03-10 23:55 ` Linus Torvalds
2015-03-12 13:10 ` Mel Gorman
2015-03-12 16:20 ` Linus Torvalds
2015-03-12 18:49 ` Mel Gorman
2015-03-17 7:06 ` Dave Chinner
2015-03-17 16:53 ` Linus Torvalds
2015-03-17 20:51 ` Dave Chinner
2015-03-17 21:30 ` Linus Torvalds
2015-03-17 22:08 ` Dave Chinner
2015-03-18 16:08 ` Linus Torvalds
2015-03-18 17:31 ` Linus Torvalds
2015-03-18 22:23 ` Dave Chinner
2015-03-19 14:10 ` Mel Gorman
2015-03-19 18:09 ` Linus Torvalds
2015-03-19 21:41 ` Linus Torvalds
2015-03-19 22:41 ` Dave Chinner
2015-03-19 23:05 ` Linus Torvalds
2015-03-19 23:23 ` Dave Chinner
2015-03-20 0:23 ` Dave Chinner
2015-03-20 1:29 ` Linus Torvalds
2015-03-20 4:13 ` Dave Chinner
2015-03-20 17:02 ` Linus Torvalds
2015-03-23 12:01 ` Mel Gorman
2015-03-20 10:12 ` Mel Gorman
2015-03-20 9:56 ` Mel Gorman
2015-03-08 20:40 ` Mel Gorman
2015-03-09 21:02 ` Mel Gorman
2015-03-10 13:08 ` Mel Gorman
2015-03-08 9:41 ` Ingo Molnar
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150307205616.GZ3087@suse.de \
--to=mgorman@suse.de \
--cc=akpm@linux-foundation.org \
--cc=aneesh.kumar@linux.vnet.ibm.com \
--cc=david@fromorbit.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=mingo@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=xfs@oss.sgi.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).