From: Michal Hocko <mhocko@kernel.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>,
Vlastimil Babka <vbabka@suse.cz>,
"Kirill A. Shutemov" <kirill@shutemov.name>,
Andrew Morton <akpm@linux-foundation.org>,
Arnd Bergmann <arnd@arndb.de>,
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
Pavel Emelyanov <xemul@virtuozzo.com>,
linux-mm <linux-mm@kvack.org>,
lkml <linux-kernel@vger.kernel.org>,
Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE
Date: Tue, 30 May 2017 16:39:41 +0200 [thread overview]
Message-ID: <20170530143941.GK7969@dhcp22.suse.cz> (raw)
In-Reply-To: <20170530140456.GA8412@redhat.com>
On Tue 30-05-17 16:04:56, Andrea Arcangeli wrote:
> On Tue, May 30, 2017 at 12:39:30PM +0200, Michal Hocko wrote:
> > On Tue 30-05-17 13:19:22, Mike Rapoport wrote:
> > > On Tue, May 30, 2017 at 09:44:08AM +0200, Michal Hocko wrote:
> > > > On Wed 24-05-17 17:27:36, Mike Rapoport wrote:
> > > > > On Wed, May 24, 2017 at 01:18:00PM +0200, Michal Hocko wrote:
> > > > [...]
> > > > > > Why cannot khugepaged simply skip over all VMAs which have userfault
> > > > > > regions registered? This would sound like a less error prone approach to
> > > > > > me.
> > > > >
> > > > > khugepaged does skip over VMAs which have userfault. We could register the
> > > > > regions with userfault before populating them to avoid collapses in the
> > > > > transition period.
> > > >
> > > > Why cannot you register only post-copy regions and "manually" copy the
> > > > pre-copy parts?
> > >
> > > We can register only post-copy regions, but this will cause VMA
> > > fragmentation. Now we register the entire VMA with userfaultfd, no matter
> > > how many pages were dirtied there since the pre-dump. If we register only
> > > post-copy regions, we will split out the VMAs for those regions.
> >
> > Is this really a problem, though?
>
> It would eventually get -ENOMEM or at best create lots of unnecessary
> vmas (at least UFFDIO_COPY would never risk to trigger -ENOMEM).
I sysctl for the mapcount can be increased, right? I also assume that
those vmas will get merged after the post copy is done.
> The only attractive alternative is to use UFFDIO_COPY for precopy too
> after pre-registering the whole range in uffd (which would happen
> later anyway to start postcopy).
>
> > It would be good to measure that though. You are proposing a new user
> > API and the THP api is quite convoluted already so there better be a
> > very good reason to add a new API. So far I can only see that it would
> > be more convinient to add another madvise command and that is rather
> > insufficient justification IMHO. Also do you expect somebody else would
> > use new madvise? What would be the usecase?
>
> UFFDIO_COPY while not being a major slowdown for sure, it's likely
> measurable at the microbenchmark level because it would add a
> enter/exit kernel to every 4k memcpy. It's not hard to imagine that as
> measurable. How that impacts the total precopy time I don't know, it
> would need to be benchmarked to be sure.
Yes, please!
> The main benefit of this
> madvise is precisely to skip those enter/exit kernel that UFFDIO_COPY
> would add. Even if the impact on the total precopy time wouldn't be
> measurable (i.e. if it's network bound load), the madvise that allows
> using memcpy after setting VM_NOHUGEPAGE, would free up some CPU
> cycles in the destination that could be used by other processes.
I understand that part but it sounds awfully one purpose thing to me.
Are we going to add other MADVISE_RESET_$FOO to clear other flags just
because we can race in this specific use case?
> About the proposed madvise, it just clear bits, but it doesn't change
> at all how those bits are computed in THP code. So I don't see it as
> convoluted.
But we already have MADV_HUGEPAGE, MADV_NOHUGEPAGE and prctl to
enable/disable thp. Doesn't that sound little bit too much for a single
feature to you?
--
Michal Hocko
SUSE Labs
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next prev parent reply other threads:[~2017-05-30 14:39 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1495433562-26625-1-git-send-email-rppt@linux.vnet.ibm.com>
[not found] ` <20170522114243.2wrdbncilozygbpl@node.shutemov.name>
[not found] ` <20170522133559.GE27382@rapoport-lnx>
[not found] ` <20170522135548.GA8514@dhcp22.suse.cz>
[not found] ` <20170522142927.GG27382@rapoport-lnx>
[not found] ` <a9e74c22-1a07-f49a-42b5-497fee85e9c9@suse.cz>
[not found] ` <20170524075043.GB3063@rapoport-lnx>
2017-05-24 7:58 ` [PATCH] mm: introduce MADV_CLR_HUGEPAGE Vlastimil Babka
2017-05-24 10:39 ` Mike Rapoport
2017-05-24 11:18 ` Michal Hocko
2017-05-24 14:25 ` Pavel Emelyanov
2017-05-24 14:27 ` Mike Rapoport
2017-05-24 15:22 ` Andrea Arcangeli
2017-05-30 7:44 ` Michal Hocko
[not found] ` <20170530074408.GA7969-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-30 10:19 ` Mike Rapoport
2017-05-30 10:39 ` Michal Hocko
[not found] ` <20170530103930.GB7969-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-30 14:04 ` Andrea Arcangeli
2017-05-30 14:39 ` Michal Hocko [this message]
[not found] ` <20170530143941.GK7969-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-30 14:56 ` Michal Hocko
[not found] ` <20170530145632.GL7969-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-30 16:06 ` Andrea Arcangeli
2017-05-31 6:30 ` Vlastimil Babka
2017-05-31 8:24 ` Michal Hocko
2017-05-31 9:27 ` Mike Rapoport
2017-05-31 10:24 ` Michal Hocko
[not found] ` <20170531082414.GB27783-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-31 10:22 ` Michal Hocko
2017-06-01 11:00 ` Mike Rapoport
2017-06-01 12:27 ` Michal Hocko
2017-05-30 15:43 ` Andrea Arcangeli
2017-05-31 12:08 ` Michal Hocko
[not found] ` <20170531120822.GL27783-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-05-31 12:39 ` Mike Rapoprt
2017-05-31 14:18 ` Andrea Arcangeli
[not found] ` <20170531141809.GB302-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2017-05-31 14:32 ` Michal Hocko
2017-05-31 15:46 ` Andrea Arcangeli
2017-06-01 6:58 ` Mike Rapoport
[not found] ` <8FA5E4C2-D289-4AF5-AA09-6C199E58F9A5-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>
2017-05-31 14:19 ` Michal Hocko
2017-06-01 6:53 ` Mike Rapoport
2017-06-01 8:09 ` Michal Hocko
[not found] ` <20170601080909.GD32677-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-06-01 8:35 ` Mike Rapoport
2017-06-01 13:45 ` Andrea Arcangeli
2017-06-02 9:11 ` Mike Rapoport
2017-05-31 9:08 ` Mike Rapoport
2017-05-31 12:05 ` Michal Hocko
2017-05-31 12:25 ` Mike Rapoprt
2017-05-24 11:31 ` Vlastimil Babka
2017-05-24 14:28 ` Pavel Emelyanov
2017-05-24 14:54 ` Vlastimil Babka
2017-05-24 15:13 ` Mike Rapoport
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