From: Minchan Kim <minchan@kernel.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
Miguel de Dios <migueldedios@google.com>,
Wei Wang <wvw@google.com>, Johannes Weiner <hannes@cmpxchg.org>,
Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH] mm: release the spinlock on zap_pte_range
Date: Tue, 30 Jul 2019 21:39:35 +0900 [thread overview]
Message-ID: <20190730123935.GB184615@google.com> (raw)
In-Reply-To: <20190730123237.GR9330@dhcp22.suse.cz>
On Tue, Jul 30, 2019 at 02:32:37PM +0200, Michal Hocko wrote:
> On Tue 30-07-19 21:11:10, Minchan Kim wrote:
> > On Mon, Jul 29, 2019 at 10:35:15AM +0200, Michal Hocko wrote:
> > > On Mon 29-07-19 17:20:52, Minchan Kim wrote:
> > > > On Mon, Jul 29, 2019 at 09:45:23AM +0200, Michal Hocko wrote:
> > > > > On Mon 29-07-19 16:10:37, Minchan Kim wrote:
> > > > > > In our testing(carmera recording), Miguel and Wei found unmap_page_range
> > > > > > takes above 6ms with preemption disabled easily. When I see that, the
> > > > > > reason is it holds page table spinlock during entire 512 page operation
> > > > > > in a PMD. 6.2ms is never trivial for user experince if RT task couldn't
> > > > > > run in the time because it could make frame drop or glitch audio problem.
> > > > >
> > > > > Where is the time spent during the tear down? 512 pages doesn't sound
> > > > > like a lot to tear down. Is it the TLB flushing?
> > > >
> > > > Miguel confirmed there is no such big latency without mark_page_accessed
> > > > in zap_pte_range so I guess it's the contention of LRU lock as well as
> > > > heavy activate_page overhead which is not trivial, either.
> > >
> > > Please give us more details ideally with some numbers.
> >
> > I had a time to benchmark it via adding some trace_printk hooks between
> > pte_offset_map_lock and pte_unmap_unlock in zap_pte_range. The testing
> > device is 2018 premium mobile device.
> >
> > I can get 2ms delay rather easily to release 2M(ie, 512 pages) when the
> > task runs on little core even though it doesn't have any IPI and LRU
> > lock contention. It's already too heavy.
> >
> > If I remove activate_page, 35-40% overhead of zap_pte_range is gone
> > so most of overhead(about 0.7ms) comes from activate_page via
> > mark_page_accessed. Thus, if there are LRU contention, that 0.7ms could
> > accumulate up to several ms.
>
> Thanks for this information. This is something that should be a part of
> the changelog. I am sorry to still poke into this because I still do not
I will include it.
> have a full understanding of what is going on and while I do not object
> to drop the spinlock I still suspect this is papering over a deeper
> problem.
I couldn't come up with better solution. Feel free to suggest it.
>
> If mark_page_accessed is really expensive then why do we even bother to
> do it in the tear down path in the first place? Why don't we simply set
> a referenced bit on the page to reflect the young pte bit? I might be
> missing something here of course.
commit bf3f3bc5e73
Author: Nick Piggin <npiggin@suse.de>
Date: Tue Jan 6 14:38:55 2009 -0800
mm: don't mark_page_accessed in fault path
Doing a mark_page_accessed at fault-time, then doing SetPageReferenced at
unmap-time if the pte is young has a number of problems.
mark_page_accessed is supposed to be roughly the equivalent of a young pte
for unmapped references. Unfortunately it doesn't come with any context:
after being called, reclaim doesn't know who or why the page was touched.
So calling mark_page_accessed not only adds extra lru or PG_referenced
manipulations for pages that are already going to have pte_young ptes anyway,
but it also adds these references which are difficult to work with from the
context of vma specific references (eg. MADV_SEQUENTIAL pte_young may not
wish to contribute to the page being referenced).
Then, simply doing SetPageReferenced when zapping a pte and finding it is
young, is not a really good solution either. SetPageReferenced does not
correctly promote the page to the active list for example. So after removing
mark_page_accessed from the fault path, several mmap()+touch+munmap() would
have a very different result from several read(2) calls for example, which
is not really desirable.
Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Johannes Weiner <hannes@saeurebad.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
next prev parent reply other threads:[~2019-07-30 12:39 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-29 7:10 [PATCH] mm: release the spinlock on zap_pte_range Minchan Kim
2019-07-29 7:45 ` Michal Hocko
2019-07-29 8:20 ` Minchan Kim
2019-07-29 8:35 ` Michal Hocko
2019-07-30 12:11 ` Minchan Kim
2019-07-30 12:32 ` Michal Hocko
2019-07-30 12:39 ` Minchan Kim [this message]
2019-07-30 12:57 ` Michal Hocko
2019-07-31 5:44 ` Minchan Kim
2019-07-31 7:21 ` Michal Hocko
2019-08-06 10:55 ` Minchan Kim
2019-08-09 12:43 ` [RFC PATCH] mm: drop mark_page_access from the unmap path Michal Hocko
2019-08-09 17:57 ` Mel Gorman
2019-08-09 18:34 ` Johannes Weiner
2019-08-12 8:09 ` Michal Hocko
2019-08-12 15:07 ` Johannes Weiner
2019-08-13 10:51 ` Michal Hocko
2019-08-26 12:06 ` Michal Hocko
2019-08-27 16:00 ` Johannes Weiner
2019-08-27 18:41 ` Michal Hocko
2019-07-30 19:42 ` [PATCH] mm: release the spinlock on zap_pte_range Andrew Morton
2019-07-31 6:14 ` Minchan Kim
2019-08-06 7:05 ` [mm] 755d6edc1a: will-it-scale.per_process_ops -4.1% regression kernel test robot
[not found] ` <20190806080415.GG11812@dhcp22.suse.cz>
2019-08-06 11:00 ` Minchan Kim
2019-08-06 11:11 ` Michal Hocko
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