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From: Andrea Arcangeli <aarcange@redhat.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Stephen Rothwell <sfr@canb.auug.org.au>,
	linux-mm@kvack.org, linux-next@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup
Date: Fri, 3 Jun 2016 15:51:54 +0200	[thread overview]
Message-ID: <20160603135154.GD29930@redhat.com> (raw)
In-Reply-To: <20160602122109.GM1995@dhcp22.suse.cz>

On Thu, Jun 02, 2016 at 02:21:10PM +0200, Michal Hocko wrote:
> Testing with the patch makes some sense as well, but I would like to
> hear from Andrea whether the approach is good because I am wondering why
> he hasn't done that before - it feels so much simpler than the current
> code.

The down_write in the exit path comes from __ksm_exit. If you don't
like it there I'd suggest to also remove it from __ksm_exit.

This is a proposed cleanup correct?

The first thing that I can notice is that khugepaged_test_exit() then
can only be called and provide the expected retval, after
atomic_inc_not_zero(mm_users). Also note mmget_not_zero() should be
used instead.

However the code still uses khugepaged_test_exit in __khugepage_enter
that won't increase the mm_users, so then the patch relaxes that check
too much, albeit only for a debug check not strictly a bug.

The cons of this change purely that it'll decrease the responsiveness
in releasing the RAM of a killed task a bit.

To me the fewer time we hold the mm_users the better and I don't see
an obvious runtime improvement coming from this change. It's a bit
simpler yes, but the down_write in the exit path is well understood,
ksm does the same thing and it's in a slow path (it only happens if
the mm that exited is the current one under scan by either ksmd or
khugepaged, so normally the down_write is not executed in the exit
path and the "mm" is collected right away both as a mm_users and
mm_count).

In short I think it's a tradeoff: pros) removes down_write in a slow
path of the the mm exit which may simplify the code a bit, cons) it
could increase the latency in freeing memory as result of a task
exiting or being killed during the khugepaged scan, for example while
the THP is being allocated. While compaction runs to allocate the THP
in collapse_huge_page, if the task is killed currently the memory is
released right away, without waiting for the allocation to succeed or
fail.

I don't see a big enough problem with the down_write in a slow path of
khugepaged_exit to justify the increased latency in releasing memory.

I was very happy by Oleg's patch reducing the mm_users holding of
userfaultfd too. That was controlled by userland so it would only be
an issue for non-cooperative usage which isn't upstream yet, and it
was also much wider than this one would become with the patch applied,
but I liked the direction.

If prefer instead to remove the down_write, you probably could move
the test_exit before the down_read/write to bail out before taking the
lock: you don't need the mmap_sem to do test_exit anymore. The only
reason the text_exit would remain in fact is just to reduce the
latency of the memory freeing, it then becomes a voluntary preempt
cond_resched() to release the memory to make a parallel ;), but unable
to let the kernel free the memory while the THP allocation runs.

Thanks,
Andrea

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  reply	other threads:[~2016-06-03 13:51 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-01  3:11 linux-next: Tree for Jun 1 Stephen Rothwell
2016-06-02  1:48 ` [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup Sergey Senozhatsky
2016-06-02  9:21   ` Michal Hocko
2016-06-02 12:08     ` Sergey Senozhatsky
2016-06-02 12:21       ` Michal Hocko
2016-06-03 13:51         ` Andrea Arcangeli [this message]
2016-06-03 14:46           ` Michal Hocko
2016-06-03 15:10             ` Andrea Arcangeli
2016-06-07  7:34               ` Michal Hocko
2016-06-08  8:19               ` Vlastimil Babka
2016-06-03  7:15     ` Sergey Senozhatsky
2016-06-03  7:25       ` Michal Hocko
2016-06-03  8:43         ` Sergey Senozhatsky
2016-06-03  9:55           ` Michal Hocko
2016-06-03 10:05             ` Michal Hocko
2016-06-03 13:38               ` Sergey Senozhatsky
2016-06-03 13:45                 ` Michal Hocko
2016-06-03 13:49                   ` Michal Hocko
2016-06-04  7:51                     ` Sergey Senozhatsky
2016-06-06  8:39                       ` Michal Hocko
2016-06-02 13:24   ` Vlastimil Babka
2016-06-02 18:58     ` Ebru Akagunduz
2016-06-03  1:00       ` Sergey Senozhatsky
2016-06-03  1:29         ` Sergey Senozhatsky
2016-06-03  4:14           ` Sergey Senozhatsky
2016-06-03 12:28     ` [PATCH] mm, thp: fix locking inconsistency in collapse_huge_page Ebru Akagunduz
2016-06-06 13:05       ` Vlastimil Babka
2016-06-09  3:51         ` Sergey Senozhatsky

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