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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Bob Peterson <rpeterso@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Steven Whitehouse <swhiteho@redhat.com>,
	Andrew Lutomirski <luto@kernel.org>,
	Andreas Gruenbacher <agruenba@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-mm <linux-mm@kvack.org>,
	Mel Gorman <mgorman@techsingularity.net>
Subject: Re: [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for a page bit
Date: Sun, 25 Dec 2016 13:51:17 -0800	[thread overview]
Message-ID: <CA+55aFzqgtz-782MmLOjQ2A2nB5YVyLAvveo6G_c85jqqGDA0Q@mail.gmail.com> (raw)
In-Reply-To: <20161225030030.23219-3-npiggin@gmail.com>

On Sat, Dec 24, 2016 at 7:00 PM, Nicholas Piggin <npiggin@gmail.com> wrote:
> Add a new page flag, PageWaiters, to indicate the page waitqueue has
> tasks waiting. This can be tested rather than testing waitqueue_active
> which requires another cacheline load.

Ok, I applied this one too. I think there's room for improvement, but
I don't think it's going to help to just wait another release cycle
and hope something happens.

Example room for improvement from a profile of unlock_page():

   46.44 │      lock   andb $0xfe,(%rdi)
   34.22 │      mov    (%rdi),%rax

this has the old "do atomic op on a byte, then load the whole word"
issue that we used to have with the nasty zone lookup code too. And it
causes a horrible pipeline hickup because the load will not forward
the data from the (partial) store.

 Its' really a misfeature of our asm optimizations of the atomic bit
ops. Using "andb" is slightly smaller, but in this case in particular,
an "andq" would be a ton faster, and the mask still fits in an imm8,
so it's not even hugely larger.

But it might also be a good idea to simply use a "cmpxchg" loop here.
That also gives atomicity guarantees that we don't have with the
"clear bit and then load the value".

Regardless, I think this is worth more people looking at and testing.
And merging it is probably the best way for that to happen.

                Linus

  reply	other threads:[~2016-12-25 21:51 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-12-25  3:00 [PATCH 0/2] PageWaiters again Nicholas Piggin
2016-12-25  3:00 ` [PATCH 1/2] mm: Use owner_priv bit for PageSwapCache, valid when PageSwapBacked Nicholas Piggin
2016-12-25  5:13   ` Hugh Dickins
2016-12-25  3:00 ` [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for a page bit Nicholas Piggin
2016-12-25 21:51   ` Linus Torvalds [this message]
2016-12-26  1:16     ` Nicholas Piggin
2016-12-26 19:07       ` Linus Torvalds
2016-12-27 11:19         ` Nicholas Piggin
2016-12-27 18:58           ` Linus Torvalds
2016-12-27 19:23             ` Linus Torvalds
2016-12-27 19:24               ` Linus Torvalds
2016-12-27 19:40                 ` Linus Torvalds
2016-12-27 20:17                   ` Linus Torvalds
2016-12-28  3:53             ` Nicholas Piggin
2016-12-28 19:17               ` Linus Torvalds
2016-12-29  4:08                 ` Nicholas Piggin
2016-12-29  4:16                   ` Linus Torvalds
2016-12-29  5:26                     ` Nicholas Piggin
2017-01-03 10:24                       ` Mel Gorman
2017-01-03 12:29                         ` Nicholas Piggin
2017-01-03 17:18                           ` Mel Gorman
2016-12-29 22:16                     ` [PATCH] mm/filemap: fix parameters to test_bit() Olof Johansson

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