From: Andrew Morton <akpm@linux-foundation.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>, Oleg Nesterov <oleg@redhat.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Vlastimil Babka <vbabka@suse.cz>, Jan Kara <jack@suse.cz>,
Michal Hocko <mhocko@suse.cz>, Hugh Dickins <hughd@google.com>,
Dave Hansen <dave.hansen@intel.com>,
Linux Kernel <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Linux-FSDevel <linux-fsdevel@vger.kernel.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
David Howells <dhowells@redhat.com>
Subject: Re: [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5
Date: Wed, 21 May 2014 14:50:00 -0700 [thread overview]
Message-ID: <20140521145000.f130f8779f7641d0d8afcace@linux-foundation.org> (raw)
In-Reply-To: <20140521213354.GL2485@laptop.programming.kicks-ass.net>
On Wed, 21 May 2014 23:33:54 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, May 21, 2014 at 02:26:22PM -0700, Andrew Morton wrote:
> > > +static inline void
> > > +__prepare_to_wait(wait_queue_head_t *q, wait_queue_t *wait,
> > > + struct page *page, int state, bool exclusive)
> >
> > Putting MM stuff into core waitqueue code is rather bad. I really
> > don't know how I'm going to explain this to my family.
>
> Right, so we could avoid all that and make the functions in mm/filemap.c
> rather large and opencode a bunch of wait.c stuff.
>
The world won't end if we do it Mel's way and it's probably the most
efficient. But ugh. This stuff does raise the "it had better be a
useful patch" bar.
> Which is pretty much what I initially pseudo proposed.
Alternative solution is not to merge the patch ;)
> > > + __ClearPageWaiters(page);
> >
> > We're freeing the page - if someone is still waiting on it then we have
> > a huge bug? It's the mysterious collision thing again I hope?
>
> Yeah, so we only clear that bit when at 'unlock' we find there are no
> more pending waiters, so if the last unlock still had a waiter, we'll
> leave the bit set.
Confused. If the last unlock had a waiter, that waiter will get woken
up so there are no waiters any more, so the last unlock clears the flag.
um, how do we determine that there are no more waiters? By looking at
the waitqueue. But that waitqueue is hashed, so it may contain waiters
for other pages so we're screwed? But we could just go and wake up the
other-page waiters anyway and still clear PG_waiters?
um2, we're using exclusive waitqueues so we can't (or don't) wake all
waiters, so we're screwed again?
(This process is proving to be a hard way of writing Mel's changelog btw).
If I'm still on track here, what happens if we switch to wake-all so we
can avoid the dangling flag? I doubt if there are many collisions on
that hash table?
If there *are* a lot of collisions, I bet it's because a great pile of
threads are all waiting on the same page. If they're trying to lock
that page then wake-all is bad. But if they're just waiting for IO
completion (probable) then it's OK.
I'll stop now.
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next prev parent reply other threads:[~2014-05-21 21:50 UTC|newest]
Thread overview: 103+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-13 9:45 [PATCH 00/19] Misc page alloc, shmem, mark_page_accessed and page_waitqueue optimisations v3r33 Mel Gorman
2014-05-13 9:45 ` [PATCH 01/19] mm: page_alloc: Do not update zlc unless the zlc is active Mel Gorman
2014-05-13 9:45 ` [PATCH 02/19] mm: page_alloc: Do not treat a zone that cannot be used for dirty pages as "full" Mel Gorman
2014-05-13 9:45 ` [PATCH 03/19] jump_label: Expose the reference count Mel Gorman
2014-05-13 9:45 ` [PATCH 04/19] mm: page_alloc: Use jump labels to avoid checking number_of_cpusets Mel Gorman
2014-05-13 10:58 ` Peter Zijlstra
2014-05-13 12:28 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 05/19] mm: page_alloc: Calculate classzone_idx once from the zonelist ref Mel Gorman
2014-05-13 22:25 ` Andrew Morton
2014-05-14 6:32 ` Mel Gorman
2014-05-14 20:29 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 06/19] mm: page_alloc: Only check the zone id check if pages are buddies Mel Gorman
2014-05-13 9:45 ` [PATCH 07/19] mm: page_alloc: Only check the alloc flags and gfp_mask for dirty once Mel Gorman
2014-05-13 9:45 ` [PATCH 08/19] mm: page_alloc: Take the ALLOC_NO_WATERMARK check out of the fast path Mel Gorman
2014-05-13 9:45 ` [PATCH 09/19] mm: page_alloc: Use word-based accesses for get/set pageblock bitmaps Mel Gorman
2014-05-22 9:24 ` Vlastimil Babka
2014-05-22 18:23 ` Andrew Morton
2014-05-22 18:45 ` Vlastimil Babka
2014-05-13 9:45 ` [PATCH 10/19] mm: page_alloc: Reduce number of times page_to_pfn is called Mel Gorman
2014-05-13 13:27 ` Vlastimil Babka
2014-05-13 14:09 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 11/19] mm: page_alloc: Lookup pageblock migratetype with IRQs enabled during free Mel Gorman
2014-05-13 13:36 ` Vlastimil Babka
2014-05-13 14:23 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 12/19] mm: page_alloc: Use unsigned int for order in more places Mel Gorman
2014-05-13 9:45 ` [PATCH 13/19] mm: page_alloc: Convert hot/cold parameter and immediate callers to bool Mel Gorman
2014-05-13 9:45 ` [PATCH 14/19] mm: shmem: Avoid atomic operation during shmem_getpage_gfp Mel Gorman
2014-05-13 9:45 ` [PATCH 15/19] mm: Do not use atomic operations when releasing pages Mel Gorman
2014-05-13 9:45 ` [PATCH 16/19] mm: Do not use unnecessary atomic operations when adding pages to the LRU Mel Gorman
2014-05-13 9:45 ` [PATCH 17/19] fs: buffer: Do not use unnecessary atomic operations when discarding buffers Mel Gorman
2014-05-13 11:09 ` Peter Zijlstra
2014-05-13 12:50 ` Mel Gorman
2014-05-13 13:49 ` Jan Kara
2014-05-13 14:30 ` Mel Gorman
2014-05-13 14:01 ` Peter Zijlstra
2014-05-13 14:46 ` Mel Gorman
2014-05-13 13:50 ` Jan Kara
2014-05-13 22:29 ` Andrew Morton
2014-05-14 6:12 ` Mel Gorman
2014-05-13 9:45 ` [PATCH 18/19] mm: Non-atomically mark page accessed during page cache allocation where possible Mel Gorman
2014-05-13 14:29 ` Theodore Ts'o
2014-05-20 15:49 ` [PATCH] mm: non-atomically mark page accessed during page cache allocation where possible -fix Mel Gorman
2014-05-20 19:34 ` Andrew Morton
2014-05-21 12:09 ` Mel Gorman
2014-05-21 22:11 ` Andrew Morton
2014-05-22 0:07 ` Mel Gorman
2014-05-22 5:35 ` Prabhakar Lad
2014-05-13 9:45 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Mel Gorman
2014-05-13 12:53 ` Mel Gorman
2014-05-13 14:17 ` Peter Zijlstra
2014-05-13 15:27 ` Paul E. McKenney
2014-05-13 15:44 ` Peter Zijlstra
2014-05-13 16:14 ` Paul E. McKenney
2014-05-13 18:57 ` Oleg Nesterov
2014-05-13 20:24 ` Paul E. McKenney
2014-05-14 14:25 ` Oleg Nesterov
2014-05-13 18:22 ` Oleg Nesterov
2014-05-13 18:18 ` Oleg Nesterov
2014-05-13 18:24 ` Peter Zijlstra
2014-05-13 18:52 ` Paul E. McKenney
2014-05-13 19:31 ` Oleg Nesterov
2014-05-13 20:32 ` Paul E. McKenney
2014-05-14 16:11 ` Oleg Nesterov
2014-05-14 16:17 ` Peter Zijlstra
2014-05-16 13:51 ` [PATCH 0/1] ptrace: task_clear_jobctl_trapping()->wake_up_bit() needs mb() Oleg Nesterov
2014-05-16 13:51 ` [PATCH 1/1] " Oleg Nesterov
2014-05-21 9:29 ` Peter Zijlstra
2014-05-21 19:19 ` Andrew Morton
2014-05-21 19:18 ` [PATCH 0/1] " Andrew Morton
2014-05-14 19:29 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Oleg Nesterov
2014-05-14 20:53 ` Mel Gorman
2014-05-15 10:48 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v4 Mel Gorman
2014-05-15 13:20 ` Peter Zijlstra
2014-05-15 13:29 ` Peter Zijlstra
2014-05-15 15:34 ` Oleg Nesterov
2014-05-15 15:45 ` Peter Zijlstra
2014-05-15 16:18 ` Mel Gorman
2014-05-15 15:03 ` Oleg Nesterov
2014-05-15 21:24 ` Andrew Morton
2014-05-21 12:15 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5 Mel Gorman
2014-05-21 13:02 ` Peter Zijlstra
2014-05-21 15:33 ` Mel Gorman
2014-05-21 16:08 ` Peter Zijlstra
2014-05-21 21:26 ` Andrew Morton
2014-05-21 21:33 ` Peter Zijlstra
2014-05-21 21:50 ` Andrew Morton [this message]
2014-05-22 0:07 ` Mel Gorman
2014-05-22 7:20 ` Peter Zijlstra
2014-05-22 10:40 ` [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock_page fastpath v7 Mel Gorman
2014-05-22 10:56 ` Peter Zijlstra
2014-05-22 13:00 ` Mel Gorman
2014-05-22 14:40 ` Mel Gorman
2014-05-22 15:04 ` Peter Zijlstra
2014-05-22 15:36 ` Mel Gorman
2014-05-22 16:58 ` [PATCH] mm: filemap: Avoid unnecessary barriers and waitqueue lookups in unlock_page fastpath v8 Mel Gorman
2014-05-22 6:45 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5 Peter Zijlstra
2014-05-22 8:46 ` Mel Gorman
2014-05-22 17:47 ` Andrew Morton
2014-05-22 19:53 ` Mel Gorman
2014-05-21 23:35 ` Mel Gorman
2014-05-13 16:52 ` [PATCH 19/19] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath Peter Zijlstra
2014-05-14 7:31 ` Mel Gorman
2014-05-19 8:57 ` [PATCH] mm: Avoid unnecessary atomic operations during end_page_writeback Mel Gorman
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