From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.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: Thu, 22 May 2014 08:45:29 +0200 [thread overview]
Message-ID: <20140522064529.GI30445@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20140521145000.f130f8779f7641d0d8afcace@linux-foundation.org>
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On Wed, May 21, 2014 at 02:50:00PM -0700, Andrew Morton wrote:
> On Wed, 21 May 2014 23:33:54 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> Alternative solution is not to merge the patch ;)
There is always that.. :-)
> > 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?
Ah, so leave it set. Then when we do an uncontended wakeup, that is a
wakeup where there are _no_ waiters left, we'll iterate the entire
hashed queue, looking for a matching page.
We'll find none, and only then clear the bit.
> (This process is proving to be a hard way of writing Mel's changelog btw).
Agreed :/
> 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?
Wake-all will be ugly and loose a herd of waiters, all racing to
acquire, all but one of whoem will loose the race. It also looses the
fairness, its currently a FIFO queue. Wake-all will allow starvation.
> 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.
Yeah, I'm not entirely sure on the rationale for adding PG_waiters to
writeback completion, and yes PG_writeback is a wake-all.
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next prev parent reply other threads:[~2014-05-22 6:45 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
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 ` Peter Zijlstra [this message]
2014-05-22 8:46 ` [PATCH] mm: filemap: Avoid unnecessary barries and waitqueue lookups in unlock_page fastpath v5 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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