From: Linus Torvalds <torvalds@linux-foundation.org>
To: Christoph Lameter <cl@linux-foundation.org>
Cc: Andi Kleen <andi@firstfloor.org>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
Minchan Kim <minchan.kim@gmail.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Peter Zijlstra <peterz@infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"hugh.dickins" <hugh.dickins@tiscali.co.uk>,
Nick Piggin <nickpiggin@yahoo.com.au>,
Ingo Molnar <mingo@elte.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
Date: Tue, 5 Jan 2010 10:56:37 -0800 (PST) [thread overview]
Message-ID: <alpine.LFD.2.00.1001051052150.3630@localhost.localdomain> (raw)
In-Reply-To: <alpine.DEB.2.00.1001051235200.2246@router.home>
On Tue, 5 Jan 2010, Christoph Lameter wrote:
>
> There will not be much spinning. The cacheline will be held exclusively by
> one processor. A request by other processors for shared access to the
> cacheline will effectively stop the execution on those processors until
> the cacheline is available.
AT WHICH POINT THEY ALL RACE TO GET IT TO SHARED MODE, only to then have
one of them actually see that it got a ticket!
Don't you see the problem? The spinlock (with ticket locks) essentially
does the "xadd" atomic increment anyway, but then it _waits_ for it. All
totally pointless, and all just making for problems, and wasting CPU time
and causing more cross-node traffic.
In contrast, a native xadd-based rwsem will basically do the atomic
increment (that ticket spinlocks also do) and then just go on with its
life. Never waiting for all the other CPU's to also do their ticket
spinlocks.
The "short critical region" you talk about is totally meaningless, since
the contention will mean that everybody is waiting and hitting that
cacheline for a long time regardless - they'll be waiting for O(n)
_other_ CPU's to get the lock first!
Linus
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next prev parent reply other threads:[~2010-01-05 18:57 UTC|newest]
Thread overview: 121+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-01-04 18:24 [RFC][PATCH 0/8] Speculative pagefault -v3 Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 1/8] mm: Remove pte reference from fault path Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 2/8] mm: Speculative pagefault infrastructure Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 3/8] mm: Add vma sequence count Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 4/8] mm: RCU free vmas Peter Zijlstra
2010-01-05 2:43 ` Paul E. McKenney
2010-01-05 8:28 ` Peter Zijlstra
2010-01-05 16:05 ` Paul E. McKenney
2010-01-04 18:24 ` [RFC][PATCH 5/8] mm: Speculative pte_map_lock() Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 6/8] mm: handle_speculative_fault() Peter Zijlstra
2010-01-05 0:25 ` KAMEZAWA Hiroyuki
2010-01-05 3:13 ` Linus Torvalds
2010-01-05 8:17 ` Peter Zijlstra
2010-01-05 8:57 ` Peter Zijlstra
2010-01-05 15:34 ` Linus Torvalds
2010-01-05 15:40 ` Al Viro
2010-01-05 16:10 ` Linus Torvalds
2010-01-06 15:41 ` Peter Zijlstra
2010-01-05 9:37 ` Peter Zijlstra
2010-01-05 23:35 ` Linus Torvalds
2010-01-05 4:29 ` Minchan Kim
2010-01-05 4:43 ` KAMEZAWA Hiroyuki
2010-01-05 5:10 ` Linus Torvalds
2010-01-05 5:30 ` KAMEZAWA Hiroyuki
2010-01-05 7:39 ` KAMEZAWA Hiroyuki
2010-01-05 15:26 ` Linus Torvalds
2010-01-05 16:14 ` Linus Torvalds
2010-01-05 17:25 ` Andi Kleen
2010-01-05 17:47 ` Christoph Lameter
2010-01-05 18:00 ` Andi Kleen
2010-01-05 17:55 ` Linus Torvalds
2010-01-05 18:13 ` Christoph Lameter
2010-01-05 18:25 ` Linus Torvalds
2010-01-05 18:46 ` Christoph Lameter
2010-01-05 18:56 ` Linus Torvalds [this message]
2010-01-05 19:15 ` Christoph Lameter
2010-01-05 19:28 ` Linus Torvalds
2010-01-05 18:55 ` Paul E. McKenney
2010-01-05 19:08 ` Linus Torvalds
2010-01-05 19:23 ` Paul E. McKenney
2010-01-05 20:29 ` Peter Zijlstra
2010-01-05 20:46 ` Linus Torvalds
2010-01-05 21:00 ` Linus Torvalds
2010-01-05 23:29 ` Paul E. McKenney
2010-01-06 0:22 ` KAMEZAWA Hiroyuki
2010-01-06 1:37 ` Linus Torvalds
2010-01-06 2:52 ` KAMEZAWA Hiroyuki
2010-01-06 3:27 ` Linus Torvalds
2010-01-06 3:56 ` KAMEZAWA Hiroyuki
2010-01-06 4:20 ` Linus Torvalds
2010-01-06 7:06 ` KAMEZAWA Hiroyuki
2010-01-06 7:49 ` Minchan Kim
2010-01-06 9:39 ` Linus Torvalds
2010-01-07 1:00 ` KAMEZAWA Hiroyuki
2010-01-08 16:53 ` Peter Zijlstra
2010-01-08 17:22 ` Linus Torvalds
2010-01-08 17:43 ` Christoph Lameter
2010-01-08 17:52 ` Linus Torvalds
2010-01-08 18:33 ` Christoph Lameter
2010-01-08 18:46 ` Andi Kleen
2010-01-08 18:56 ` Christoph Lameter
2010-01-08 19:10 ` Andi Kleen
2010-01-08 19:11 ` Linus Torvalds
2010-01-08 19:28 ` Andi Kleen
2010-01-08 19:39 ` Linus Torvalds
2010-01-08 19:42 ` Linus Torvalds
2010-01-08 21:36 ` Linus Torvalds
2010-01-08 21:46 ` Christoph Lameter
2010-01-08 22:43 ` Linus Torvalds
2010-01-08 22:43 ` Linus Torvalds
2010-01-09 14:47 ` Ed Tomlinson
2010-01-10 5:27 ` Nitin Gupta
2010-01-05 15:14 ` Christoph Lameter
2010-01-05 8:18 ` Peter Zijlstra
2010-01-05 6:00 ` Minchan Kim
2010-01-05 4:48 ` Linus Torvalds
2010-01-05 6:09 ` Minchan Kim
2010-01-05 6:09 ` KAMEZAWA Hiroyuki
2010-01-05 6:24 ` Minchan Kim
2010-01-05 8:35 ` Peter Zijlstra
2010-01-05 13:45 ` Arjan van de Ven
2010-01-05 14:15 ` Andi Kleen
2010-01-05 15:17 ` Christoph Lameter
2010-01-06 3:22 ` Arjan van de Ven
2010-01-07 16:11 ` Christoph Lameter
2010-01-07 16:19 ` Linus Torvalds
2010-01-07 16:31 ` Linus Torvalds
2010-01-07 16:34 ` Paul E. McKenney
2010-01-07 16:36 ` Christoph Lameter
2010-01-08 4:49 ` Arjan van de Ven
2010-01-08 5:00 ` Linus Torvalds
2010-01-08 15:51 ` Christoph Lameter
2010-01-09 15:55 ` Arjan van de Ven
2010-01-07 17:22 ` Peter Zijlstra
2010-01-07 17:36 ` Linus Torvalds
2010-01-07 17:49 ` Linus Torvalds
2010-01-07 18:00 ` Peter Zijlstra
2010-01-07 18:15 ` Linus Torvalds
2010-01-07 21:49 ` Peter Zijlstra
2010-01-07 18:44 ` Linus Torvalds
2010-01-07 19:20 ` Paul E. McKenney
2010-01-07 20:06 ` Linus Torvalds
2010-01-07 20:25 ` Paul E. McKenney
2010-01-07 19:24 ` Christoph Lameter
2010-01-07 20:08 ` Linus Torvalds
2010-01-07 20:13 ` Linus Torvalds
2010-01-07 21:44 ` Peter Zijlstra
2010-01-07 22:33 ` Linus Torvalds
2010-01-08 0:23 ` KAMEZAWA Hiroyuki
2010-01-08 0:25 ` KAMEZAWA Hiroyuki
2010-01-08 0:39 ` Linus Torvalds
2010-01-08 0:41 ` Linus Torvalds
2010-01-07 23:51 ` Rik van Riel
2010-01-04 18:24 ` [RFC][PATCH 7/8] mm,x86: speculative pagefault support Peter Zijlstra
2010-01-04 18:24 ` [RFC][PATCH 8/8] mm: Optimize pte_map_lock() Peter Zijlstra
2010-01-04 21:41 ` [RFC][PATCH 0/8] Speculative pagefault -v3 Rik van Riel
2010-01-04 21:46 ` Peter Zijlstra
2010-01-04 23:20 ` Rik van Riel
2010-01-04 21:59 ` Christoph Lameter
2010-01-05 0:28 ` KAMEZAWA Hiroyuki
2010-01-05 2:26 ` Minchan Kim
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