From: David Howells <dhowells@redhat.com>
To: Michel Lespinasse <walken@google.com>
Cc: dhowells@redhat.com,
Linus Torvalds <torvalds@linux-foundation.org>,
Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Mike Waychison <mikew@google.com>,
Suleiman Souhlal <suleiman@google.com>,
Ying Han <yinghan@google.com>
Subject: Re: [PATCH 05/12] x86 rwsem: take advantage of new RWSEM_WAITING_BIAS semantics
Date: Wed, 12 May 2010 13:10:29 +0100 [thread overview]
Message-ID: <20188.1273666229@redhat.com> (raw)
In-Reply-To: <1273634462-2672-6-git-send-email-walken@google.com>
Michel Lespinasse <walken@google.com> wrote:
> static inline void __up_write(struct rw_semaphore *sem)
> {
> ...
> LOCK_PREFIX " xadd %1,(%2)\n\t"
> - /* tries to transition
> - 0xffff0001 -> 0x00000000 */
> - " jz 1f\n"
> + /* substracts 0xffff0001, returns the old value */
> + " cmp %4,%1\n\t"
> + /* are there waiting threads and no active threads ? */
> + " jne 1f\n"
> " call call_rwsem_wake\n"
It looks like you're betting on there being fast-path contention. If the
common case is no fast-path contention, then you'd be better off putting the
comparison out of line in the medium-path and retaining the JZ instruction.
The same goes for __up_read(): you could retain the JNS there and put the
comparison out of line into the medium-path.
Doing this also saves you some code space.
David
next prev parent reply other threads:[~2010-05-12 12:11 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-05-12 3:20 [PATCH 00/12] rwsem changes + down_read_unfair() proposal Michel Lespinasse
2010-05-12 3:20 ` [PATCH 01/12] rwsem: test for no active locks in __rwsem_do_wake undo code Michel Lespinasse
2010-05-12 3:20 ` [PATCH 02/12] rwsem: use single atomic update for sem count when waking up readers Michel Lespinasse
2010-05-12 3:20 ` [PATCH 03/12] rwsem: let RWSEM_WAITING_BIAS represent any number of waiting threads Michel Lespinasse
2010-05-12 3:20 ` [PATCH 04/12] rwsem: consistently use adjustment variable Michel Lespinasse
2010-05-12 3:20 ` [PATCH 05/12] x86 rwsem: take advantage of new RWSEM_WAITING_BIAS semantics Michel Lespinasse
2010-05-12 3:20 ` [PATCH 06/12] rwsem: wake queued readers when other readers are active Michel Lespinasse
2010-05-12 3:20 ` [PATCH 07/12] rwsem: wake queued readers when writer blocks on active read lock Michel Lespinasse
2010-05-12 3:20 ` [PATCH 08/12] rwsem: smaller wrappers around rwsem_down_failed_common Michel Lespinasse
2010-05-12 3:20 ` [PATCH 09/12] generic rwsem: implement down_read_unfair Michel Lespinasse
2010-05-12 3:21 ` [PATCH 10/12] rwsem: down_read_unfair infrastructure support Michel Lespinasse
2010-05-12 3:21 ` [PATCH 11/12] x86 rwsem: down_read_unfair implementation Michel Lespinasse
2010-05-12 3:21 ` [PATCH 12/12] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files Michel Lespinasse
2010-05-12 22:53 ` KOSAKI Motohiro
2010-05-12 23:35 ` Michel Lespinasse
2010-05-13 0:32 ` KOSAKI Motohiro
2010-05-12 10:39 ` [PATCH 01/12] rwsem: test for no active locks in __rwsem_do_wake undo code David Howells
2010-05-12 11:01 ` [PATCH 02/12] rwsem: use single atomic update for sem count when waking up readers David Howells
2010-05-13 0:54 ` Michel Lespinasse
2010-05-12 11:36 ` David Howells
2010-05-12 11:45 ` [PATCH 04/12] rwsem: consistently use adjustment variable David Howells
2010-05-13 1:12 ` Michel Lespinasse
2010-05-12 12:10 ` David Howells [this message]
2010-05-12 12:22 ` [PATCH 06/12] rwsem: wake queued readers when other readers are active David Howells
2010-05-13 2:39 ` Michel Lespinasse
2010-05-13 5:41 ` Michel Lespinasse
2010-05-12 12:33 ` [PATCH 07/12] rwsem: wake queued readers when writer blocks on active read lock David Howells
2010-05-12 12:36 ` [PATCH 08/12] rwsem: smaller wrappers around rwsem_down_failed_common David Howells
2010-05-12 12:42 ` David Howells
2010-05-13 2:54 ` Michel Lespinasse
2010-05-12 12:46 ` [PATCH 09/12] generic rwsem: implement down_read_unfair David Howells
2010-05-12 13:08 ` [PATCH 11/12] x86 rwsem: down_read_unfair implementation David Howells
2010-05-12 13:10 ` [PATCH 12/12] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files David Howells
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