Messages in this thread Patch in this message |  | | Date | Wed, 13 Feb 2013 04:07:24 -0800 | From | tip-bot for Rik van Riel <> | Subject | [tip:core/locking] x86/smp: Implement proportional backoff for ticket spinlocks |
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Commit-ID: 4c13d231339a8dc7ca056476fa10d95369080c8c Gitweb: http://git.kernel.org/tip/4c13d231339a8dc7ca056476fa10d95369080c8c Author: Rik van Riel <riel@redhat.com> AuthorDate: Wed, 6 Feb 2013 15:04:42 -0500 Committer: Ingo Molnar <mingo@kernel.org> CommitDate: Wed, 13 Feb 2013 09:06:29 +0100
x86/smp: Implement proportional backoff for ticket spinlocks
Add simple fixed value proportional backoff for ticket spinlocks.
By pounding on the cacheline with the spin lock less often, bus traffic is reduced. In cases of a data structure with embedded spinlock, the lock holder has a better chance of making progress.
If we are next in line behind the current holder of the lock, we do a fast spin, so as not to waste any time when the lock is released.
The number 50 is likely to be wrong for many setups, and this patch is mostly to illustrate the concept of proportional backup. The next patch automatically tunes the delay value.
Signed-off-by: Rik van Riel <riel@redhat.com> Signed-off-by: Michel Lespinasse <walken@google.com> Acked-by: Rafael Aquini <aquini@redhat.com> Cc: eric.dumazet@gmail.com Cc: lwoodman@redhat.com Cc: knoel@redhat.com Cc: chegu_vinod@hp.com Cc: raghavendra.kt@linux.vnet.ibm.com Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/20130206150442.1ffd017c@cuia.bos.redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org> --- arch/x86/kernel/smp.c | 23 ++++++++++++++++++++--- 1 file changed, 20 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kernel/smp.c b/arch/x86/kernel/smp.c index 20da354..aa743e9 100644 --- a/arch/x86/kernel/smp.c +++ b/arch/x86/kernel/smp.c @@ -117,11 +117,28 @@ static bool smp_no_nmi_ipi = false; */ void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc) { + __ticket_t head = inc.head, ticket = inc.tail; + __ticket_t waiters_ahead; + unsigned loops; + for (;;) { - cpu_relax(); - inc.head = ACCESS_ONCE(lock->tickets.head); + waiters_ahead = ticket - head - 1; + /* + * We are next after the current lock holder. Check often + * to avoid wasting time when the lock is released. + */ + if (!waiters_ahead) { + do { + cpu_relax(); + } while (ACCESS_ONCE(lock->tickets.head) != ticket); + break; + } + loops = 50 * waiters_ahead; + while (loops--) + cpu_relax(); - if (inc.head == inc.tail) + head = ACCESS_ONCE(lock->tickets.head); + if (head == ticket) break; } }
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