Commit-ID: 4c13d231339a8dc7ca056476fa10d95369080c8c Gitweb: http://git.kernel.org/tip/4c13d231339a8dc7ca056476fa10d95369080c8c Author: Rik van Riel <[email protected]> AuthorDate: Wed, 6 Feb 2013 15:04:42 -0500 Committer: Ingo Molnar <[email protected]> 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 <[email protected]> Signed-off-by: Michel Lespinasse <[email protected]> Acked-by: Rafael Aquini <[email protected]> Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: [email protected] Cc: Linus Torvalds <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Link: http://lkml.kernel.org/r/[email protected] Signed-off-by: Ingo Molnar <[email protected]> --- 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; } } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
