From 4e21502d37a503d4311e2bdb46a66aef783a7cfe Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Tue, 18 Dec 2018 18:13:59 +0100 Subject: locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound commit b247be3fe89b6aba928bf80f4453d1c4ba8d2063 upstream. On x86, atomic_cond_read_relaxed will busy-wait with a cpu_relax() loop, so it is desirable to increase the number of times we spin on the qspinlock lockword when it is found to be transitioning from pending to locked. According to Waiman Long: | Ideally, the spinning times should be at least a few times the typical | cacheline load time from memory which I think can be down to 100ns or | so for each cacheline load with the newest systems or up to several | hundreds ns for older systems. which in his benchmarking corresponded to 512 iterations. Suggested-by: Waiman Long Signed-off-by: Will Deacon Acked-by: Peter Zijlstra (Intel) Acked-by: Waiman Long Cc: Linus Torvalds Cc: Thomas Gleixner Cc: boqun.feng@gmail.com Cc: linux-arm-kernel@lists.infradead.org Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-5-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Sasha Levin --- arch/x86/include/asm/qspinlock.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'arch/x86') diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h index cf4cdf508ef4..2cb6624acaec 100644 --- a/arch/x86/include/asm/qspinlock.h +++ b/arch/x86/include/asm/qspinlock.h @@ -6,6 +6,8 @@ #include #include +#define _Q_PENDING_LOOPS (1 << 9) + #define queued_spin_unlock queued_spin_unlock /** * queued_spin_unlock - release a queued spinlock -- cgit v1.2.3