[STABLE,v4.9,09/10] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound

Message ID 20181218221049.6816-10-bigeasy@linutronix.de
State New
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  • Untitled series #17381
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Commit Message

Sebastian Andrzej Siewior Dec. 18, 2018, 10:10 p.m.
From: Will Deacon <will.deacon@arm.com>

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 <longman@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Acked-by: Waiman Long <longman@redhat.com>

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
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 <mingo@kernel.org>

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

 arch/x86/include/asm/qspinlock.h | 2 ++
 1 file changed, 2 insertions(+)



diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h
index e07cc206919d4..8b1ba1607091c 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -5,6 +5,8 @@ 
 #include <asm-generic/qspinlock_types.h>
 #include <asm/paravirt.h>
+#define _Q_PENDING_LOOPS	(1 << 9)
 #define	queued_spin_unlock queued_spin_unlock
  * queued_spin_unlock - release a queued spinlock