[tip:locking/core] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound

[tip-bot for Will Deacon]

Commit-ID:  b247be3fe89b6aba928bf80f4453d1c4ba8d2063
Gitweb:     https://git.kernel.org/tip/b247be3fe89b6aba928bf80f4453d1c4ba8d2063
Author:     Will Deacon <will.deacon@xxxxxxx>
AuthorDate: Thu, 26 Apr 2018 11:34:18 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 27 Apr 2018 09:48:47 +0200

locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound

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@xxxxxxxxxx>
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Acked-by: Waiman Long <longman@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: boqun.feng@xxxxxxxxx
Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
Cc: paulmck@xxxxxxxxxxxxxxxxxx
Link: http://lkml.kernel.org/r/1524738868-31318-5-git-send-email-will.deacon@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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 90b0b0ed8161..da1370ad206d 100644
--- a/arch/x86/include/asm/qspinlock.h
+++ b/arch/x86/include/asm/qspinlock.h
@@ -7,6 +7,8 @@
 #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