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Date:   Fri, 27 Apr 2018 02:38:35 -0700
From:   tip-bot for Will Deacon <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, peterz@...radead.org,
        longman@...hat.com, linux-kernel@...r.kernel.org,
        will.deacon@....com, mingo@...nel.org, tglx@...utronix.de,
        hpa@...or.com
Subject: [tip:locking/core] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS
 upper bound

Commit-ID:  b247be3fe89b6aba928bf80f4453d1c4ba8d2063
Gitweb:     https://git.kernel.org/tip/b247be3fe89b6aba928bf80f4453d1c4ba8d2063
Author:     Will Deacon <will.deacon@....com>
AuthorDate: Thu, 26 Apr 2018 11:34:18 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
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@...hat.com>
Signed-off-by: Will Deacon <will.deacon@....com>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Waiman Long <longman@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: boqun.feng@...il.com
Cc: linux-arm-kernel@...ts.infradead.org
Cc: paulmck@...ux.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@...nel.org>
---
 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

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