From b247be3fe89b6aba928bf80f4453d1c4ba8d2063 Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon@arm.com>
Date: Thu, 26 Apr 2018 11:34:18 +0100
Subject: [PATCH 04/12] locking/qspinlock/x86: Increase _Q_PENDING_LOOPS upper bound
Git-commit: b247be3fe89b6aba928bf80f4453d1c4ba8d2063
Patch-mainline: v4.18-rc1
References: bsc#1050549

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: Davidlohr Bueso <dbueso@suse.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 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
-- 
2.16.4