linux-kernel - [PATCH v3 04/14] locking/qspinlock/x86: Increase _Q_PENDING

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Message-Id: <1524738868-31318-5-git-send-email-will.deacon@arm.com>
Date:   Thu, 26 Apr 2018 11:34:18 +0100
From:   Will Deacon <will.deacon@....com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org, peterz@...radead.org,
        mingo@...nel.org, boqun.feng@...il.com, paulmck@...ux.vnet.ibm.com,
        longman@...hat.com, will.deacon@....com
Subject: [PATCH v3 04/14] 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.

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Suggested-by: Waiman Long <longman@...hat.com>
Signed-off-by: Will Deacon <will.deacon@....com>
---
 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
-- 
2.1.4