[PATCH] lockref: Limit number of cmpxchg loop retries

From: Jan Glauber <jglauber@cavium.com>
To: linux-kernel@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Jan Glauber <jglauber@marvell.com>,
	Will Deacon <will.deacon@arm.com>,
	Jayachandran Chandrasekharan Nair <jnair@marvell.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH] lockref: Limit number of cmpxchg loop retries
Date: Wed,  5 Jun 2019 15:48:49 +0200	[thread overview]
Message-ID: <20190605134849.28108-1-jglauber@marvell.com> (raw)
In-Reply-To: <CAHk-=wjPqcPYkiWKFc=R3+18DXqEhV+Nfbo=JWa32Xp8Nze67g@mail.gmail.com>

The lockref cmpxchg loop is unbound as long as the spinlock is not
taken. Depending on the hardware implementation of compare-and-swap
a high number of loop retries might happen.

Add an upper bound to the loop to force the fallback to spinlocks
after some time. A retry value of 100 should not impact any hardware
that does not have this issue.

With the retry limit the performance of an open-close testcase
improved between 60-70% on ThunderX2.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jan Glauber <jglauber@marvell.com>
---
 lib/lockref.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/lockref.c b/lib/lockref.c
index 3d468b53d4c9..5b34bbd3eba8 100644
--- a/lib/lockref.c
+++ b/lib/lockref.c
@@ -9,6 +9,7 @@
  * failure case.
  */
 #define CMPXCHG_LOOP(CODE, SUCCESS) do {					\
+	int retry = 100;							\
 	struct lockref old;							\
 	BUILD_BUG_ON(sizeof(old) != 8);						\
 	old.lock_count = READ_ONCE(lockref->lock_count);			\
@@ -21,6 +22,8 @@
 		if (likely(old.lock_count == prev.lock_count)) {		\
 			SUCCESS;						\
 		}								\
+		if (!--retry)							\
+			break;							\
 		cpu_relax();							\
 	}									\
 } while (0)
-- 
2.21.0


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