Commit 405963b6 authored Jun 09, 2015 by Waiman Long Committed by Ingo Molnar Jun 19, 2015

locking/qrwlock: Don't contend with readers when setting _QW_WAITING



The current cmpxchg() loop in setting the _QW_WAITING flag for writers
in queue_write_lock_slowpath() will contend with incoming readers
causing possibly extra cmpxchg() operations that are wasteful. This
patch changes the code to do a byte cmpxchg() to eliminate contention
with new readers.

A multithreaded microbenchmark running 5M read_lock/write_lock loop
on a 8-socket 80-core Westmere-EX machine running 4.0 based kernel
with the qspinlock patch have the following execution times (in ms)
with and without the patch:

With R:W ratio = 5:1

	Threads	   w/o patch	with patch	% change
	-------	   ---------	----------	--------
	   2	     990	    895		  -9.6%
	   3	    2136	   1912		 -10.5%
	   4	    3166	   2830		 -10.6%
	   5	    3953	   3629		  -8.2%
	   6	    4628	   4405		  -4.8%
	   7	    5344	   5197		  -2.8%
	   8	    6065	   6004		  -1.0%
	   9	    6826	   6811		  -0.2%
	  10	    7599	   7599		   0.0%
	  15	    9757	   9766		  +0.1%
	  20	   13767	  13817		  +0.4%

With small number of contending threads, this patch can improve
locking performance by up to 10%. With more contending threads,
however, the gain diminishes.

Signed-off-by: Waiman Long <Waiman.Long@hp.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1433863153-30722-3-git-send-email-Waiman.Long@hp.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 92ae1837

kernel/locking/qrwlock.c

+24 −4

Original line number	Diff line number	Diff line
		@@ -22,6 +22,26 @@
		#include <linux/hardirq.h>
		#include <asm/qrwlock.h>

		/*
		* This internal data structure is used for optimizing access to some of
		* the subfields within the atomic_t cnts.
		*/
		struct __qrwlock {
		union {
		atomic_t cnts;
		struct {
		#ifdef __LITTLE_ENDIAN
		u8 wmode; /* Writer mode */
		u8 rcnts[3]; /* Reader counts */
		#else
		u8 rcnts[3]; /* Reader counts */
		u8 wmode; /* Writer mode */
		#endif
		};
		};
		arch_spinlock_t lock;
		};

		/**
		* rspin_until_writer_unlock - inc reader count & spin until writer is gone
		* @lock : Pointer to queue rwlock structure
		@@ -107,10 +127,10 @@ void queue_write_lock_slowpath(struct qrwlock *lock)
		* or wait for a previous writer to go away.
		*/
		for (;;) {
		cnts = atomic_read(&lock->cnts);
		if (!(cnts & _QW_WMASK) &&
		(atomic_cmpxchg(&lock->cnts, cnts,
		cnts \| _QW_WAITING) == cnts))
		struct __qrwlock l = (struct __qrwlock )lock;

		if (!READ_ONCE(l->wmode) &&
		(cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
		break;

		cpu_relax_lowlatency();

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