linux-kernel - Re: [PATCH v3 2/2] locking/qrwlock: Don't contend with readers when setting _QW

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Date:	Thu, 18 Jun 2015 13:40:32 +0100
From:	Will Deacon <will.deacon@....com>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v3 2/2] locking/qrwlock: Don't contend with readers when
 setting _QW_WAITING

On Thu, Jun 18, 2015 at 02:33:56AM +0100, Waiman Long wrote:
> On 06/16/2015 02:02 PM, Will Deacon wrote:
> > On Mon, Jun 15, 2015 at 11:24:03PM +0100, Waiman Long wrote:
> >> 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@...com>
> >> ---
> >>   kernel/locking/qrwlock.c |   28 ++++++++++++++++++++++++----
> >>   1 files changed, 24 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> >> index d7d7557..559198a 100644
> >> --- a/kernel/locking/qrwlock.c
> >> +++ b/kernel/locking/qrwlock.c
> >> @@ -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
> >> @@ -109,10 +129,10 @@ 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;
> > Maybe you could also update the x86 implementation of queue_write_unlock
> > to write the wmode field instead of casting to u8 *?
> >
> The queue_write_unlock() function is in the header file. I don't want to 
> expose the internal structure to other files.

Then I don't see the value in the new data structure -- why not just cast
to u8 * instead? In my mind, the structure has the advantage of supporting
both big and little endian systems, but to be useful it would need to be
available in the header file for architectures that chose to override
queue_write_unlock.

As an aside, I have some patches to get this up and running on arm64
which would need something like this structure for the big-endian case.

Will
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