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@xxxxxx>
> ---
> 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 *?
Will
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