Re: [PATCH 6/6] locking/rwsem: Check for active lock before bailing on spinning

Jason Low
2015-01-27T18:16:36Z

On Sun, 2015-01-25 at 23:36 -0800, Davidlohr Bueso wrote:
> 37e9562453b (locking/rwsem: Allow conservative optimistic
> spinning when readers have lock) forced the default for
> optimistic spinning to be disabled if the lock owner was
> nil, which makes much sense for readers. However, while
> it is not our priority, we can make some optimizations
> for write-mostly workloads. We can bail the spinning step
> and still be conservative if there are any active tasks,
> otherwise there's really no reason not to spin, as the
> semaphore is most likely unlocked.
>
> This patch recovers most of a Unixbench 'execl' benchmark
> throughput by sleeping less and making better average system
> usage:
>
> before:
> CPU %user %nice %system %iowait %steal %idle
> all 0.60 0.00 8.02 0.00 0.00 91.38
>
> after:
> CPU %user %nice %system %iowait %steal %idle
> all 1.22 0.00 70.18 0.00 0.00 28.60
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
> kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
> index 88b3468..e0e9738 100644
> --- a/kernel/locking/rwsem-xadd.c
> +++ b/kernel/locking/rwsem-xadd.c
> @@ -296,23 +296,30 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
> static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> {
> struct task_struct *owner;
> - bool on_cpu = false;
> + bool ret = true;
>
> if (need_resched())
> return false;
>
> rcu_read_lock();
> owner = ACCESS_ONCE(sem->owner);
> - if (owner)
> - on_cpu = owner->on_cpu;
> - rcu_read_unlock();
> + if (!owner) {
> + long count = ACCESS_ONCE(sem->count);
> + /*
> + * If sem->owner is not set, yet we have just recently entered the
> + * slowpath with the lock being active, then there is a possibility
> + * reader(s) may have the lock. To be safe, bail spinning in these
> + * situations.
> + */
> + if (count & RWSEM_ACTIVE_MASK)
> + ret = false;
> + goto done;
> + }
>
> - /*
> - * If sem->owner is not set, yet we have just recently entered the
> - * slowpath, then there is a possibility reader(s) may have the lock.
> - * To be safe, avoid spinning in these situations.
> - */
> - return on_cpu;
> + ret = owner->on_cpu;
> +done:
> + rcu_read_unlock();
> + return ret;
> }

Acked-by: Jason Low <jason.low2@hp.com>