On Thu, 2015-07-09 at 12:32 -0400, Waiman Long wrote:
> This patch eliminates that waiting. It also has the side effect
> of reducing the chance of writer lock stealing and improving the
> fairness of the lock. Using a locking microbenchmark, a 10-threads 5M
> locking loop of mostly readers (RW ratio = 10,000:1) has the following
> performance numbers in a Haswell-EX box:
>
> Kernel Locking Rate (Kops/s)
> ------ ---------------------
> 4.1.1 15,063,081
> Patched 4.1.1 17,241,552
In any case, for such read-mostly scenarios, you'd probably want to be
using rcu ;-).
>
> Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
> ---
> kernel/locking/qrwlock.c | 12 ++++--------
> 1 files changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
> index d9c36c5..6a7a3b8 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -88,15 +88,11 @@ void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
> arch_spin_lock(&lock->lock);
>
> /*
> - * At the head of the wait queue now, wait until the writer state
> - * goes to 0 and then try to increment the reader count and get
> - * the lock. It is possible that an incoming writer may steal the
> - * lock in the interim, so it is necessary to check the writer byte
> - * to make sure that the write lock isn't taken.
> + * At the head of the wait queue now, increment the reader count
> + * and wait until the writer, if it has the lock, has gone away.
> + * At ths
^^ this
> stage, it is not possible for a writer to remain in the
> + * waiting state (_QW_WAITING). So there won't be any deadlock.
Because the writer setting _QW_WAITING is done in the slowpath,
serialized with the qrwlock->lock, right?
> */
> - while (atomic_read(&lock->cnts) & _QW_WMASK)
> - cpu_relax_lowlatency();
> -
> cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
Nit: since 'cnts' is now only the original value of lock->cnts before
adding _QR_BIAS, could we rename it to 'prev_cnts' (or something)? --
iirc you removed the need for the variable when in interrupt context.
> rspin_until_writer_unlock(lock, cnts);
Thanks,
Davidlohr
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