Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

[Will Deacon <will.deacon@arm.com>]

Hi Waiman,

On Mon, Jul 06, 2015 at 04:43:04PM +0100, Waiman Long wrote:
> Currently, a reader will check first to make sure that the writer mode
> byte is cleared before incrementing the reader count. That waiting is
> not really necessary. It increases the latency in the reader/writer
> to reader transition and reduces readers performance.
> 
> 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
> 
> Signed-off-by: Waiman Long <Waiman.Long@hp.com>

I've just finished rebasing my arm64 qrwlock stuff, but I think it will
conflict with these patches. Do you mind if I post them for review anyway,
so we can at least co-ordinate our efforts?

> ---
>  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 81bae99..ecd2d19 100644
> --- a/kernel/locking/qrwlock.c
> +++ b/kernel/locking/qrwlock.c
> @@ -88,15 +88,11 @@ void queue_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 stage, it is not possible for a writer to remain in the
> +	 * waiting state (_QW_WAITING). So there won't be any deadlock.
>  	 */
> -	while (atomic_read(&lock->cnts) & _QW_WMASK)
> -		cpu_relax_lowlatency();

Thinking about it, can we kill _QW_WAITING altogether and set (cmpxchg
from 0) wmode to _QW_LOCKED in the write_lock slowpath, polling (acquire)
rmode until it hits zero?

Will