linux-kernel - Re: [PATCH v2 1/2] locking/qrwlock: Reduce reader/writer to reader lock transfer latency

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Date:	Thu, 09 Jul 2015 13:52:38 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Waiman Long <Waiman.Long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>
Subject: Re: [PATCH v2 1/2] locking/qrwlock: Reduce reader/writer to reader
 lock transfer latency

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@...com>
> ---
>  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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