From: Will Deacon <will.deacon@arm.com>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
Thomas Gleixner <tglx@linutronix.de>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Scott J Norton <scott.norton@hp.com>,
Douglas Hatch <doug.hatch@hp.com>
Subject: Re: [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency
Date: Mon, 6 Jul 2015 19:23:53 +0100 [thread overview]
Message-ID: <20150706182353.GC1607@arm.com> (raw)
In-Reply-To: <1436197386-58635-3-git-send-email-Waiman.Long@hp.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
next prev parent reply other threads:[~2015-07-06 18:24 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-06 15:43 [PATCH 0/4] locking/qrwlock: Improve qrwlock performance Waiman Long
2015-07-06 15:43 ` [PATCH 1/4] locking/qrwlock: Better optimization for interrupt context readers Waiman Long
2015-07-06 15:43 ` [PATCH 2/4] locking/qrwlock: Reduce reader/writer to reader lock transfer latency Waiman Long
2015-07-06 18:23 ` Will Deacon [this message]
2015-07-06 19:49 ` Waiman Long
2015-07-07 9:17 ` Will Deacon
2015-07-07 11:17 ` Peter Zijlstra
2015-07-07 11:49 ` Will Deacon
2015-07-07 14:30 ` Waiman Long
2015-07-07 17:27 ` Will Deacon
2015-07-07 18:10 ` Will Deacon
2015-07-07 21:29 ` Waiman Long
2015-07-08 9:52 ` Peter Zijlstra
2015-07-08 17:19 ` Will Deacon
2015-07-06 15:43 ` [PATCH 3/4] locking/qrwlock: Reduce writer to writer " Waiman Long
2015-07-06 15:43 ` [PATCH 4/4] locking/qrwlock: Use direct MCS lock/unlock in slowpath Waiman Long
2015-07-07 11:24 ` Peter Zijlstra
2015-07-07 21:59 ` Waiman Long
2015-07-07 22:13 ` Peter Zijlstra
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