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Date:   Fri, 27 Jul 2018 14:33:32 +0100
From:   Will Deacon <will.deacon@arm.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Joe Mario <jmario@redhat.com>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v3] locking/rwsem: Exit read lock slowpath if queue empty
 & no writer
Message-ID: <20180727133331.GC28549@arm.com>
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On Tue, Jul 24, 2018 at 03:10:25PM -0400, Waiman Long wrote:
> It was discovered that a constant stream of readers with occassional
> writers pounding on a rwsem may cause many of the readers to enter the
> slowpath unnecessarily thus increasing latency and lowering performance.
> 
> In the current code, a reader entering the slowpath critical section
> will unconditionally set the WAITING_BIAS, if not set yet, and clear
> its active count even if no one is in the wait queue and no writer
> is present. This causes some incoming readers to observe the presence
> of waiters in the wait queue and hence have to go into the slowpath
> themselves.
> 
> With sufficient numbers of readers and a relatively short lock hold time,
> the WAITING_BIAS may be repeatedly turned on and off and a substantial
> portion of the readers will go into the slowpath sustaining a rather
> long queue in the wait queue spinlock and repeated WAITING_BIAS on/off
> cycle until the logjam is broken opportunistically.
> 
> To avoid this situation from happening, an additional check is added to
> detect the special case that the reader in the critical section is the
> only one in the wait queue and no writer is present. When that happens,
> it can just exit the slowpath and return immediately as its active count
> has already been set in the lock.  Other incoming readers won't observe
> the presence of waiters and so will not be forced into the slowpath.
> 
> The issue was found in a customer site where they had an application
> that pounded on the pread64 syscalls heavily on an XFS filesystem. The
> application was run in a recent 4-socket boxes with a lot of CPUs. They
> saw significant spinlock contention in the rwsem_down_read_failed() call.
> With this patch applied, the system CPU usage went down from 85% to 57%,
> and the spinlock contention in the pread64 syscalls was gone.
> 
> v3: Revise the commit log and comment again.
> v2: Add customer testing results and remove wording that may cause
>     confusion.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/locking/rwsem-xadd.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)

My nits with the commit message have been addressed, so:

Acked-by: Will Deacon <will.deacon@arm.com>

Thanks!

Will