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Date:   Thu, 26 Jul 2018 17:02:15 -0700
From:   Davidlohr Bueso <dave@stgolabs.net>
To:     Waiman Long <longman@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org, Joe Mario <jmario@redhat.com>
Subject: Re: [PATCH v3] locking/rwsem: Exit read lock slowpath if queue empty
 & no writer
Message-ID: <20180727000215.ez3el2tbbasxa5fk@linux-r8p5>
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On Tue, 24 Jul 2018, 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>

Reviewed-by: Davidlohr Bueso <dbueso@suse.de>