linux-kernel - Re: [PATCH-tip v3 05/14] locking/rwsem: Remove rwsem

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Date:   Thu, 11 Apr 2019 09:25:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH-tip v3 05/14] locking/rwsem: Remove rwsem_wake() wakeup
 optimization

On Wed, Apr 10, 2019 at 02:42:22PM -0400, Waiman Long wrote:
> With the commit 59aabfc7e959 ("locking/rwsem: Reduce spinlock contention
> in wakeup after up_read()/up_write()"), the rwsem_wake() forgoes doing
> a wakeup if the wait_lock cannot be directly acquired and an optimistic
> spinning locker is present.  This can help performance by avoiding
> spinning on the wait_lock when it is contended.
> 
> With the later commit 133e89ef5ef3 ("locking/rwsem: Enable lockless
> waiter wakeup(s)"), the performance advantage of the above optimization
> diminishes as the average wait_lock hold time become much shorter.
> 
> By supporting rwsem lock handoff, we can no longer relies on the fact
> that the presence of an optimistic spinning locker will ensure that the
> lock will be acquired by a task soon. This can lead to missed wakeup
> and application hang. So the commit 59aabfc7e959 ("locking/rwsem:
> Reduce spinlock contention in wakeup after up_read()/up_write()")
> will have to be reverted.

Does it make sense to make this patch #3 in this series? The less code
there is, the easier to review the other patches.