Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org, Davidlohr Bueso <dave@stgolabs.net>,
	Jason Low <jason.low2@hp.com>, Dave Chinner <david@fromorbit.com>,
	Scott J Norton <scott.norton@hpe.com>,
	Douglas Hatch <doug.hatch@hpe.com>
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field
Date: Tue, 10 May 2016 09:02:19 +0200	[thread overview]
Message-ID: <20160510070219.GG3408@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <57314650.5050206@hpe.com>

On Mon, May 09, 2016 at 10:24:16PM -0400, Waiman Long wrote:
> >>+static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
> >>+{
> >>+	/*
> >>+	 * We check the owner value first to make sure that we will only
> >>+	 * do a write to the rwsem cacheline when it is really necessary
> >>+	 * to minimize cacheline contention.
> >>+	 */
> >>+	if (sem->owner != (struct task_struct *)RWSEM_READER_OWNED)
> >>+		sem->owner = (struct task_struct *)RWSEM_READER_OWNED;
> >How much if anything did this optimization matter?
> 
> I hadn't run any performance test to verify the effective of this change.
> For a reader-heavy rwsem, this change should be able to save quite a lot of
> needless write to the rwsem cacheline.

Right; I was just wondering.

> >>+static inline bool rwsem_is_writer_owned(struct task_struct *owner)
> >>+{
> >>+	return (unsigned long)owner>  RWSEM_READER_OWNED;
> >>+}
> >Tad too clever that; what does GCC generate if you write the obvious:
> >
> >	return owner&&  owner != RWSEM_READER_OWNER;
> 
> You are right. GCC is intelligent enough to make the necessary optimization.
> I will revert it to this form which is more obvious.

Yay! thanks for checking. Sometimes GCC throws a wobbly and does
something unexpected, but it tends to get these 'simple' things right.