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

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Message-ID: <20160510070219.GG3408@twins.programming.kicks-ass.net>
Date:	Tue, 10 May 2016 09:02:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Dave Chinner <david@...morbit.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

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.