linux-kernel - Re: [PATCH][RFC]: mutex: adaptive spin

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Date:	Wed, 07 Jan 2009 07:32:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	paulmck@...ux.vnet.ibm.com, Gregory Haskins <ghaskins@...ell.com>,
	Ingo Molnar <mingo@...e.hu>, Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Chris Mason <chris.mason@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH][RFC]: mutex: adaptive spin

On Wed, 2009-01-07 at 11:57 +0800, Lai Jiangshan wrote:
> Peter Zijlstra wrote:
> > +void mutex_spin_or_schedule(struct mutex_waiter *waiter, long state, unsigned long *flags)
> > +{
> > +	struct mutex *lock = waiter->lock;
> > +	struct task_struct *task = waiter->task;
> > +	struct task_struct *owner = lock->owner;
> > +	struct rq *rq;
> > +
> > +	if (!owner)
> > +		goto do_schedule;
> > +
> > +	rq = task_rq(owner);
> > +
> > +	if (rq->curr != owner) {
> > +do_schedule:
> > +		__set_task_state(task, state);
> > +		spin_unlock_mutex(&lock->wait_lock, *flags);
> > +		schedule();
> > +	} else {
> > +		spin_unlock_mutex(&lock->wait_lock, *flags);
> > +		for (;;) {
> > +			/* Stop spinning when there's a pending signal. */
> > +			if (signal_pending_state(state, task))
> > +				break;
> > +
> > +			/* Owner changed, bail to revalidate state */
> > +			if (lock->owner != owner)
> > +				break;
> > +
> > +			/* Owner stopped running, bail to revalidate state */
> > +			if (rq->curr != owner)
> > +				break;
> > +
> 
> 2 questions from my immature thought:
> 
> 1) Do we need keep gcc from optimizing when we access lock->owner
>    and rq->curr in the loop?

cpu_relax() is a compiler barrier iirc.

> 2) "if (rq->curr != owner)" need become smarter.
>    schedule()
>    {
> 	select_next
> 	rq->curr = next;
> 	contex_swith
>    }
> we also spin when owner is select_next-ing in schedule().
> but select_next is not fast enough.

I'm not sure what you're saying here..
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