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

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Message-ID: <49642829.20006@cn.fujitsu.com>
Date:	Wed, 07 Jan 2009 11:57:29 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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

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?

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.


Lai.


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