linux-kernel - Re: [PATCH 1/5] irq_work: Split raised and lazy lists

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Date:	Mon, 26 May 2014 21:26:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] irq_work: Split raised and lazy lists

On Mon, May 26, 2014 at 06:53:13PM +0200, Frederic Weisbecker wrote:
> On Mon, May 26, 2014 at 05:59:44PM +0200, Peter Zijlstra wrote:
> > On Sun, May 25, 2014 at 04:29:47PM +0200, Frederic Weisbecker wrote:
> > > An irq work can be handled from two places: from the tick if the work
> > > carries the "lazy" flag and the tick is periodic, or from a self IPI.
> > > 
> > > We merge all these works in a single list and we use some per cpu latch
> > > to avoid raising a self-IPI when one is already pending.
> > > 
> > > Now we could do away with this ugly latch if only the list was only made of
> > > non-lazy works. Just enqueueing a work on the empty list would be enough
> > > to know if we need to raise an IPI or not.
> > > 
> > > Also we are going to implement remote irq work queuing. Then the per CPU
> > > latch will need to become atomic in the global scope. That's too bad
> > > because, here as well, just enqueueing a work on an empty list of
> > > non-lazy works would be enough to know if we need to raise an IPI or not.
> > > 
> > > So lets take a way out of this: split the works in two distinct lists,
> > > one for the works that can be handled by the next tick and another
> > > one for those handled by the IPI. Just checking if the latter is empty
> > > when we queue a new work is enough to know if we need to raise an IPI.
> > 
> > That ^
> > 
> > >  bool irq_work_queue(struct irq_work *work)
> > >  {
> > > +	unsigned long flags;
> > > +
> > >  	/* Only queue if not already pending */
> > >  	if (!irq_work_claim(work))
> > >  		return false;
> > >  
> > > -	/* Queue the entry and raise the IPI if needed. */
> > > -	preempt_disable();
> > > +	/* Check dynticks safely */
> > > +	local_irq_save(flags);
> > 
> > Does not mention this ^
> > 
> > 'sup?
> 
> Because it's really just a technical detail.
> If we enqueue before checking for tick stopped, we can avoid disabling irqs
> because it's fine if we just raced with an irq in-between.
> 
> But now that we enqueue _after_, we can't afford an IRQ in between.
> 
> Should I update the comments maybe?

Well, yes because it was entirely non-obvious, but maybe we can write it
such that we can avoid the irq disable, because they're expensive.

How about something like:

	if (work->flags & IRQ_WORK_LAZY) {
		if (llist_add(&work->llnode, __get_cpu_var(lazy_list)) && 
		    tick_nohz_tick_stopped())
			arch_irq_work_raise();
	} else {
		if (llist_add(&work->llnode, __get_cpu_var(raise_list)))
			arch_irq_work_raise();
	}

That way we check it after the enqueue.
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