linux-kernel - Re: [PATCH] sched: Do not release current rq lock on non contended double_lock

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Date:	Tue, 14 Jun 2016 21:42:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH] sched: Do not release current rq lock on non contended
 double_lock_balance()

On Tue, Jun 14, 2016 at 02:02:28PM -0400, Steven Rostedt wrote:
> On Tue, 14 Jun 2016 13:58:20 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> > And it does indeed make the hold time harder to analyze.
> > 
> > For instance; pull_rt_task() does:
> > 
> > 	for_each_cpu() {
> > 		double_lock_balance(this, that);
> > 		...
> > 		double_unlock_balance(this, that);
> > 	}
> > 
> > Which, with the trylock, ends up with a max possible hold time of
> > O(nr_cpus).
> 
> Sure, but I think we should try to limit that loop too, because that
> loop itself is what is triggering the large latency for me, because
> it constantly releases a spinlock and has to wait. This loop is done
> with preemption disabled.

Much worse, its done with IRQs disabled. But that affects only the local
CPU. Holding the lock that long affects all other CPUs too.

> > Unlikely, sure, but RT is a game of upper bounds etc.
> 
> Sure, but should we force worst case all the time?

How is that relevant? Either you have a bounded operation or you don't.

> We do a lot of optimization to allow for good throughput as well.

Only within keeping the upper bounds. The moment you let go of that,
you've destroyed RT.

> > So should we maybe do something like:
> > 
> > 	if (unlikely(raw_spin_is_contended(&this_rq->lock) ||
> > 	             !raw_spin_trylock(&busiest->lock))) {
> 
> Why do we care if this_rq is contended? 

To bound hold time.

> That's exactly what causes
> large latency to happen. Because when we let go of this_rq, this fast
> path becomes much slower because now it must wait for whatever is
> waiting on it to finish. The more CPUs you have, the bigger this issue
> becomes.

Yes, icky issue.

And while the numbers look pretty I'm not sure you've not introduced
another, less likely, issue.