linux-kernel - Re: [PATCH tip/sched/core v2] sched/rt: Simplify the IPI rt balancing logic

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Date:   Thu, 4 May 2017 20:42:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        John Kacur <jkacur@...hat.com>, Scott Wood <swood@...hat.com>
Subject: Re: [PATCH tip/sched/core v2] sched/rt: Simplify the IPI rt
 balancing logic

On Thu, May 04, 2017 at 01:25:38PM -0400, Steven Rostedt wrote:
> > I think you want to write that as:
> > 
> > 	struct root_domain *rd = rq->rd;
> > 	int cpu, next;
> > 
> > 	/* comment */
> > 	for (;;) { 
> > 		if (rd->rto_cpu >= nr_cpu_ids) {
> 
> If we go with your change, then this needs to be:
> 
> 		if (rd->rto_cpu < 0) {
> 
> > 			cpu = cpumask_first(rd->rto_mask);
> > 			rd->rto_cpu = cpu;
> > 			return cpu;
> > 		}

No you can leave it out entirely.

> > 
> > 		cpu = cpumask_next(rd->rto_mask);
> 
> cpumask_next() requires two parameters.

Indeed it does:

		cpu = cpumask_next(rd->rto_cpu, rd->rto_mask);

will be cpumask_first() when rto_cpu == -1, see for example
for_each_cpu().

> > > +static inline bool rto_start_trylock(atomic_t *v)
> > > +{
> > > +	return !atomic_cmpxchg(v, 0, 1);  
> > 
> > Arguably this could be: !atomic_cmpxchg_acquire(v, 0, 1);
> 
> Yes agreed. But if you remember, at the time I was basing this off of
> tip/sched/core, which didn't have atomic_cmpxchg_acquire() available.

No that's the try_cmpxchg stuff, the _acquire stuff is long in.

> Thanks for the review. I'll spin up a new patch. Unfortunately, I no
> longer have access to the behemoth machine. I'll only be testing this
> on 4 cores now, or 8 with HT.

I have something with 144 CPUs in or thereabout, if you have the
testcase handy I can give it a spin.