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

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Message-ID: <20170424125200.vxfmehm4tpdpeisc@hirez.programming.kicks-ass.net>
Date:   Mon, 24 Apr 2017 14:52:00 +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>
Subject: Re: [RFC][PATCH tip/sched/core] sched/rt: Simplify the IPI rt
 balancing logic

On Mon, Apr 24, 2017 at 08:43:18AM -0400, Steven Rostedt wrote:
> On Mon, 24 Apr 2017 10:51:54 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Fri, Apr 21, 2017 at 10:49:29PM -0400, Steven Rostedt wrote:
> > > When a CPU schedules in a lower priority task and wants to make sure
> > > overloaded CPUs know about it. It increments the rto_loop_next. Then it does
> > > an atomic_inc_return() on rto_loop_start. If the returned value is not "1",
> > > then it does atomic_dec() on rt_loop_start and returns. If the value is "1",
> > > then it will take the rto_lock to synchronize with a possible IPI being sent
> > > around to the overloaded CPUs.  
> > 
> > > +	start = atomic_inc_return(&rq->rd->rto_loop_start);
> > > +	if (start != 1)
> > > +		goto out;  
> > 
> > > +out:
> > > +	atomic_dec(&rq->rd->rto_loop_start);  
> > 
> > 
> > Did you just write a very expensive test-and-set trylock?
> 
> Probably. I didn't know we had a generic one. Where is it?
> 

There isn't. What I was getting at though is that something like:

static inline bool rto_start_trylock(atomic_t *v)
{
	int zero = 0;
	return atomic_try_cmpxchg(v, &zero, 1);
}

static void rto_start_unlock(atomic_t *v)
{
	atomic_set_release(v, 0);
}

Is more: clearer, faster and correct.


Clearer as that it better describes what it does, faster as that you
only have a single atomic, and more correct because it does a RELEASE in
the case we care about.