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

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Message-ID: <20170424084318.2d03700f@gandalf.local.home>
Date:   Mon, 24 Apr 2017 08:43:18 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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, 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?

$ git grep test_and_set |grep trylock
fs/jfs/jfs_metapage.c:#define trylock_metapage(mp) test_and_set_bit_lock(META_locked, &(mp)->flag)

I don't think the above is what you mean.

-- Steve