linux-kernel - Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able

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Message-ID: <20171005115539.7daafb66@gandalf.local.home>
Date:   Thu, 5 Oct 2017 11:55:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julia Cartwright <julia@...com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        <bigeasy@...utronix.de>, <linux-rt-users@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Dean Luick <dean.luick@...el.com>,
        Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        <linux-rdma@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>,
        Sebastian Sanchez <sebastian.sanchez@...el.com>
Subject: Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able_nort()

On Thu, 5 Oct 2017 10:37:59 -0500
Julia Cartwright <julia@...com> wrote:

> On Thu, Oct 05, 2017 at 05:27:30PM +0200, Thomas Gleixner wrote:
> > On Thu, 5 Oct 2017, Julia Cartwright wrote:  
> > > On Tue, Oct 03, 2017 at 12:49:19PM -0300, Arnaldo Carvalho de Melo wrote:  
> > > > -	preempt_disable();
> > > > +	preempt_disable_nort();
> > > >  	this_cpu_inc(*sc->buffers_allocated);  
> > > 
> > > Have you tried this on RT w/ CONFIG_DEBUG_PREEMPT?  I believe that the
> > > this_cpu_* operations perform a preemption check, which we'd trip.  
> > 
> > Good point. Changing this to migrate_disable() would do the trick.  
> 
> Wouldn't we still trip the preempt check even with migration disabled?
> In another thread I asked the same question: should the preemption
> checks here be converted to migration-checks in RT?

Is it a "preemption check"? Getting a cpu # should only care about
migration. This isn't the same as a rcu_sched check is it? That does
care about preemption.

-- Steve