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

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Date:   Thu, 5 Oct 2017 11:05:35 -0500
From:   Julia Cartwright <julia@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Oct 05, 2017 at 11:55:39AM -0400, Steven Rostedt wrote:
> 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"?

Sorry if I was unclear, more precisely: the this_cpu_* family of
accessors, w/ CONFIG_DEBUG_PREEMPT currently spits out a warning when
the caller is invoked in a context where preemption is enabled.

The check is shared w/ the smp_processor_id() check, as implemented in
lib/smp_processor_id.c.  It effectively boils down to a check of
preempt_count() and irqs_disabled().

> Getting a cpu # should only care about migration.

I think we're agreeing? :)

> This isn't the same as a rcu_sched check is it? That does care about
> preemption.

This is something totally different, I think.

   Julia