linux-kernel - Re: [PATCH v2 6/9] perf,tracing: Prepare the perf-trace interface for RCU changes

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Date:   Thu, 13 Feb 2020 13:38:25 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        rostedt@...dmis.org, mingo@...nel.org, gregkh@...uxfoundation.org,
        gustavo@...eddedor.com, tglx@...utronix.de, paulmck@...nel.org,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com,
        "Steven Rostedt (VMware)" <rosted@...dmis.org>
Subject: Re: [PATCH v2 6/9] perf,tracing: Prepare the perf-trace interface
 for RCU changes

On Thu, Feb 13, 2020 at 09:29:51AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 12, 2020 at 06:28:30PM -0500, Joel Fernandes wrote:
> > On Wed, Feb 12, 2020 at 10:01:45PM +0100, Peter Zijlstra wrote:
> > > The tracepoint interface will stop providing regular RCU context; make
> > > sure we do it ourselves, since perf makes use of regular RCU protected
> > > data.
> > > 
> > > Suggested-by: Steven Rostedt (VMware) <rosted@...dmis.org>
> > > Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > > ---
> > >  kernel/events/core.c |    5 +++++
> > >  1 file changed, 5 insertions(+)
> > > 
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -8950,6 +8950,7 @@ void perf_tp_event(u16 event_type, u64 c
> > >  {
> > >  	struct perf_sample_data data;
> > >  	struct perf_event *event;
> > > +	unsigned long rcu_flags;
> > 
> > The flags are not needed I guess, if you agree on not using in_nmi() in
> > trace_rcu_enter().
> 
> Even then we need to store the state: 'didn't do nothing' vs 'did call
> rcu_needs_to_wake_up_and_pay_attention_noaw'. That is, we only need to
> do something (expensive!) when !rcu_is_watching().

You are right, that sounds good. I was talking to Paul and we chatted that if
in_nmi() is safe (which I believe it is as we are not calling RCU before you
update the preempt counts), then in RCU we can replace the @irq with
!in_nmi() and simplify that code.  Then we can simplify this bit as well
(keep rcu_flags but only call rcu_irq_enter_irqsave() instead of
rcu_nmi_enter(). May be you can do the RCU internal bits in your v3 or should
those be separate patches? Whatever Paul and you want to do.

thanks,

 - Joel

 - Joel