linux-kernel - Re: [PATCH 2/2] tracing: Add support for critical section events

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Message-ID: <20170906084040.uhqibgnls36rdh2a@hirez.programming.kicks-ass.net>
Date:   Wed, 6 Sep 2017 10:40:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, kernel-team@...roid.com,
        Ingo Molnar <mingo@...hat.com>,
        Byungchul Park <byungchul.park@....com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] tracing: Add support for critical section events

On Tue, Sep 05, 2017 at 09:35:11AM -0700, Joel Fernandes wrote:
> On Mon, Sep 4, 2017 at 11:52 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Sep 04, 2017 at 08:26:13PM -0700, Joel Fernandes wrote:
> >
> >> Apologies, I meant (without the "off"):
> >>
> >> subsystem: atomic_section
> >> events:
> >>   irqs_disable
> >>   irqs_enable
> >>   preempt_disable
> >>   preempt_enable
> >>
> >> and additionally (similar to what my patch does):
> >>   preemptirq_enable
> >>   preemptirq_disable
> >>
> >
> > What do you need the last for?
> 
> The last 2 events above behave as 'disable' means either preempt or
> irq got disabled, and 'enable' means *both* preempt and irq are
> enabled (after either one of them was disabled).
> 
> This has the advantage of not generating events when we're already in
> an atomic section when using these events, for example acquiring spin
> locks in an interrupt handler might increase the preempt count and
> generate 'preempt_disable' events, but not preemptirq_disable events.
> This has the effect of reducing the spam in the traces when all we
> care about is being in an atomic section or not. These events happen a
> lot so to conserve space in the trace buffer, the user may want to
> just enable the latter 2 events. Does that sound Ok to you?

Hurm,... how about placing a filter on the other 4, such that we only
emit the event on 0<->x state transitions? IIRC tracing already has
filter bits and eBPF bits on that allow something like that.

That avoids having to introduce more tracepoints and gets you the same
results.