From: Peter Zijlstra <peterz@infradead.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Nicholas Piggin <npiggin@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tracing/irqtrace: only call trace_hardirqs_on/off when state changes
Date: Tue, 1 May 2018 21:19:51 +0200 [thread overview]
Message-ID: <20180501191951.GJ12217@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180501144620.1e832a09@gandalf.local.home>
On Tue, May 01, 2018 at 02:46:20PM -0400, Steven Rostedt wrote:
> On Fri, 17 Nov 2017 02:15:06 +1000
> Nicholas Piggin <npiggin@gmail.com> wrote:
>
> > In local_irq_save and local_irq_restore, only call irq tracing when
> > the flag state acutally changes. It is not unexpected for the state
> > to go disable->disable.
> >
> > This allows the irq tracing code to better track superfluous
> > enables and disables, and in future could issue warnings. For the
> > most part they are harmless, but they can indicate that the caller
> > has lost track of its irq state.
>
> I missed this before (that was a busy time, I missed a lot of emails
> then :-/ ).
>
> Anyway, this makes sense.
>
> Peter?
I'm confused. The patch calls the trace hooks less often, so how can it
then better track superfluous calls?
> > @@ -110,7 +110,8 @@ do { \
> > #define local_irq_save(flags) \
> > do { \
> > raw_local_irq_save(flags); \
> > - trace_hardirqs_off(); \
> > + if (!raw_irqs_disabled_flags(flags)) \
> > + trace_hardirqs_off(); \
> > } while (0)
Here we only call the trace hook when we actually did an ON->OFF change
and loose the call on OFF->OFF.
> > @@ -118,9 +119,11 @@ do { \
> > do { \
> > if (raw_irqs_disabled_flags(flags)) { \
> > raw_local_irq_restore(flags); \
> > - trace_hardirqs_off(); \
> > + if (!irqs_disabled()) \
> > + trace_hardirqs_off(); \
Only call on ON->OFF, ignore OFF->OFF.
> > } else { \
> > - trace_hardirqs_on(); \
> > + if (irqs_disabled()) \
> > + trace_hardirqs_on(); \
> > raw_local_irq_restore(flags); \
> > } \
> > } while (0)
Only call on OFF->ON, ignore ON->ON.
Now, lockdep only minimally tracks these otherwise redundant operations;
see redundant_hardirqs_{on,off} counters, and loosing that doesn't seen
like a big issue.
But I'm confused how this helps track superfluous things, it looks like
it explicitly tracks _less_ superfluous transitions.
next prev parent reply other threads:[~2018-05-01 19:20 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-16 16:15 [PATCH] tracing/irqtrace: only call trace_hardirqs_on/off when state changes Nicholas Piggin
2018-05-01 18:46 ` Steven Rostedt
2018-05-01 19:19 ` Peter Zijlstra [this message]
2018-05-01 19:38 ` Steven Rostedt
2018-05-01 19:48 ` Peter Zijlstra
2018-05-01 20:00 ` Steven Rostedt
2018-05-01 21:15 ` Joel Fernandes
2018-05-02 0:12 ` Nicholas Piggin
2018-07-11 1:44 ` Steven Rostedt
2018-07-11 5:58 ` Joel Fernandes
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