linux-kernel - Re: [PATCH] tracing/irqtrace: only call trace_hardirqs

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Date:   Tue, 1 May 2018 14:15:14 -0700
From:   Joel Fernandes <joel.opensrc@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/irqtrace: only call trace_hardirqs_on/off when
 state changes

On Tue, May 1, 2018 at 1:00 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 1 May 2018 21:48:38 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
>
>> On Tue, May 01, 2018 at 03:38:40PM -0400, Steven Rostedt wrote:
>> > On Tue, 1 May 2018 21:19:51 +0200
>> > Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> > > 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.
>> >
>> > I think it is about triggering on OFF->OFF a warning, as that would
>> > only happen if we have:
>> >
>> >     local_irq_save(flags);
>> >     [..]
>> >     local_irq_disable();
>> >
>>
>> Ahh, ok. Yes, that is easier to do with these changes. The alternative
>> is to add more information to the tracehooks such that we can do the
>> same internally, but whatever.
>>
>> Yeah, I'm fine with the proposed change, but maybe improve the Changelog
>> a little for slow people like me :-)
>
> Great!
>
> Nicholas,
>
> I know this is an old patch (from last November), but want to send it
> again with a proper change log and signed off by?

I actually wrote the exact same patch yesterday with changes Matsami
suggested. However I decided not to send it, since it didn't have any
performance improvement (which was the reason I wrote it).

Also with my recent set, I don't think it will help detect repeated
calls to trace_hardirqs_off because we are handling that recursive
case by using per-cpu variable and bailing out if there is a
recursion, before even calling into lockdep.

I have mixed feelings about this patch, I am Ok with this patch but I
suggest its sent with the follow-up patch that shows its use of this.
And also appreciate if such a follow-up patch is rebased onto the IRQ
tracepoint work: https://patchwork.kernel.org/patch/10373129/

What do you think?

thanks,

 - Joel