linux-kernel - Re: [PATCH 4/8] sched,rcu,tracing: Mark preempt_count

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Message-ID: <20200212101415.3615d66c@gandalf.local.home>
Date:   Wed, 12 Feb 2020 10:14:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        mingo@...nel.org, joel@...lfernandes.org,
        gregkh@...uxfoundation.org, gustavo@...eddedor.com,
        tglx@...utronix.de, paulmck@...nel.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com
Subject: Re: [PATCH 4/8] sched,rcu,tracing: Mark preempt_count_{add,sub}()
 notrace

On Wed, 12 Feb 2020 16:02:11 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Feb 12, 2020 at 09:24:17AM -0500, Steven Rostedt wrote:
> > On Wed, 12 Feb 2020 10:32:14 +0100
> > Peter Zijlstra <peterz@...radead.org> wrote:
> >   
> > > Because of the requirement that no tracing happens until after we've
> > > incremented preempt_count, see nmi_enter() / trace_rcu_enter(), mark
> > > these functions as notrace.  
> > 
> > I actually depend on these function being traced.  
> 
> Why? They already have a tracepoint inside.

Only when enabled.

> 
> > We do have
> > "preempt_enable_notrace()" and "preempt_disable_notrace()" for places
> > that shouldn't be traced. Can't we use those? (or simply
> > __preempt_count_add()) in the nmi_enter() code instead? (perhaps create
> > a preempt_count_add_notrace()).  
> 
> My initial patch has __preempt_count_add/sub() in, but then I figured
> someone would go complain the tracepoint would go missing.

Fine, but what bug are you trying to fix? I haven't seen one mentioned
yet. Function tracing has recursion protection, and tracing
preempt_count in nmi_enter() causes no problems. What's the problem you
are trying to solve?

-- Steve