linux-kernel - Re: [PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks() and stack_tracer

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Date:   Fri, 7 Apr 2017 10:58:26 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks()
 and stack_tracer_disable()

On Fri, 7 Apr 2017 07:43:35 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Fri, Apr 07, 2017 at 10:01:06AM -0400, Steven Rostedt wrote:
> > Paul,
> > 
> > Here's my latest. You OK with it?  
> 
> Given your update to 3/5, I suspect that we could live with it.  I am
> expecting some complaints about increases in idle-entry latency, but might
> be best to wait for complaints rather than complexifying too proactively.

We only added a this_cpu_inc() and this_cpu_dec() which are very fast
operations. I highly doubt it will be measurable. Although, I'm talking
about x86, IIRC, the this_cpu_inc/dec were be poorly written for other
archs in the past. I'm not sure if that was fixed though.

> 
> That said, there isn't supposed to be any tracing during the now very
> small interval where RCU's idle-entry is incomplete.  Mightn't it be
> better to (under CONFIG_PROVE_RCU or some such) give splats if tracing
> showed up in that interval?
> 

Again, tracing is not the issue. I do function tracing in that location
without any problems. The issue here was the stack tracer.

Maybe we can create a new variable that is more cache local to the RCU
code.

What about calling it "rcu_disabled"? Then tracing that depends on RCU
can simply check that.

s/stack_trace_disable/disable_rcu/
s/stack_trace_enable/enable_rcu/

export a per cpu variable rcu_disabled

Then I can have the stack tracer check that variable as well. And we
could even put in a WARN_ON(this_cpu_read(rcu_disabled) in the
TRACE_EVENT() macros.

Thoughts?

-- Steve