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 08:11:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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, Apr 07, 2017 at 10:58:26AM -0400, Steven Rostedt wrote:
> 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 is an issue for CPUs that don't have a to-memory increment
instruction.  How about __this_cpu_inc() and __this_cpu_dec(), given
that preemption is disabled?

> > 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?

At this point, if you can use the "__" versions, the change should be
small.  With that change, if no one else complains, I am OK.

							Thanx, Paul