linux-kernel - Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi

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Message-ID: <20190829163255.GA15605@linux.ibm.com>
Date:   Thu, 29 Aug 2019 09:32:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Frederic Weisbecker <fweisbec@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>, kernel-team@...roid.com,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC v1 2/2] rcu/tree: Remove dynticks_nmi_nesting counter

On Thu, Aug 29, 2019 at 09:59:07AM -0400, Joel Fernandes wrote:
> On Wed, Aug 28, 2019 at 08:43:36PM -0700, Paul E. McKenney wrote:
> [snip] 
> > On the tracing patch...  That patch might be a good idea regardless,
> > but I bet that the reason that you felt the sudden need for it was due
> > to the loss of information in your eventual ->dynticks_nesting field.
> > After all, the value 0x1 might be an interrupt from idle, or it might
> > just as easily be a task running in the kernel at process level.
> 
> True, however what really triggered me to do it was the existing code which
> does not distinguish between entry/exit from USER and IDLE.
> 
> > The reason the patch might nevertheless be a good idea is that redundant
> > information can be helpful when debugging.  Especially when debugging
> > new architecture-specific code, which is when RCU's dyntick-idle warnings
> > tend to find bugs.
> 
> Sure, and also that it is more readable to ordinary human beings than "++="
> and "--=" :-D.

And those considerations did figure into my deciding that the tracing
change was likely a good thing in any case.  ;-)

							Thanx, Paul