linux-kernel - Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

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Message-Id: <20181015210856.GE2674@linux.ibm.com>
Date:   Mon, 15 Oct 2018 14:08:56 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Nikolay Borisov <nborisov@...e.com>, linux-kernel@...r.kernel.org,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about
 disabling preemption

On Mon, Oct 15, 2018 at 01:15:56PM -0700, Joel Fernandes wrote:
> On Mon, Oct 15, 2018 at 12:54:26PM -0700, Paul E. McKenney wrote:
> [...]
> > > > In any case, please don't spin for milliseconds with preemption disabled.
> > > > The real-time guys are unlikely to be happy with you if you do this!
> > > 
> > > Well just to clarify, I was just running Oleg's test which did this. This
> > > test was mentioned in the original documentation that I deleted. Ofcourse I
> > > would not dare do such a thing in production code :-D. I guess to Oleg's
> > > defense, he did it to very that synchronize_rcu() was not blocked on
> > > preempt-disable sections which was a different test.
> > 
> > Understood!  Just pointing out that RCU's tolerating a given action does
> > not necessarily mean that it is a good idea to take that action.  ;-)
> 
> Makes sense :-) thanks.

Don't worry, that won't happen again.  ;-)

> > > > > > +		pr_crit("SPIN done!\n");
> > > > > > +		preempt_enable();
> > > > > > +		break;
> > > > > > +	case 777:
> > > > > > +		pr_crit("SYNC start\n");
> > > > > > +		synchronize_rcu();
> > > > > > +		pr_crit("SYNC done!\n");
> > > > > 
> > > > > But you are using the console printing infrastructure which is rather
> > > > > heavyweight. Try replacing pr_* calls with trace_printk so that you
> > > > > write to the lock-free ring buffer, this will reduce the noise from the
> > > > > heavy console printing infrastructure.
> > > > 
> > > > And this might be a problem as well.
> > > 
> > > This was not the issue (or atleast not fully the issue) since I saw the same
> > > thing with trace_printk. It was exactly what you said - which is the
> > > excessively long preempt disabled times.
> > 
> > One approach would be to apply this patch against (say) v4.18, which
> > does not have consolidated grace periods.  You might then be able to
> > tell if the pr_crit() calls make any difference.
> 
> I could do that, yeah. But since the original problem went away due to
> disabling preempts for a short while, I will move on and continue to focus on
> updating other parts of the documenation. Just to mention I
> brought this up because I thought its better to do that than not to, just
> incase there is any lurking issue with the consolidation. Sorry if that ended
> up with me being noisy.

Not a problem, no need to apologize!

> Just curious, while I am going through the documentation, is there anything
> in particular that particularly sticks out to you that needs updating? I
> think I am around 50% there with the last several rounds of doc patches but I
> have lot more to go through. "Just keep doing what you're doing" is also a
> perfectly valid answer ;-)

It is the things needing updating that I do not yet know about that worry
the most, so "Just keep doing what you're doing" seems most appropriate.  ;-)

							Thanx, Paul