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

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Message-Id: <20181015023328.GP2674@linux.ibm.com>
Date:   Sun, 14 Oct 2018 19:33:28 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     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 Sun, Oct 14, 2018 at 07:13:49PM -0700, Joel Fernandes wrote:
> On Sun, Oct 14, 2018 at 07:08:27PM -0700, Joel Fernandes wrote:
> > On Sun, Oct 14, 2018 at 04:17:31PM -0700, Paul E. McKenney wrote:
> > > On Sun, Oct 14, 2018 at 02:29:55PM -0700, Joel Fernandes (Google) wrote:
> > > > The Requirements.html document says "Disabling Preemption Does Not Block
> > > > Grace Periods". However this is no longer true with the RCU
> > > > consolidation. Lets remove the obsolete (non-)requirement entirely.
> > > > 
> > > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > 
> > > Good catch, queued, thank you!
> > 
> > Thanks! By the way after I sent the patch, I also tried Oleg's experiment to
> > confirm that this is indeed obsolete.  :)
> > 
> > One thing interesting came up when I tried synchronize_rcu_expedited()
> > instead of synchronize_rcu() in Oleg's experiment, I still saw a multiple
> > millisecond delay between when the rcu read section completely and the
> > synchronize_rcu_expedited returns:
> > 
> > For example, with synchronize_rcu_expedited, the 'SPIN done' and the 'SYNC
> > done' are about 3 millisecond apart:
> > [   77.599142] SPIN start
> > [   77.601595] SYNC start
> > [   82.604950] SPIN done!
> > [   82.607836] SYNC done!
> >  I saw anywhere from 2-6 milliseconds.
> > 
> > The reason I bring this up is according to Requirements.html: In some cases,
> > the multi-millisecond synchronize_rcu() latencies are unacceptable. In these
> > cases, synchronize_rcu_expedited() may be used instead,.. so either I messed
> > something up in the experiment, or I need to update this part of the document ;-)

In normal testing, 2-6 milliseconds is indeed excessive.  Could you please
point me at Oleg's experiment?  Also, what CONFIG_PREEMPT setting were
you using?  (My guess is CONFIG_PREEMPT=y.)

> So I realized I'm running in Qemu so it could also be a scheduling delay of
> the vcpu thread. So apologies about the noise if the experiment works fine
> for you.

I used rcuperf, which might not be doing the same thing as Oleg's
experiment.

							Thanx, Paul