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

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Message-ID: <20181015020827.GA217384@joelaf.mtv.corp.google.com>
Date:   Sun, 14 Oct 2018 19:08:27 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
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 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 ;-)

thanks,

 - Joel