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SubjectRe: [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@joelfernandes.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


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