Subject: Re: [PATCH RFC] v4 somewhat-expedited "big hammer" RCU grace periods

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 8 May 2009 11:05:25 -0700

On Fri, May 08, 2009 at 07:28:31PM +0200, Eric Dumazet wrote:
> Paul E. McKenney a écrit :
> > Fourth cut of "big hammer" expedited RCU grace periods.  This uses
> > a kthread that schedules itself on all online CPUs in turn, thus
> > forcing a grace period.  The synchronize_sched(), synchronize_rcu(),
> > and synchronize_bh() primitives wake this kthread up and then wait for
> > it to force the grace period.
> > 
> > As before, this does nothing to expedite callbacks already registered
> > with call_rcu() or call_rcu_bh(), but there is no need to.  Just maps
> > to synchronize_rcu() and a new synchronize_rcu_bh() on preemptable RCU,
> > which has more complex grace-period detection -- this can be fixed later.
> > 
> > Passes light rcutorture testing.  Grace periods take around 200
> > microseconds on an 8-CPU Power machine.  This is a good order of magnitude
> > better than v3, but an order of magnitude slower than v2.  Furthermore,
> > it will get slower the more CPUs you have, and eight CPUs is not all
> > that many these days.  So this implementation still does not cut it.
> > 
> > Once again, I am posting this on the off-chance that I made some stupid
> > mistake that someone might spot.  Absent that, I am taking yet another
> > different approach, namely setting up per-CPU threads that are awakened
> > via smp_call_function(), permitting the quiescent states to be waited
> > for in parallel.
> > 
> 
> I dont know, dont we have possibility one cpu is dedicated for the use
> of a cpu hungry real time thread ?
> 
> krcu_sched_expedited() would dead lock or something ?

Good point!!!

One approach would be to use a prio-99 RT per-CPU thread that sleeps
unless/until an expedited grace period is required.

Aggressive real-time workloads would need to avoid doing things (like
changing networking configuration) that require expedited grace periods.

Seem reasonable?

							Thanx, Paul

> > Shortcomings:
> > 
> > o	Too slow!!!  Thinking in terms of using per-CPU kthreads.
> > 
> > o	The wait_event() calls result in 120-second warnings, need
> > 	to use something like wait_event_interruptible().  There are
> > 	probably other corner cases that need attention.
> > 
> > o	Does not address preemptable RCU.
> > 
> > Changes since v3:
> > 
> > o	Use a kthread that schedules itself on each CPU in turn to
> > 	force a grace period.  The synchronize_rcu() primitive
> > 	wakes up the kthread in order to avoid messing with affinity
> > 	masks on user tasks.
> > 
> > o	Tried a number of additional variations on the v3 approach, none
> > 	of which helped much.
> > 
> > Changes since v2:
> > 
> > o	Use reschedule IPIs rather than a softirq.
> > 
> > Changes since v1:
> > 
> > o	Added rcutorture support, and added exports required by
> > 	rcutorture.
> > 
> > o	Added comment stating that smp_call_function() implies a
> > 	memory barrier, suggested by Mathieu.
> > 
> > o	Added #include for delay.h.
> > 
> > Signed-off-by: Paul E. McKenney <[email-masked]>
> > ---
> > 
> >  include/linux/rcuclassic.h |   16 +++
> >  include/linux/rcupdate.h   |   24 ++---
> >  include/linux/rcupreempt.h |   10 ++
> >  include/linux/rcutree.h    |   13 ++
> >  kernel/rcupdate.c          |  103 +++++++++++++++++++++++
> >  kernel/rcupreempt.c        |    1 
> >  kernel/rcutorture.c        |  200 ++++++++++++++++++++++++---------------------
> >  7 files changed, 261 insertions(+), 106 deletions(-)
> > 
> 
> > +/*
> > + * Kernel thread that processes synchronize_sched_expedited() requests.
> > + * This is implemented as a separate kernel thread to avoid the need
> > + * to mess with other tasks' cpumasks.
> > + */
> > +static int krcu_sched_expedited(void *arg)
> > +{
> > +	int cpu;
> > +
> > +	do {
> > +		wait_event(need_sched_expedited_wq, need_sched_expedited);
> > +		need_sched_expedited = 0;
> > +		get_online_cpus();
> > +		for_each_online_cpu(cpu) {
> > +			sched_setaffinity(0, &cpumask_of_cpu(cpu));
> > +			schedule();
> 
> <<no return>>
> 
> > +		}
> > +		put_online_cpus();
> > +		sched_expedited_done = 1;
> > +		wake_up(&sched_expedited_done_wq);
> > +	} while (!kthread_should_stop());
> > +	return 0;
> > +}
> 
>