Messages in this thread | | | Date | Thu, 15 Mar 2012 14:50:54 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH RFC] rcu: Make rcu_barrier() less disruptive |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > On Thu, Mar 15, 2012 at 11:21:59AM -0700, Paul E. McKenney wrote: > > On Thu, Mar 15, 2012 at 01:45:27PM -0400, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > The rcu_barrier() primitive interrupts each and every CPU, registering > > > > a callback on every CPU. Once all of these callbacks have been invoked, > > > > rcu_barrier() knows that every callback that was registered before > > > > the call to rcu_barrier() has also been invoked. > > > > > > > > However, there is no point in registering a callback on a CPU that > > > > currently has no callbacks, most especially if that CPU is in a > > > > deep idle state. This commit therefore makes rcu_barrier() avoid > > > > interrupting CPUs that have no callbacks. Doing this requires reworking > > > > the handling of orphaned callbacks, otherwise callbacks could slip through > > > > rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had > > > > not yet interrupted to a CPU that rcu_barrier() had already interrupted. > > > > This reworking was needed anyway to take a first step towards weaning > > > > RCU from the CPU_DYING notifier's use of stop_cpu(). > > > > > > Quoting Documentation/RCU/rcubarrier.txt: > > > > > > "We instead need the rcu_barrier() primitive. This primitive is similar > > > to synchronize_rcu(), but instead of waiting solely for a grace > > > period to elapse, it also waits for all outstanding RCU callbacks to > > > complete. Pseudo-code using rcu_barrier() is as follows:" > > > > > > The patch you propose seems like a good approach to make rcu_barrier > > > less disruptive, but everyone need to be aware that rcu_barrier() would > > > quit having the side-effect of doing the equivalent of > > > "synchronize_rcu()" from now on: within this new approach, in the case > > > where there are no pending callbacks, rcu_barrier() could, AFAIU, return > > > without waiting for the current grace period to complete. > > > > > > Any use of rcu_barrier() that would assume that a synchronize_rcu() is > > > implicit with the rcu_barrier() execution would be a bug anyway, but > > > those might only show up after this patch is applied. I would therefore > > > recommend to audit all rcu_barrier() users to ensure none is expecting > > > rcu_barrier to act as a synchronize_rcu before pushing this change. > > > > Good catch! > > > > I am going to chicken out and explicitly wait for a grace period if there > > were no callbacks. Having rcu_barrier() very rarely be a quick no-op does > > sound like a standing invitation for subtle non-reproducible bugs. ;-) > > I take it back... > > After adopting callbacks (rcu_adopt_orphan_cbs()), _rcu_barrier() > unconditionally posts a callback on the current CPU and waits for it. > So _rcu_barrier() actually does always wait for a grace period.
Ah ok, that should handle it then.
> > Yes, I could be more dainty and make rcu_adopt_orphan_cbs() return an > indication of whether there were any callbacks, and then post the callback > only if either there were some callbacks adopted or if there were no calls > to smp_call_function_single(). But that adds complexity for almost no > benefit -- and no one can accuse _rcu_barrier() of being a fastpath! ;-) > > Or am I missing something here?
Nope, I think it all makes sense.
Thanks,
Mathieu
> > Thanx, Paul >
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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