Messages in this thread | | | Date | Thu, 18 Oct 2012 09:35:25 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] rcu: restore correct batch limiting |
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On Thu, Oct 18, 2012 at 06:10:28PM +0200, Eric Dumazet wrote: > On Thu, 2012-10-18 at 08:25 -0700, Paul E. McKenney wrote: > > On Thu, Oct 18, 2012 at 02:44:50PM +0200, Eric Dumazet wrote: > > > > > > > Having 2 billions callbacks on one cpu would be problematic, I really > > > hope nobody relies on this ;) > > > > Fair point! ;-) > > > > But just making everything long makes it quite easy to analyze. > > > > > I guess the 10/infinity switch should be smarter. > > > > > > something like the following maybe : > > > > > > rdp->blimit = max(blimit, rdp->qlen >> 6); > > > > > > (if queue is big, dont wait to hit 10000 before allowing more items to > > > be handled per round) > > > > The -rt guys would not be amused. :-( > > > > But for non-realtime use, increasing rcutree.blimit either at boot or > > via sysfs could make sense. It is also likely that I will move callback > > processing to a kthread at some point, which would allow some additional > > flexibility. > > > > Ah, I now realize the loop can exceed blimit, but is it true for BH > variant ? (Not really a problem for 3.6/3.7 kernels, but prior ones)
Yep, applies to all the RCU flavors.
> if (++count >= bl && > (need_resched() || > (!is_idle_task(current) && !rcu_is_callbacks_kthread()))) > break; > > I wonder if ksoftirqd should be included as well...
This would be safe only if ksoftirqd could be guaranteed to be the lowest-priority process on the given CPU, which I do not believe to be the case. The problem is that if ksoftirqd is not the lowest-priority process on the given CPU, we can seriously delay that other process for no good reason. The fact that ksoftirqd does local_bh_disable() means that the scheduler cannot preempt it, either. :-(
> > Furthermore, it would be easy to have one default for non-rt and another > > for -rt, if that would help. > > > > > Signed-off-by: Eric Dumazet <edumazet@google.com> > > > > > > Please dont forget stable teams. (3.2 + ) > > > > Added both, please see below! > > Seems fine to me, thanks Paul !
Thank you for everything on this one!
Thanx, Paul
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