Messages in this thread | | | Subject | Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs | From | Peter Zijlstra <> | Date | Wed, 31 Aug 2011 11:17:25 +0200 |
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On Wed, 2011-08-31 at 00:24 +0200, Frederic Weisbecker wrote: > On Tue, Aug 30, 2011 at 10:58:38PM +0200, Peter Zijlstra wrote: > > On Tue, 2011-08-30 at 17:42 +0200, Peter Zijlstra wrote: > > > On Tue, 2011-08-30 at 17:33 +0200, Frederic Weisbecker wrote: > > > > > See all that is still kernelspace ;-) I think I know what you mean to > > > > > say though, but seeing as you note there is even now a known shortcoming > > > > > I'm not very confident its a solid construction. What will help us find > > > > > such holes? > > > > > > > > This: https://lkml.org/lkml/2011/6/23/744 > > > > > > > > It's in one of Paul's branches and should make it for the next merge window. > > > > This should detect any of such holes. I made that on purpose for the nohz cpusets > > > > when I saw how much error prone that can be with rcu :) > > > > > > OK, good ;-) > > > > > > > > I would much rather we not rely on such fragile things too much.. this > > > > > RCU stuff wants way more thought, as it stands your patch-set doesn't do > > > > > anything useful IMO. > > > > > > > > Not sure what you mean. Well that Rcu thing for sure is fragile but we have > > > > the tools ready to find the problems. > > > > > > Right that thing you linked above does catch abuse, still your current > > > proposal means that due to RCU it will basically never disable the tick. > > > > So how about something like: > > > > Assuming we are in rcu_nohz state; on kernel enter we leave rcu_nohz but > > don't start the tick, instead we assign another cpu to run our state > > machine. > > The nohz CPU still has to notice its own quiescent states.
Why? rcu-sched can use a context-switch counter, rcu-preempt doesn't even need that. Remote cpus can notice those just fine.
> Now it could be > an optimization to ask another CPU to handle all the rest once that quiescent > state is found. That doesn't solve our main problem though which is to > reliably report quiescent states when asked for.
No, seriously, RCU should not, ever, need to re-enable the tick. Imagine a HPC workload where the system cores are also responsible for all IO and all the adaptive-nohz cores are simply crunching numbers. In that scenario you'll have a very high rcu usage because the system cores are all very busy arranging work for the computation cores.
> > On kernel exit we 'donate' all our rcu state to a willing victim (the > > same that earlier was kind enough to drive our state) and undo our > > entire GP accounting and re-enter rcu_nohz state. > > That's already what does rcu_enter_nohz().
Almost but not quite, it doesn't donate the callbacks for example (something it does do on hotplug -- and therefore any assumption the callback will in fact run on the cpu you submit it on is already broken).
> > If between that time we did restart the tick, we take back our rcu state > > and skip the donate and rcu_nohz enter on kernel exit. > > That's also what is done in this patchset.
Its not, since you don't hand of the grace period detectoring you don't take it back now do you..
> As soon as we re-enter the kernel > or the tick had to be restarted before we re-enter the kernel,
Another impossibility, you can only restart the tick from the kernel.
> we call > rcu_exit_nohz() that pulls back the CPU to the whole RCU machinery.
But you then also start the tick again..
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