Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to idle enter/exit APIs | From | Peter Zijlstra <> | Date | Tue, 30 Aug 2011 22:58:38 +0200 |
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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.
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.
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.
I really should go read all those docs Paul send me to see how insane the above is.
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