Messages in this thread | | | Date | Sun, 12 Oct 2008 15:46:29 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH, RFC] v7 scalable classic RCU implementation |
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On Sun, Oct 12, 2008 at 05:52:56PM +0200, Manfred Spraul wrote: > Paul E. McKenney wrote: >> +/* >> + * If the specified CPU is offline, tell the caller that it is in >> + * a quiescent state. Otherwise, whack it with a reschedule IPI. >> + * Grace periods can end up waiting on an offline CPU when that >> + * CPU is in the process of coming online -- it will be added to the >> + * rcu_node bitmasks before it actually makes it online. Because this >> + * race is quite rare, we check for it after detecting that the grace >> + * period has been delayed rather than checking each and every CPU >> + * each and every time we start a new grace period. >> + */ > > What about using CPU_DYING and CPU_STARTING? > > Then this race wouldn't exist anymore.
Because I don't want to tie RCU too tightly to the details of the online/offline implementation. It is too easy for someone to make a "simple" change and break things, especially given that the online/offline code still seems to be adjusting a bit.
So I might well use CPU_DYING and CPU_STARTING, but I would still keep the check offlined CPUs in the force_quiescent_state() processing.
>> +static void force_quiescent_state(struct rcu_state *rsp, int relaxed) >> +{ >> + [snip] >> + case RCU_FORCE_QS: >> + >> + /* Check dyntick-idle state, send IPI to laggarts. */ >> + if (rcu_process_dyntick(rsp, >> dyntick_recall_completed(rsp), >> + rcu_implicit_dynticks_qs)) >> + goto unlock_ret; >> + >> + /* Leave state in case more forcing is required. */ >> + >> + break; > > Hmm - your code must loop multiple times over the cpus. > I've use a different approach: More forcing is only required for a nohz cpu > when it was hit within a long-running interrupt. > Thus I've added a '->kick_poller' flag, rcu_irq_exit() reports back when > the long-running interrupt completes. Never more than one loop over the > outstanding cpus is required.
Do you send a reschedule IPI to CPUs that are not in dyntick idle mode, but who have failed to pass through a quiescent state?
In my case, more forcing is required only for a nohz CPU in a long-running interrupt (as with your approach), for sending the aforementioned IPI, and for checking for offlined CPUs as noted above.
Thanx, Paul
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