Messages in this thread | | | Date | Sun, 21 Sep 2008 13:09:51 +0200 | From | Manfred Spraul <> | Subject | Re: [PATCH, RFC] v4 scalable classic RCU implementation |
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Hi Paul,
Some further thoughts about design differences between your and my implementation:
- rcutree's qsmaskinit is the worst-case list of cpus that could be in rcu read side critical sections. - rcustate's cpu_total is the accurate list of cpus that could be in rcu read side critical sections.
Both variables are read rarely: for rcu_state, twice per grace period.
rcutree fixes up cpus that are "incorrectly" listed in qsmaskinit with force_quiescent_state(). It forces rcutree to use a cpu bitmask for qsmask and it forces rcutree to store the "done" information in a global structure. Additionately, in the worst case force_quiescent_state() must loop over all cpus. rcustate can use per-cpu structures and a global atomic_t. There is no loop over all cpus. That's a big advantage, thus I think it's worth the effort to maintain an accurate list. Unfortunately, I don't have an efficient implementation for the accurate list.
Some random ideas: - cpu_total is only read rarely. Thus it would be ok if the read operation is expensive [e.g. collect data from multiple cachelines, acquire spinlocks...] - updates to cpu_total happen with every interrupt on an idle system with no_hz. Thus it must be very scalable, preferably per-cpu data. And: Updates are far more frequent than grace periods. - updates to cpu_total happen nearly never without no_hz. Especially: far less frequent than grace periods.
What about adding an "invalid" flag to cpu_total? The "real" data is stored in per-cpu structures. - when a cpu enters/leaves nohz, then it invalidates the global cpu_total and updates a per-cpu structure - when the state machine needs the number of rcu-tracked cpus, then it checks if the global cpu_total is valid. If it's valid, then cpu_total is used directly. Otherwise the per-cpu structures are enumerated and the new value is stored as cpu_total.
What do you think?
-- Manfred
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