Messages in this thread | | | Date | Wed, 5 Sep 2012 16:44:43 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC tip/core/rcu] Add callback-free CPUs |
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On Wed, Sep 05, 2012 at 11:48:40PM +0200, Peter Zijlstra wrote: > On Wed, 2012-09-05 at 14:39 -0700, Paul E. McKenney wrote: > > RCU callback execution can add significant OS jitter and also can degrade > > scheduling latency. This commit therefore adds the ability for selected > > CPUs ("rcu_nocbs=" boot parameter) to have their callbacks offloaded to > > kthreads. If the "rcu_nocb_poll" boot parameter is also specified, these > > kthreads will do polling, removing the need for the offloaded CPUs to do > > wakeups. At least one CPU must be doing normal callback processing: > > currently CPU 0 cannot be selected as a no-CBs CPU. In addition, attempts > > to offline the last normal-CBs CPU will fail. > > > > This is an experimental patch, so just FYI for the moment. Known > > shortcomings include: > > > > o The counters should be atomic_long_t rather than atomic_t. > > > > o No-CBs CPUs can be configured only at boot time. > > > > o Only a modest number of CPUs can be configured as no-CBs CPUs. > > Definitely a few tens, perhaps a few hundred, but no way thousands. > > > > o At least one CPU must remain a normal-CBs CPU. > > > > o Not much in the way of energy-efficiency features, though there > > are some natural energy savings inherent in the implementation > > > > o The per-no-CBs-CPU kthreads are not subject to RCU priority boosting. > > > > o Care is required when setting the kthreads to RT priority. > > > > Later versions will address some of them, but others are likely to remain. > > My LPC feedback in writing... > > So I see RCU as consisting of two parts: > A) Grace period tracking, > 2) Running the callbacks. > > This series seems to conflate the two, it talks of doing the callbacks > elsewhere (kthread), but it also moves the grace period detectoring into > the same kthread. > > The latter part is what complicates the thing. I'd suggest doing the > very simple callbacks only implementation first and leaving the grace > period machinery in the tick. > > Its typically the callbacks that consume most CPU time, whereas the > grace period computations, while tricky and subtle, are relatively > cheap. > > In particular, it solves the need to wait for grace periods from the > kthread (and bounce that no-nocb cpu to make progress), and it makes the > atomic list operations stuff a lot easier.
I was excited by this possibility when you first mentioned it, but the low-OS-jitter fans are going to need the grace-period computation to be offloaded as well. So if I use your (admittedly much simpler) approach, I get to rewrite it when Frederic's adaptive-ticks work goes in. Given that this is probably happening relatively soon, it would be better if I just did the implementation that will be needed long-term, rather than rewriting.
Though I am sure that people will be sad about fewer RCU patches. ;-)
Thanx, Paul
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