On Thu, Jun 16, 2011 at 09:03:35AM +0200, Ingo Molnar wrote: > > * Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > Ingo Molnar <mingo@xxxxxxx> wrote: > > > > > > I have this fix queued up currently: > > > > > > 09223371deac: rcu: Use softirq to address performance regression > > > > I really don't think that is even close to enough. > > Yeah. > > > It still does all the callbacks in the threads, and according to > > Peter, about half the rcu time in the threads remained.. > > You are right - things that are a few percent on a 24 core machine > will definitely go exponentially worse on larger boxen. We'll get rid > of the kthreads entirely. I did indeed at one time have access to larger test systems than I do now, and I clearly need to fix that. :-/ > The funny thing about this workload is that context-switches are > really a fastpath here and we are using anonymous IRQ-triggered > softirqs embedded in random task contexts as a workaround for that. The other thing that the IRQ-triggered softirqs do is to get the callbacks invoked in cases where a CPU-bound user thread is never context switching. Of course, one alternative might be to set_need_resched() to force entry into the scheduler as needed. > [ I think we'll have to revisit this issue and do it properly: > quiescent state is mostly defined by context-switches here, so we > could do the RCU callbacks from the task that turns a CPU > quiescent, right in the scheduler context-switch path - perhaps > with an option for SCHED_FIFO tasks to *not* do GC. I considered this approach for TINY_RCU, but dropped it in favor of reducing the interlocking between the scheduler and RCU callbacks. Might be worth revisiting, though. If SCHED_FIFO task omit RCU callback invocation, then there will need to be some override for CPUs with lots of SCHED_FIFO load, probably similar to RCU's current blimit stuff. > That could possibly be more cache-efficient than softirq execution, > as we'll process a still-hot pool of callbacks instead of doing > them only once per timer tick. It will also make the RCU GC > behavior HZ independent. ] Well, the callbacks will normally be cache-cold in any case due to the grace-period delay, but on the other hand, both tick-independence and the ability to shield a given CPU from RCU callback execution might be quite useful. The tick currently does the following for RCU: 1. Informs RCU of user-mode execution (rcu_sched and rcu_bh quiescent state). 2. Informs RCU of non-dyntick idle mode (again, rcu_sched and rcu_bh quiescent state). 3. Kicks the current CPU's RCU core processing as needed in response to actions from other CPUs. Frederic's work avoiding ticks in long-running user-mode tasks might take care of #1, and it should be possible to make use of the current dyntick-idle APIs to deal with #2. Replacing #3 efficiently will take some thought. > In any case the proxy kthread model clearly sucked, no argument about > that. Indeed, I lost track of the global nature of real-time scheduling. :-( Whatever does the boosting will need to have process context and can be subject to delays, so that pretty much needs to be a kthread. But it will context-switch quite rarely, so should not be a problem. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>