Messages in this thread |  | | | Date | Thu, 16 Jun 2011 10:16:44 -0700 | | From | "Paul E. McKenney" <> | | Subject | Re: [GIT PULL] Re: REGRESSION: Performance regressions from switching anon_vma->lock to mutex |
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On Thu, Jun 16, 2011 at 09:03:35AM +0200, Ingo Molnar wrote: > > * Linus Torvalds <torvalds@linux-foundation.org> wrote: > > > > > > > Ingo Molnar <mingo@elte.hu> 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
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