Messages in this thread | | | Date | Tue, 25 Jul 2017 17:05:40 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 02/15] rcu: Use timer as backstop for NOCB deferred wakeups |
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On Tue, Jul 25, 2017 at 06:17:10PM -0400, Steven Rostedt wrote: > On Tue, 25 Jul 2017 12:18:14 -0700 > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > On Tue, Jul 25, 2017 at 02:12:20PM -0400, Steven Rostedt wrote: > > > On Mon, 24 Jul 2017 14:44:31 -0700 > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote: > > > > > > > The handling of RCU's no-CBs CPUs has a maintenance headache, namely > > > > that if call_rcu() is invoked with interrupts disabled, the rcuo kthread > > > > wakeup must be defered to a point where we can be sure that scheduler > > > > locks are not held. Of course, there are a lot of code paths leading > > > > from an interrupts-disabled invocation of call_rcu(), and missing any > > > > one of these can result in excessive callback-invocation latency, and > > > > potentially even system hangs. > > > > > > What about using irq_work? That's what perf and ftrace use for such a > > > case. > > > > I hadn't looked at irq_work before, thank you for the pointer! > > > > I nevertheless believe that timers work better in this particular case > > because they can be cancelled (which appears to be the common case), they > > Is the common case here that it doesn't trigger? That is, the > del_timer() will be called?
If you have lots of call_rcu() invocations, many of them will be invoked with interrupts enabled, and a later one with interrupts enabled will take care of things for the earlier ones. So there can be workloads where this is the case.
> > normally are not at all time-critical, and because running in softirq > > is just fine -- no need to run out of the scheduling-clock interrupt. > > irq_work doesn't always use the scheduling clock. IIRC, it will simply > trigger a interrupt (if the arch supports it), and the work will be > done when interrupts are enabled (the interrupt that will do the work > will trigger)
Ah, OK, so scheduling clock is just the backstop. Still, softirq is a bit nicer to manage than hardirq.
> > Seem reasonable? > > Don't know. With irq_work, you just call it and forget about it. No > need to mod or del timers.
But I could have a series of call_rcu() invocations with interrupts disabled, so I would need to interact somehow with the irq_work handler. Either that or dynamically allocate the needed data structure.
Or am I missing something here?
Thanx, Paul
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