On Tue, 25 Jul 2017 17:05:40 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> wrote:
> 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" <paul...@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" <paul...@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. Note, only the first irq_work called will take action. The other callers will see that a irq_work is pending and will not reivoke one. > > > > 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. Still requires a hard interrupt (timer) (thinking of NOHZ FULL where this does matter). > > > > 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? You treat it just like you are with the timer code. You have a irq_work struct attached to your rdp descriptor. And call irq_work_run() when interrupts are disabled. If it hasn't already been invoked it will invoke one. Then the irq_work handler will look at the rdp attached to the irq_work (container_of()), and then wake the associated thread. It is much lighter weight than a timer setup. -- Steve
