>> But I was choosing to ignore it mainly to reduce the overhead of a >> perf intr in general. A subsequent real interrupt could go thru thru >> the gyrations of preemption etc. > > So that's dangerous thinking... People that run a PREEMPT kernel > generally tend to care about latency (esp. when combined with > PREEMPT_RT). > > And ignoring a preemption point gets these people upset (and missed > preemptions are a royal friggin pain to debug). Which implies that this patch goes to trash ! Unless we think that running instrumentation (perf) on production systems will not yield the same behavior in general. >>> What do you (on ARC) do about irq_work ? >> >> Nothing ATM. What I meant was lack of support for arch_irq_work_raise(). But given that we don't have NMIs (yet), this need *not* be a must as things could actually be scheduled in the regular intr return path ? At any rate arch_irq_work_raise() is not relevant for this discussion since NMIs are not involved. > So the reason I'm asking is that some architectures that don't have NMIs > call irq_work_run() at the very end of their perf-interrupt handler (ARM > does this for instance). But on ARC, we don't call irq_work_run() in perf intr return path and that seem to imply it is broken - as in latency to service a perf induced preemption. > And the thing is, _that_ can and does do things like wakeups and will > thus require doing the PREEMPT thing. Reassures that this patch has to go to trash anyways, but I'm more worried about perf intr return for ARC in general. >> Although I'm sure it is, can you please explain how irq_work is relevant in >> the context of this patch. > > Since the perf interrupt (in general) cannot call a whole lot of things > for it needs to assume running from NMI context, it needs to defer > things to a more regular context. It does this with irq_work. And so do places such as flush_smp_call_function_queue() where the cross-core IPI could be an NMI. > So for instance, when the output buffer reaches its watermark, we'll > raise the irq_work to issue the wakeup of tasks that poll() on that. Cool, thx for the explanation. Perhaps I should put it in a Documentation/irq_work.txt pr some such ! Thx, -Vineet
