On Tue, Nov 14, 2017 at 03:01:26PM -0800, Vineet Gupta wrote: > On 11/14/2017 02:28 AM, Peter Zijlstra wrote: > > On Tue, Nov 07, 2017 at 02:13:04PM -0800, Vineet Gupta wrote: > > > In the more likely case of returning to kernel from perf interrupt, do a > > > fast path returning w/o bothering about CONFIG_PREEMPT etc > > > > I think this needs more explaining and certainly also deserves a code > > comment. > > Sure ! It was a quick hack mainly to solicit feedback. > > > > Is the argument something along these lines? > > > > Assumes the interrupt will never set TIF_NEED_RESCHED; > > therefore no preemption is ever required on return from > > the interrupt. > > No. I don't think we can assume that. Well, given we run that code from NMI context on a number of platforms (x86 being one of them) it can not in fact do things like wakeups. So the pure perf-interrupt part should never set TIF_NEED_RESCHED. I think we can actually make that assumption. > 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). > > What do you (on ARC) do about irq_work ? > > Nothing ATM. 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). And the thing is, _that_ can and does do things like wakeups and will thus require doing the PREEMPT thing. > 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. 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.
