On Tuesday 23 February 2016 03:28 PM, Peter Zijlstra wrote: > On Tue, Feb 23, 2016 at 10:51:42AM +0530, Vineet Gupta wrote: >> On Friday 19 February 2016 12:17 PM, Vineet Gupta wrote: >>> Hi Peter, >>> >>> I've been debugging a csd_lock_wait() deadlock on SMP+PREEMPT ARC HS38x2 and it >>> turned out to be lot more interesting than I'd hoped for. This is stock v4.4 >>> >>> Trouble starts with an IPI to self which doesn't get delivered as the inter-core >>> interrupt providing h/w is not capable of IPI to self (which I found as part of >>> debugging this). Subsequent IPIs from other cores to this core get elided as well >>> due to the IPI coalescing optimization in arch/arc/kernel/smp.c: ipi_send_msg_one() >>> >>> There are ways to use a different h/w mechanism to solve the trigger issue and I'd >>> hoped to just implement arch_irq_work_raise(). > > Yes, there are other architectures that use other means for self-IPI, > IIRC PowerPC has to program their timer in the past to generate a local > interrupt. > >>> But the trouble is the call stack >>> for this issue: IPI to self is triggered from >>> >>> sys_sched_setscheduler >>> __balance_callback >>> pull_rt_task >>> irq_work_queue_on <-- called with @cpu == self >>> >>> Looking into irq_work.c, irq_work_queue() is what is semantically needed, >>> specifically arch_irq_work_raise() will not be called, which means I need >>> arch_send_call_function_single_ipi() to be able to IPI to self cpu also. Is that >>> expected from arch code.... >> >> What I actually meant was is it OK for irq_work_queue_on() to be called locally >> (is this a sched bug/optimization(. Further if it is OK to be called, does it need >> to do behave more like irq_work_queue() i.e. call arch_irq_work_raise() or >> arch_send_call_function_single_ipi() is expected to handle sending IPI to self ! > > Right, so I'm not actually sure we started out with this requirement. > But you're not the first to run into this, see: > > lkml.kernel.org/r/CAJZ5v0gLankSuziQq25qTCyNqeOX43yD9jnJu_XXwbdyajfmKg at mail.gmail.com > > Initially I think irq_work_queue_on() was only used remotely, but I > think it makes sense to allow the current cpu, esp. since people seem to > be using it like that. So it seems Russell's questions in the thread above stands still. IMO we need to massage irq_work_queue_on() to handle the case of called for local cpu. This will automatically take care of CONFIG_SMP kernel running on UP hardware. > > Now the distinct difference between arch_irq_work_raise() and > arch_send_call_function_single_ipi() is that arch_irq_work_raise() > should be NMI-safe. > > So on x86 it has to be extra careful about the lapic state, whereas the > regular IPI code doesn't. > > I seem to have forgotten the status of NMIs on ARC, but this is > something to make a note of. >
