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(). 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 ! > > Just wanted to understand before writing patches... > > Test case triggering is harmless looking LTP: trace_sched -c 1 > It is kind of scheduler fizzer as it triggers a whole bunch of sched activity. > > Thx, > -Vineet >
