Re: [LKP] [perf] WARNING: CPU: 0 PID: 1457 at kernel/events/core.c:890 add_event_to_ctx()

On Sun, Jan 25, 2015 at 04:34:28AM +0000, Fengguang Wu wrote: Hi Fengguang, Thanks very much for the report. [...] What seems to be happening is: * An event is created in the context of task A on CPU0. As the task's hw context is empty of events, we call perf_pmu_ctx_activate. This adds the cpuctx of the relevant HW PMU to the active_ctx_list. Note that we checked the task's ctx for emptiness, then added the PMU's hw context. * An event is created (as a result of a clone()) in the context of task B on CPU0, and we do the same thing, finding the ctx empty of events we add the HW PMU's cpuctx to the active_ctx_list. As it's already in there, the WARN_ON(!list_empty(&cpuctx->active_ctx_list)) explodes. So I guess what we actually want to do is to turn the active_ctx list into a list of perf_event_contexts rather than perf_event_cpu_contexts, and add/remove from the list when a context is scheduled in/out (or updated empty<->nonempty). That way we remove chances for erroneous add/remove, and we don't need to treat task and CPU contexts separately in perf_event_task_tick. I managed to get the reproducer running on a box at home, so I'll spin a potential fix against that for a while, and send that out if I don't see explosions. Peter, I guess you'll drop this patch for now? Thanks, Mark.