Messages in this thread | | | Date | Mon, 20 May 2013 15:01:47 +0530 | From | "Srivatsa S. Bhat" <> | Subject | Re: NOHZ: WARNING: at arch/x86/kernel/smp.c:123 native_smp_send_reschedule, round 2 |
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On 05/20/2013 01:40 PM, Frederic Weisbecker wrote: > 2013/5/20 Borislav Petkov <bp@alien8.de>: >> On Mon, May 20, 2013 at 11:16:33AM +0800, Michael Wang wrote: >>> I suppose the reason is that the cpu we passed to >>> mod_delayed_work_on() has a chance to become offline before we >>> disabled irq, what about check it before send resched ipi? like: >> >> I think this is only addressing the symptoms - what we should be doing >> instead is asking ourselves why are we even scheduling work on a cpu if >> the machine goes offline? >> >> I don't know though who should be responsible for killing all that >> work - the workqueue itself or the guy who created it, i.e. cpufreq >> governor... >> >> Hmmm. > > Let's look at this portion of cpu_down(): > > err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu)); > if (err) { > /* CPU didn't die: tell everyone. Can't complain. */ > smpboot_unpark_threads(cpu); > cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu); > goto out_release; > } > BUG_ON(cpu_online(cpu)); > > /* > * The migration_call() CPU_DYING callback will have removed all > * runnable tasks from the cpu, there's only the idle task left now > * that the migration thread is done doing the stop_machine thing. > * > * Wait for the stop thread to go away. > */ > while (!idle_cpu(cpu)) > cpu_relax(); > /* This actually kills the CPU. */ > __cpu_die(cpu); > > /* CPU is completely dead: tell everyone. Too late to complain. */ > cpu_notify_nofail(CPU_DEAD | mod, hcpu); > > check_for_tasks(cpu); > > The CPU is considered offline after the take_cpu_down stop machine job > completes. But the struct timer_list timers are migrated later through > CPU_DEAD notification. Only once that's completed we check for illegal > residual tasks in the CPU. So there is a little window between the > stop machine thing and __cpu_die() where a timer can fire with > cpu_online(cpu) == 1. >
Nope, the dying CPU is removed from the cpu_online_mask in the very first stages of stop_machine(), specifically in the __cpu_disable() function. __cpu_die() is just a dummy.
> Now concerning the workqueue I don't know. I guess the per cpu ones > are not migrated due to their affinity. Apparently they can still wake > up and execute works due to the timers...
The interesting thing is that the cpufreq governor actually _cancels_ the queued work in CPU_DOWN_PREPARE stage, as far as I understand.
cpufreq_cpu_callback() -> __cpufreq_remove_dev() -> __cpufreq_governor(data, CPUFREQ_GOV_STOP); -> od_cpufreq_governor_dbs() -> cpufreq_governor_dbs(), which has the following case statement:
case CPUFREQ_GOV_STOP: if (dbs_data->cdata->governor == GOV_CONSERVATIVE) cs_dbs_info->enable = 0;
gov_cancel_work(dbs_data, policy);
mutex_lock(&dbs_data->mutex); mutex_destroy(&cpu_cdbs->timer_mutex);
mutex_unlock(&dbs_data->mutex);
break;
But recently I removed the call to __cpufreq_remove_dev() in the suspend/resume path (tasks frozen), in commit a66b2e503 (cpufreq: Preserve sysfs files across suspend/resume). So I'm curious to know if this is affecting in any way.
So Boris, do you see the warnings during regular hotplug also (via sysfs) or only during suspend/shutdown? [Actually shutdown doesn't freeze tasks, so that is already a hint that this warning can be triggered via sysfs also, but it would be good to get a confirmation.]
And Viresh, in the regular hotplug paths, the call to gov_cancel_work() is supposed to kill any pending workqueue functions pertaining to offline CPUs right? Could there be a synchronization bug somewhere due to which this might not be happening properly?
Regards, Srivatsa S. Bhat
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