On Thursday, July 11, 2013 10:43:45 AM Michael Wang wrote:
> Hi, Sergey
>
> On 07/11/2013 07:13 AM, Sergey Senozhatsky wrote:
> [snip]
> >
> >
> > Please kindly review the following patch.
> >
> >
> >
> > Remove cpu device only upon succesful cpu down on CPU_POST_DEAD event,
> > so we can kill off CPU_DOWN_FAILED case and eliminate potential extra
> > remove/add path:
> >
> > hotplug lock
> > CPU_DOWN_PREPARE: __cpufreq_remove_dev
> > CPU_DOWN_FAILED: cpufreq_add_dev
> > hotplug unlock
> >
> > Since cpu still present on CPU_DEAD event, cpu stats table should be
> > kept longer and removed later on CPU_POST_DEAD as well.
> >
> > Because CPU_POST_DEAD action performed with hotplug lock released, CPU_DOWN
> > might block existing gov_queue_work() user (blocked on get_online_cpus())
> > and unblock it with one of policy->cpus offlined, thus cpu_is_offline()
> > check is performed in __gov_queue_work().
> >
> > Besides, existing gov_queue_work() hotplug guard extended to protect all
> > __gov_queue_work() calls: for both all_cpus and !all_cpus cases.
> >
> > CPUFREQ_GOV_START performs direct __gov_queue_work() call because hotplug
> > lock already held there, opposing to previous gov_queue_work() and nested
> > get/put_online_cpus().
>
> Nice to know you have some idea on solving the issue ;-)
>
> I'm not sure whether I catch the idea, but seems like you are trying
> to re-organize the timing of add/remove device.
>
> I'm sure that we have more than one way to solve the issues, but what
> we need is the cure of root...
>
> As Srivatsa discovered, the root issue may be:
> gov_cancel_work() failed to stop all the work after it's return.
>
> And Viresh also confirmed that this is not by-designed.
>
> Which means gov_queue_work() invoked by od_dbs_timer() is supposed to
> never happen after CPUFREQ_GOV_STOP notify, the whole policy should
> stop working at that time.
>
> But it failed to, and the work concurrent with cpu dying caused the
> first problem.
>
> Thus I think we should focus on this and suggested below fix, I'd like
> to know your opinions :)
>
> Regards,
> Michael Wang
>
> diff --git a/drivers/cpufreq/cpufreq_governor.c
> b/drivers/cpufreq/cpufreq_governor.c
> index dc9b72e..a64b544 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -178,13 +178,14 @@ void gov_queue_work(struct dbs_data *dbs_data, struct
> cpufreq_policy *policy,
> {
> int i;
>
> + if (dbs_data->queue_stop)
> + return;
> +
> if (!all_cpus) {
> __gov_queue_work(smp_processor_id(), dbs_data, delay);
> } else {
> - get_online_cpus();
> for_each_cpu(i, policy->cpus)
> __gov_queue_work(i, dbs_data, delay);
> - put_online_cpus();
> }
> }
> EXPORT_SYMBOL_GPL(gov_queue_work);
> @@ -193,12 +194,27 @@ static inline void gov_cancel_work(struct dbs_data
> *dbs_data,
> struct cpufreq_policy *policy)
> {
> struct cpu_dbs_common_info *cdbs;
> - int i;
> + int i, round = 2;
>
> + dbs_data->queue_stop = 1;
> +redo:
> + round--;
> for_each_cpu(i, policy->cpus) {
> cdbs = dbs_data->cdata->get_cpu_cdbs(i);
> cancel_delayed_work_sync(&cdbs->work);
> }
> +
> + /*
> + * Since there is no lock to prvent re-queue the
> + * cancelled work, some early cancelled work might
> + * have been queued again by later cancelled work.
> + *
> + * Flush the work again with dbs_data->queue_stop
> + * enabled, this time there will be no survivors.
> + */
> + if (round)
> + goto redo;
Well, what about doing:
for (round = 2; round; round--)
for_each_cpu(i, policy->cpus) {
cdbs = dbs_data->cdata->get_cpu_cdbs(i);
cancel_delayed_work_sync(&cdbs->work);
}
instead?
> + dbs_data->queue_stop = 0;
> }
>
> /* Will return if we need to evaluate cpu load again or not */
> diff --git a/drivers/cpufreq/cpufreq_governor.h
> b/drivers/cpufreq/cpufreq_governor.h
> index e16a961..9116135 100644
> --- a/drivers/cpufreq/cpufreq_governor.h
> +++ b/drivers/cpufreq/cpufreq_governor.h
> @@ -213,6 +213,7 @@ struct dbs_data {
> unsigned int min_sampling_rate;
> int usage_count;
> void *tuners;
> + int queue_stop;
>
> /* dbs_mutex protects dbs_enable in governor start/stop */
> struct mutex mutex;
>
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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