Re: [dm-devel] dm-writeboost testing

[Akira Hayakawa]

Hi, Mikulas,

I am sorry to say that
I don't have such machines to reproduce the problem.

But agree with that I am dealing with workqueue subsystem
in a little bit weird way.
I should clean them up.

For example,
free_cache() routine below is
a deconstructor of the cache metadata
including all the workqueues.

void free_cache(struct wb_cache *cache)
{
        cache->on_terminate = true;

        /* Kill in-kernel daemons */
        cancel_work_sync(&cache->sync_work);
        cancel_work_sync(&cache->recorder_work);
        cancel_work_sync(&cache->modulator_work);

        cancel_work_sync(&cache->flush_work);
        destroy_workqueue(cache->flush_wq);

        cancel_work_sync(&cache->barrier_deadline_work);

        cancel_work_sync(&cache->migrate_work);
        destroy_workqueue(cache->migrate_wq);
        free_migration_buffer(cache);

        /* Destroy in-core structures */
        free_ht(cache);
        free_segment_header_array(cache);

        free_rambuf_pool(cache);
}

cancel_work_sync() before destroy_workqueue()
can probably be removed because destroy_workqueue() first
flush all the works.

Although I prepares independent workqueue
for each flush_work and migrate_work
other four works are queued into the system_wq
through schedule_work() routine.
This asymmetricity is not welcome for
architecture-portable code.
Dependencies to the subsystem should be minimized.
In detail, workqueue subsystem is really changing
about its concurrency support so
trusting only the single threaded workqueue
will be a good idea for stability.

To begin with,
these works are never out of queue
until the deconstructor is called
but they are repeating running and sleeping.
Queuing these kind of works to system_wq
may be unsupported.

So,
my strategy is to clean them up in a way that
1. all daemons are having their own workqueue
2. never use cancel_work_sync() but only calls destroy_workqueue()
   in the deconstructor free_cache() and error handling in resume_cache().

Could you please run the same test again
after I fixed these points
to see whether it is still reproducible?


> On 3.11.3 on PA-RISC without preemption, the device unloads (although it 
> takes many seconds and vmstat shows that the machine is idle during this 
> time)
This behavior is benign but probably should be improved.
In said free_cache() it first turns `on_terminate` flag to true
to notify all the daemons that we are shutting down.
Since the `update_interval` and `sync_interval` are 60 seconds by default
we must wait for them to finish for a while.

Akira
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