Re: [PATCH 2/3] workqueue: use dedicated creater kthread for all pools

[Lai Jiangshan]

On 07/29/2014 02:55 AM, Tejun Heo wrote:
> Hello, Lai.
> 
> On Sat, Jul 26, 2014 at 11:04:50AM +0800, Lai Jiangshan wrote:
>> There are some problems with the managers:
>>   1) The last idle worker prefer managing to processing.
>>      It is better that the processing of work items should be the first
>>      priority to make the whole system make progress earlier.
>>   2) managers among different pools can be parallel, but actually
>>      their major work are serialized on the kernel thread "kthreadd".
>>      These managers are sleeping and wasted when the system is lack
>>      of memory.
> 
> It's a bit difficult to get excited about this patchset given that
> this is mostly churn without many actual benefits.  Sure, it
> consolidates one-per-pool managers into one kthread_worker but it was
> one-per-pool already.  That said, I don't hate it and it may be
> considered an improvement.  I don't know.

It prefers to processing works rather than creating worker without any
loss of the guarantee.

processing works makes directly progress for the system.
creating worker makes delay and indirectly progress.

> 
>> This patch introduces a dedicated creater kthread which offloads the
>> managing from the workers, thus every worker makes effort to process work
>> rather than create worker, and there is no manager wasting on sleeping
>> when the system is lack of memory.  This dedicated creater kthread causes
>> a little more work serialized than before, but it is acceptable.
> 
> I do dislike the idea of creating this sort of hard serialization
> among separate worker pools.  It's probably acceptable but why are we
> doing this?  


I was about to use per-cpu kthread_worker, but I changed to use single
global kthread_worker after I had noticed the kthread_create() is already
serialized in kthreadd.

> To save one thread per pool and shed 30 lines of code
> while adding 40 lines to kthread_worker?

Countings are different between us!
Simplicity was also my aim in this patchset and total LOC was reduced.

> 
>>   1) the creater_work
>> Every pool has a struct kthread_work creater_work to create worker, and
>> the dedicated creater kthread processes all these creater_works of
>> all pools. struct kthread_work has itself execution exclusion, so we don't
>> need the manager_arb to handle the creating exclusion any more.
> 
> This explanation can be a bit misleading, I think.  We just don't have
> multiple workers trying to become managers anymore.
> 
>> put_unbound_pool() uses the flush_kthread_work() to synchronize with
>> the creating rather than uses the manager_arb.
>>
>>   2) the cooldown timer
>> The cooldown timer is introduced to implement the cool-down mechanism
>> rather than to causes the creater to sleep.  When the create_worker()
>> fails, the cooldown timer is requested and it will restart the creater_work.
> 
> Why?  Why doesn't the creator sleep?
> 
> ...
>>   5) the routine of the creater_work
>> The creater_work creates a worker in these two conditions:
>>   A) pool->nr_idle == 0
>>      A new worker is needed to be created obviously even there is no
>>      work item pending.  The busy workers may sleep and the pool can't
>>      serves the future new work items if no new worker is standby or
>>      being created.
> 
> This is kinda silly when the duty of worker creation is served by an
> external entity.  Why would a pool need any idle worker?

The idle worker must be ready or being prepared for wq_worker_sleeping()
or chained-wake-up.

percpu-kthreadd can serve for wq_worker_sleeping() in this case, but it is
not a good idle to introduce percpu-kthreadd now since no other user.

> insert_work() may as well just queue worker creation on demand.

Yes, it can save some workers for idle-unbound-pool.

> 
>>   8) init_workqueues()
>> The struct kthread_worker kworker_creater is initialized earlier than
>> worker_pools in init_workqueues() so that kworker_creater_thread is
>> created than all early kworkers.  Although the early kworkers are not
>> depends on kworker_creater_thread, but this initialization order makes
>> the pid of kworker_creater_thread smaller than kworkers which
>> seems more smooth.
> 
> Just don't create idle workers?

It does a good idea.

Thanks for review and comments.
Lai
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