Re: [PATCH 13/13] workqueue: record global worker ID instead of pool ID in work->data when off-queue

Tejun Heo
2013-02-07T22:02:34Z

Hey, again.

On Fri, Feb 01, 2013 at 02:41:36AM +0800, Lai Jiangshan wrote:
> Every time, when we need to find the executing worker from work,
> we need 2 steps:
> find the pool from the idr by pool id.
> find the worker from the hash table of the pool.
>
> Now we merge them as one step: find the worker directly from the idr by worker gwid.
> (lock_pool_executing_work(). If the work is onq, we still use hash table.)
>
> It makes the code more straightforward.
>
> In future, we may add percpu worker gwid <--> worker mapping cache when needed.
>
> And we are planing to add non-std worker_pool, we still don't know how to
> implement worker_pool_by_id() for non-std worker_pool, this patch solves it.
>
> This patch slows down the very-slow-path destroy_worker(), if it is blamed,
> we will move the synchronize_rcu() out.
>
> This patch adds a small overhead(rcu_read_[un]lock) in fast path
> lock_pool_executing_work(). if it is blamed, we will use rcu_sched instead.
> (local_irq_disable() implies rcu_read_lock_sched(), so we can remove
> this overhead.)

So, I've been looking at the last six patches and am not really sure
whether I want this. At least not in the current form. We end up
locking, unlocking and then locking again the same thing in hot path.
Having a proper locking API is all nice and good *if* it actually can
support use case at hand. The proposed one can't.

Also, if worker is RCU protected then pool should be too (otherwise
you can't do worker->pool->lock), so it doesn't really simplify that
aspect either.

I do like work items pointing back to workers instead of pools, so I
think I'll try that differently.

Thanks.

--
tejun