linux-kernel - Re: [PATCH 1/2] workqueue: remove unneeded test before wake up next worker

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Message-ID: <20140718220514.GC1819@htj.dyndns.org>
Date:	Fri, 18 Jul 2014 18:05:14 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] workqueue: remove unneeded test before wake up next
 worker

On Wed, Jul 16, 2014 at 06:09:58PM +0800, Lai Jiangshan wrote:
> In this code:
> 	if ((worker->flags & WORKER_UNBOUND) && need_more_worker(pool))
> 		wake_up_worker(pool);
> 
> the first test is unneeded. Even the first test is removed, it doesn't affect
> the wake-up logic when WORKER_UNBOUND. And it will not introduce any useless
> wake-up when !WORKER_UNBOUND since the nr_running >= 1 except only one case.
> It will introduce useless/redundant wake-up when cpu_intensive, but this
> case is rare and next patch will also remove this redundant wake-up.
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
>  kernel/workqueue.c |    7 ++-----
>  1 files changed, 2 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index f8d54c1..6d11b9a 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2047,11 +2047,8 @@ __acquires(&pool->lock)
>  	if (unlikely(cpu_intensive))
>  		worker_set_flags(worker, WORKER_CPU_INTENSIVE, true);
>  
> -	/*
> -	 * Unbound pool isn't concurrency managed and work items should be
> -	 * executed ASAP.  Wake up another worker if necessary.
> -	 */
> -	if ((worker->flags & WORKER_UNBOUND) && need_more_worker(pool))
> +	/* Wake up another worker if necessary. */
> +	if (need_more_worker(pool))
>  		wake_up_worker(pool);

What does this buy us?  Sure, it may achieve about the same operation
but it's a lot more confusing.  need_more_worker() is specifically for
concurrency management.  Applying it to unmanaged workers could lead
to okay behavior but conflating the two to save one test on worker
flags doesn't seem like a good trade-off to me.

Thanks.

-- 
tejun
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