linux-kernel - Re: [PATCH v2 3/5] mm: use a dedicated workqueue for the free workers

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Date:   Thu, 23 Mar 2017 14:53:09 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Aaron Lu <aaron.lu@...el.com>
CC:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>
Subject: Re: [PATCH v2 3/5] mm: use a dedicated workqueue for the free workers

On Wed, Mar 22, 2017 at 09:43:04PM +0800, Aaron Lu wrote:
> On Wed, Mar 22, 2017 at 05:55:12PM +0900, Minchan Kim wrote:
> > On Wed, Mar 22, 2017 at 04:41:04PM +0800, Aaron Lu wrote:
> > > My understanding of the unbound workqueue is that it will create a
> > > thread pool for each node, versus each CPU as in the bound workqueue
> > > case, and use threads from the thread pool(create threads if not enough)
> > > to do the work.
> > 
> > Yes, that was my understand so I read code and found that
> > 
> > insert_work:
> >         ..
> >         if (__need_more_worker(pool))
> >                 wake_up_worker(pool); 
> > 
> > so I thought if there is a running thread in that node, workqueue
> > will not wake any other threads so parallelism should be max 2.
> > AFAIK, if the work goes sleep, scheduler will spawn new worker
> > thread so the active worker could be a lot but I cannot see any
> > significant sleepable point in that work(ie, batch_free_work).
> 
> Looks like worker_thread() will spawn new worker through manage_worker().
> 
> Note that pool->nr_running will always be zero for an unbound workqueue
> and thus need_more_worker() will return true as long as there are queued
> work items in the pool.

Aha, it solves my wonder. Thanks a lot!