linux-kernel - Re: [PATCH] bdi: Move cgroup bdi_writeback to a dedicated low concurrency workqueue

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Message-ID: <20180523231736.GT1718769@devbig577.frc2.facebook.com>
Date:   Wed, 23 May 2018 16:17:36 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>, kernel-team@...com
Subject: Re: [PATCH] bdi: Move cgroup bdi_writeback to a dedicated low
 concurrency workqueue

Hello, Rik.

On Wed, May 23, 2018 at 06:03:15PM -0400, Rik van Riel wrote:
> Dumb question.  Does setting max_active to 1 mean
> that every cgwb_release_workfn() ends up forcing
> another RCU grace period on the whole system, while
> today you might have a bunch of them waiting on the
> same RCU grace period advance?
> 
> Would it be faster to have some number (up to 16?)
> push RCU once, at the same time, instead of having
> each of them push RCU into a next grace period one
> after another?

Oh yeah, you're absolutely right.  This would end up doing a lot of
back-to-back synchronize_rcu_expedited() calls which can't be good.
I'll send a patch to push it upto 16.

Thanks.

-- 
tejun