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Date:   Wed, 23 May 2018 16:17:36 -0700
From:   Tejun Heo <tj@kernel.org>
To:     Rik van Riel <riel@surriel.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-kernel@vger.kernel.org,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Jan Kara <jack@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>, kernel-team@fb.com
Subject: Re: [PATCH] bdi: Move cgroup bdi_writeback to a dedicated low
 concurrency workqueue
Message-ID: <20180523231736.GT1718769@devbig577.frc2.facebook.com>
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 <1527112995.7898.31.camel@surriel.com>
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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