linux-kernel - Re: [PATCH rfc] workqueue: honour cond

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Message-ID: <20201109140141.GE7496@mtj.duckdns.org>
Date:   Mon, 9 Nov 2020 09:01:41 -0500
From:   "tj@...nel.org" <tj@...nel.org>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "neilb@...e.de" <neilb@...e.de>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "jiangshanlai@...il.com" <jiangshanlai@...il.com>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vincent.guittot@...aro.org" <vincent.guittot@...aro.org>
Subject: Re: [PATCH rfc] workqueue: honour cond_resched() more effectively.

Hello,

On Mon, Nov 09, 2020 at 01:50:40PM +0000, Trond Myklebust wrote:
> > I'm thinking the real problem is that you're abusing workqueues. Just
> > don't stuff so much work into it that this becomes a problem. Or
> > rather,
> > if you do, don't lie to it about it.
> 
> If we can't use workqueues to call iput_final() on an inode, then what
> is the point of having them at all?
> 
> Neil's use case is simply a file that has managed to accumulate a
> seriously large page cache, and is therefore taking a long time to
> complete the call to truncate_inode_pages_final(). Are you saying we
> have to allocate a dedicated thread for every case where this happens?

I think the right thing to do here is setting CPU_INTENSIVE or using an
unbound workqueue. Concurrency controlled per-cpu workqueue is unlikely to
be a good fit if the work can run long enough to need cond_resched(). Better
to let the scheduler handle it. Making workqueue warn against long-running
concurrency managed per-cpu work items would be great. I'll put that on my
todo list but if anyone is interested please be my guest.

Thanks.

-- 
tejun