linux-kernel - Re: Can we get a general timed LRU built on the workqueue subsys?

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Message-ID: <YModuoRpG47DSaXG@slm.duckdns.org>
Date:   Wed, 16 Jun 2021 11:50:18 -0400
From:   Tejun Heo <tj@...nel.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: Can we get a general timed LRU built on the workqueue subsys?

Hello, David.

On Tue, Jun 08, 2021 at 04:14:21PM +0100, David Howells wrote:
> Would it be practical to get some sort of timed LRU built on top of the
> workqueue such that I can, say, queue an item on it without using the
> delayed_work struct?
> 
> The reason I'd like this is that I want to avoid using delayed_work because it
> would increase the size of the fscache_cookie struct by 50% (110% with
> lockdep), and then you'd have to multiply that by the number of cookies on the
> system - could be tens or hundreds of thousands; could be a million+ in some
> applications.
> 
> Really, only one timer should be necessary for the entire queue if every item
> in the queue has the same timeout length, since it would only need to keep
> track of the item at the front of the queue.  This timer could be managed with
> timer_reduce() rather than mod_timer() or del_timer()+add_timer().
> 
> Each item in the queue would need a timestamp to say when it expires, say:
> 
> 	struct work_lru {
> 		struct work_struct work;
> 		unsigned long expires_at;
> 	};
> 
> There are other places I could use such a thing too, not just for fscache
> cookies.

No objection from me but if reducing the size of delayed_work is meaningful
enough I kinda wonder whether this can be generalized so that all
delayed_works are smaller. There's no fundmental reason to have these
smaller ones separate, right?

Thanks.

-- 
tejun