linux-kernel - Re: [PATCH] task_work: remove fifo ordering guarantee

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Date:	Mon, 31 Aug 2015 08:02:08 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH] task_work: remove fifo ordering guarantee


* Oleg Nesterov <oleg@...hat.com> wrote:

> On 08/29, Ingo Molnar wrote:
> >
> > So I'm wondering, is there any strong reason why we couldn't use a double linked
> > list and still do FIFO and remove that silly linear list walking hack?
> 
> This will obviously enlarge callback_head, and it is often embedded.
> But this is minor.
> 
> If we use a double linked list we can't do task_work_add() lockless.
> So we will need another spinlock_t in task_struct. We can't use pi_lock.

The fact that the O(N) overhead was measured in real apps to be in the 
milliseconds IMHO weakens cycle-level concerns about also having a spinlock next 
to the list head. (There's no additional cacheline bouncing concerns with the 
spinlock: the head of a LIFO list is essentially a bouncing cacheline.)

If there's some other solution, sure, but LIFO queues tend to be trouble down the 
line.

Thanks,

	Ingo
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