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

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Message-ID: <20150831125139.GC31015@redhat.com>
Date:	Mon, 31 Aug 2015 14:51:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
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

On 08/31, Ingo Molnar wrote:
>
> * 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.)

I agree. I just tried to explain that we need a bit more changes than
just s/callback_head/list_head/ in task_struct.

And. The fact that this O(N) overhead was measured means that we have
more overhead with offload-fput-to-exit_task_work which would be nice
to remove as well.

Oleg.

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