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

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Message-ID: <20150829124921.GA14973@redhat.com>
Date:	Sat, 29 Aug 2015 14:49:21 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Maciej Żenczykowski <maze@...gle.com>
Subject: Re: [PATCH] task_work: remove fifo ordering guarantee

On 08/28, Eric Dumazet wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> In commit f341861fb0b ("task_work: add a scheduling point in
> task_work_run()") I fixed a latency problem adding a cond_resched()
> call.
>
> Later, commit ac3d0da8f329 added yet another loop to reverse a list,
> bringing back the latency spike :
>
> I've seen in some cases this loop taking 275 ms, if for example a
> process with 2,000,000 files is killed.
>
> We could add yet another cond_resched() in the reverse loop,

Can't we do this?

> or we
> can simply remove the reversal, as I do not think anything
> would depend on order of task_work_add() submitted works.

Personally I'd prefer to keep the fifo ordering. It just makes
more sense imho. Even if currently nobody depends on it (although
I am not sure about out-of-tree modules, say, systemtap).

Let's look keyctl_session_to_parent(). It does task_work_cancel()
but only because we can not trust user-space. Otherwise we could
remove it and just do task_work_add(), but this needs fifo.

Fifo just looks more sane to me.

Oleg.

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