Date: Sat, 29 Aug 2015 14:49:21 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Maciej =?utf-8?Q?=C5=BBenczykowski?= <maze@google.com>
Subject: Re: [PATCH] task_work: remove fifo ordering guarantee
Message-ID: <20150829124921.GA14973@redhat.com>
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On 08/28, Eric Dumazet wrote:
>
> From: Eric Dumazet <edumazet@google.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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