linux-kernel - Re: [PATCH 0/3] posix timers: Extend kernel API to report more info about timers

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Message-ID: <CA+RrjuURQTCP8yMa4nWxNQg=qxxsu6vrko2UVK4f8pFYbK6SYQ@mail.gmail.com>
Date:	Wed, 20 Feb 2013 17:21:29 -0800
From:	Matthew Helsley <matt.helsley@...il.com>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] posix timers: Extend kernel API to report more info
 about timers

On Thu, Feb 14, 2013 at 8:18 AM, Pavel Emelyanov <xemul@...allels.com> wrote:
> Hi.
>
> I'm working on the checkpoint-restore project (http://criu.org), briefly
> it's aim is to collect information about process' state and saving it so
> that later it is possible to recreate the processes in the very same state
> as they were, using the collected information.
>
> One part of the task's state is the posix timers that this task has created.
> Currently kernel doesn't provide any API for getting information about
> what timers are currently created by process and in which state they are.
> I'd like to extend the posix timers API to provide more information about
> timers.
>
> Another problem with timers is the timer ID. Currently IDs are generated
> from global IDR and this makes it impossible to restore a timer from
> the saved state in general, as the required ID may be already busy at the
> time of restore.
>
> That said, I propose to
>
> 1. Change the way timer IDs are generated. This was done some time ago, so
>    I'm just re-sending this patch;

Seems fine in principle. Aside: I noticed there were some
important-looking patches to the idr usage in timer id allocation
today...

> 2. Add a system call that will list timer IDs created by the calling process;

If timers were listed in /proc like fds then you wouldn't need this
syscall. If we keep adding new syscalls like this CRIU will be
needlessly x86-specific when it could have been written more portably.

> 3. Add a system call that will allow to get the sigevent information about
>    particular timer in the sigaction-like manner.

You mentioned "extending the POSIX timer API". Isn't that something
best left to standards bodies lest your changes conflict with theirs?
Again, if this were a /proc interface you wouldn't have that issue
(you'll have others ;)).

>
> This is actually an RFC to start discussion about how the described problems
> can be addressed. Thus, if the approach with new system calls is not acceptable,
> I'm OK to implement this in any other form.

My preference is for "other form" for the reasons above.

Cheers,
    -Matt Helsley
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