linux-kernel - Re: [RFC] Per-thread getrusage

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Date:	Mon, 28 Jan 2008 12:38:17 +0300
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Vinay Sridhar <vinay@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, libc-alpha@...rceware.org,
	drepper@...hat.com, wli@...omorphy.com, sripathik@...ibm.com,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [RFC] Per-thread getrusage

Andrew Morton wrote:
> On Mon, 28 Jan 2008 10:48:23 +0300 Pavel Emelyanov <xemul@...nvz.org> wrote:
> 
>> Andrew Morton wrote:
>>> 	On Thu, 17 Jan 2008 13:57:05 +0530 Vinay Sridhar <vinay@...ux.vnet.ibm.com> wrote:
>>>
>>>> Hi All,
>>>>
>>>> Last year, there was discussion about per-thread getrusage by adding
>>>> RUSAGE_THREAD flag to getrusage(). Please refer to the thread
>>>> http://lkml.org/lkml/2007/4/4/308. Ulrich had suggested that we should
>>>> design a better user-space API. Specifically, we need a
>>>> pthread_getrusage interface in the thread library, which accepts
>>>> pthread_t, converts pthread_t into the corresponding tid and passes it
>>>> down to the syscall.
>>>>
>>>> There are two ways to implement this in the kernel:
>>>> 1) Introduce an additional parameter 'tid' to sys_getrusage() and put
>>>> code in glibc to handle getrusage() and pthread_getrusage() calls
>>>> correctly.
>>>> 2) Introduce a new system call to handle pthread_getrusage() and leave
>>>> sys_getrusage() untouched.
>>>>
>>>> We implemented the second idea above, simply because it avoids touching
>>>> any existing code. We have implemented a new syscall, thread_getrusage()
>>>> and we have exposed pthread_getrusage() API to applications.
>>>>
>>>> Could you please share your thoughts on this? Does the approach look
>>>> alright? The code is hardly complete. It is just a prototype that works
>>>> on IA32 at the moment.
>>>>
>>>> ...
>>>>
>>>> +asmlinkage long sys_thread_getrusage(int tid, struct rusage __user *ru);
>>> What happens if `tid' refers to a thread in a different pid namespace?
>>>
>> That's impossible. I explicitly deny namespace creation in case the
>> CLONE_THREAD is specified. So all threads of a single process always
>> live in one pid namespace.
>>
> 
> If the code was using find_task_by_vpid() then OK (I guess).  But it is

Yup, find_task_by_vpid() will find the proper (i.e. in your namespace) task.

> looking the tids up in the init_pid_ns.  Which I assume means that if it's
> in a new namespace and is looking up a sibling thread it will simply fail?

If it looks in the init_pid_ns, then it can either fail or obtain a task 
from different namespace. The find_task_by_pid_ns() was intended to be used
in proc mainly, to get tasks from the namespace pointed by the super-block
being explored.

Please excuse my lamentable ignorance, but which code does such things with
init_pid_ns? I followed the 'per-thread rusage' thread and didn't find any.

> Or am I missing something?
> 

Thanks,
Pavel
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