linux-kernel - Re: [PATCH 0/24] kernel: add a netlink interface to get information about processes (v2)

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Date:	Tue, 7 Jul 2015 09:24:05 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	David Ahern <dsahern@...il.com>
Cc:	Andrew Vagin <avagin@...n.com>, Andrey Vagin <avagin@...nvz.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Roger Luethi <rl@...lgate.ch>, Arnd Bergmann <arnd@...db.de>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Pavel Odintsov <pavel.odintsov@...il.com>
Subject: Re: [PATCH 0/24] kernel: add a netlink interface to get information
 about processes (v2)

On Tue, Jul 7, 2015 at 9:17 AM, David Ahern <dsahern@...il.com> wrote:
> On 7/7/15 9:56 AM, Andy Lutomirski wrote:
>>
>> Netlink is fine for these use cases (if they were related to the
>> netns, not the pid ns or user ns), and it works.  It's still tedious
>> -- I bet that if you used a syscall, the user code would be
>> considerable shorter, though. :)
>>
>> How would this be a problem if you used plain syscalls?  The user
>> would make a request, and the syscall would tell the user that their
>> result buffer was too small if it was, in fact, too small.
>
>
> It will be impossible to tell a user what sized buffer is needed. The size
> is largely a function of the number of tasks and number of maps per thread
> group and both of those will be changing. With the growing size of systems
> (I was sparc systems with 1024 cpus) the workload can be 10's of thousands
> of tasks each with a lot of maps (e.g., java workloads). That amounts to a
> non-trivial amount of data that has to be pushed to userspace.
>
> One of the benefits of the netlink approach is breaking the data across
> multiple messages and picking up where you left off. That infrastructure is
> already in place.
>

How does picking up where you left off work?  I assumed the interface
was something along the lines of "give me information starting from
pid N", but maybe I missed something.

--Andy
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