Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

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Jay Lan wrote:

Balbir Singh wrote:
Andrew Morton wrote:
On Fri, 09 Jun 2006 16:21:46 +0530
Balbir Singh <[email protected]> wrote:


Andrew Morton wrote:

On Fri, 09 Jun 2006 03:41:04 -0400
Shailabh Nagar <[email protected]> wrote:



Hence, this patch introduces a configuration parameter
   /sys/kernel/taskstats_tgid_exit
through which a privileged user can turn on/off sending of
per-tgid stats on
task exit.
That seems a bit clumsy.  What happens if one consumer wants the
per-tgid
stats and another does not?
For all subsystems that re-use the taskstats structure from the exit
path,
we have the issue that you mentioned. Thats because several
statistics co-exist
in the same structure. These subsystems can keep their tgid-stats
empty by not
filling up anything in fill_tgid() or using this patch to
selectively enable/disable
tgid stats.

For other subsystems, they could pass tgidstats as NULL to
taskstats_exit_send().

I don't understand.  If a subsystem exists then it fills in its slots in
the taskstats structure, doesn't it?

No other subsystem needs a global knob, does it?

You see the problem - if one userspace package wants the tgid-stats and
another concurrently-running one does now, what do we do?  Just leave it
enabled and run a bit slower?
Another option is to get the package to define their own taskstats
genetlink attribute and fill it up in taskstats_exit_send(). This
would be similar to
TASKSTATS_TYPE_AGGR_PID/TGID.

They can make this attribute independent of the taskstats structure
and fill
it based on their policy (per-pid or per-tgid). But the current interface
users like CSA want to build on top of the taskstats structure.

That was my question to you from the beginning: do you propose a common
interface based on taskstats or genetlink?
Actually its both, at this time. Preference is for packages to use taskstats unless they have absolutely nothing in common with the stats already in struct taskstats...at which point they could choose to use a different structure and ship it (alongwith the taskstats structure) using a different netlink attribute. Since the processing of data happens via attributes, existing users (like the daemons of CSA, delay accounting
etc.) can simply ignore that extra attribute coming along.

If CSA defines its own taskstats genetlink attirbute, does it listen to
the same socket as delayacct? If yes, then the socket will be jammed with
duplicate information before long.
Very true. The use of a common taskstats ensures that no duplication needs to occur and as long
as performance is not an issue, extending taskstats is the preferable way.

What Balbir was pointing out is that the current taskstats interface is flexible enough, on account of its use of netlink attributes, to allow other users of the interface to define their own attributes. But this is not
something that should be pursued at this stage.

Is it an option to make per-tgid data a unicast? Ie, your daemon
periodically
polling the per-tgid stats?
That option already exists (daemon can do a GET of per-tgid stats).
However, the reason it won't work is because the stats accumalated between the last poll and the exit of the task will get lost. That was the motivation behind the "push" of stats from the kernel in the
first place.

As I noted in the other mail, since the per-tgid stats are actually sent out very few times in practice (because of the optimization to not send it when the exiting thread is the only one in its thread group),
the extra data/overhead is unlikely to be an issue.

--Shailabh

Thanks,
- jay


If so, how much slower?  Your changelog says some potential users don't
need the tgid-stats, but so what?  I assume this patch is a performance
thing?  If so, has it been quantified?



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