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

[Shailabh Nagar <nagar@watson.ibm.com>]

Andrew Morton wrote:

> On Fri, 09 Jun 2006 16:21:46 +0530
> Balbir Singh <balbir@in.ibm.com> wrote:
>
>  
>
> > Andrew Morton wrote:
> >    
> >
> > > On Fri, 09 Jun 2006 03:41:04 -0400
> > > Shailabh Nagar <nagar@watson.ibm.com> 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?
> > >      
Then the tgid stat sending on exit will need to be turned on for everyone.

> > 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?
>  
It can choose not to, by not inserting its "fill my fields" function 
inside the do..while_each_thread
loop within fill_tgid. So while they would still necessarily receive the 
per-tgid taskstats struct on exit
(because some other subsystem needs it), they can atleast save on 
filling up their part of the struct
if they don't need it.

> No other subsystem needs a global knob, does it?
>  
I didn't understand.

> 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?
>  
Yes, thats what will have to be done. If one user wants, all users will 
need to get the stats. They can
limit their impact by not processing the parts of the netlink message 
that correspond to the per-tgid stats
(since the per-tgid stats are sent as a separate attribute, thats easy 
to do).

This patch covers the use case where someone like CSA is the only user 
(delay accounting is turned off)
and wants to reduce the performance impact of the kernel allocating, 
sending and userspace discarding
the per-tgid stats.

> 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?  
Yes, its a performance optimization.

> If so, has it been quantified?
>  
No :-(
Will try to get some numbers.
Jay/Chris, if you can try to do that too, for the kind of usage that is 
typical of CSA,
that would be great.

--Shailabh

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