Re: [PATCH] Mark timer_stats as incompatible with multiple pid namespaces

[Ingo Molnar <mingo@elte.hu>]

* Eric W. Biederman <ebiederm@xmission.com> wrote:

> > What the heck??? Please solve this properly instead of hiding it. 
> > /proc/timer_stats is damn useful and it's a must-have for powertop 
> > to work.
> 
> Hmm.  Perhaps the dependency conflict should go in the other direction 
> then.
> 
> My goal is to document the issue while a proper fix is being written. 
> I have known about this for all of about 1 day now.  It was added 
> since last time I went through the kernel and made a thorough sweep of 
> pid users.
> 
> What the proper fix is isn't even obvious at this point. Possibly it 
> is making /proc/timer_stats disappear in child pid namespaces.
>   Which we don't currently have the infrastructure fore.
>
> Possibly it is reworking the stats collection so we store a struct pid 
> * instead of a pid_t value.  So we would know if the reader of the 
> value can even see processes you have collected stats for.

the problem is, this interface stores historic PIDs too - i.e. PIDs of 
tasks that might have exited already.

the proper way would be to also store the namespace address, and to 
filter out non-matching entries. (If leakage of this data across 
namespace creation is of any concern then flush out existing namespace 
data when a namespace is destroyed)

	Ingo