Re: [patch] Performance Counters for Linux, v3

[Ingo Molnar <mingo@elte.hu>]

* stephane eranian <eranian@googlemail.com> wrote:

> Hi,
> 
> Given the level of abstractions you are using for the API, and given 
> your argument that the kernel can do the HW resource scheduling better 
> than anybody else.
> 
> What happens in the following test case:
> 
>    - 2-way system (cpu0, cpu1)
> 
>    - on cpu0, two processes P1, P2, each self-monitoring and counting event E1.
>      Event E1 can only be measured on counter C1.
> 
>    - on cpu1, there is a cpu-wide session, monitoring event E1, thus using C1
> 
>    - the scheduler decides to migrate P1 onto CPU1. You now have a
>      conflict on C1.
> 
> How is this managed?

If there's a single unit of sharable resource [such as an event counter, 
or a physical CPU], then there's just three main possibilities: either 
user 1 gets it all, or user 2 gets it all, or they share it.

We've implemented the essence of these variants, with sharing the resource 
being the sane default, and with the sysadmin also having a configuration 
vector to reserve the resource to himself permanently. (There could be 
more variations of this.)

What is your point?

	Ingo