Re: perfmon2 context: thread_struct vs. task_struct?

[Andrew Morton <akpm@osdl.org>]

Stephane Eranian <eranian@hpl.hp.com> wrote:
>
> Hello,
> 
> The perfmon2 subsystem maintains a structure per-thread
> that contains the save-area for the performance counters
> and related software state. The save area is used on
> context-switch. There can only be one context per thread.
> The context is dynamically allocated as such it does not
> consume memory in each thread. It is dyanmically attached
> to the thread to monitor.
> 
> In the current implementation, the context (pfm_context)
> pointer is implemented in the thread_struct on the basis
> that this is somewhat related to machine state.
> 
> Historically, the perfmon subsystem only existed on IA-64
> which made the thread_struct (an arch-specific structure)
> the obvious place to put this.
> 
> Nowadays, perfmon2 supports most major architectures. It
> may make sense to move the void *pfm_context pointer from 
> the thread_struct into the task_struct. This would save
> some indirections, make it readily available to other
> archiectures when it is ported.
> 
> I admit I am not quite clear as to what goes where between
> thread_struct and task_struct.
> 
> Would it make sense  to move the pointer to the perfmon2
> context into the task_struct?

I'd say so, yes.  Especialy if the struct is the same on all architectures,
is referred to from non-arch-specific code and is absent if
CONFIG_PERFMON=n.