[Devel] Re: [ckrm-tech] [PATCH 4/13] BC: context handling

[Paul Menage]

On 11/23/06, Pavel Emelianov <xemul at openvz.org> wrote:
>
> We can do the following:
>
>   if (tsk == current)
>       /* fast way */
>       tsk->exec_bc = bc;
>   else
>       /* slow way */
>       stop_machine_run(...);
>
> What do you think?

How about having two pointers per task:

- exec_bc, which is the one used for charging
- real_bc, which is the task's actual beancounter

at the start of irq, do

current->exec_bc = &init_bc;

at the end of irq, do

current->exec_bc = current->real_bc;

When moving a task to a different bc do:

task->real_bc = new_bc;
atomic_cmpxchg(&task->exec_bc, old_bc, new_bc);

(with appropriate memory barriers). So if the task is in an irq with a
modified exec_bc pointer, we do nothing, otherwise we update exec_bc
to point to the new real_bc.

Paul