Paul Menage wrote:
> On 11/23/06, Pavel Emelianov <[email protected]> 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);
You mean moving is like this:
old_bc = task->real_bc;
task->real_bc = new_bc;
cmpxchg(&tsk->exec_bc, old_bc, new_bc);
? Then this won't work:
Initialisation:
current->exec_bc = init_bc;
current->real_bc = init_bc;
...
IRQ:
current->exec_bc = init_bc;
...
old_bc = tsk->real_bc; /* init_bc */
tsk->real_bc = bc1;
cx(tsk->exec_bc, init_bc, bc1); /* ok */
...
Here at the middle of an interrupt
we have bc1 set as exec_bc on task
which IS wrong!
...
current->exec_bc =
current->real_bc;
We need some way to be sure that task isn't running at
the moment we change it's beancounter. Otherwise we're
risking that we'll spoil some temporary context.
> (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
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