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

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Message-ID: <6599ad830611230053m7182698cu897abe5d19471aff@mail.gmail.com>
Date:	Thu, 23 Nov 2006 00:53:51 -0800
From:	"Paul Menage" <menage@...gle.com>
To:	"Pavel Emelianov" <xemul@...nvz.org>
Cc:	"Kirill Korotaev" <dev@...ru>, "Andrew Morton" <akpm@...l.org>,
	ckrm-tech@...ts.sourceforge.net,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	matthltc@...ibm.com, hch@...radead.org,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>, oleg@...sign.ru,
	devel@...nvz.org
Subject: Re: [ckrm-tech] [PATCH 4/13] BC: context handling

On 11/23/06, Pavel Emelianov <xemul@...nvz.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
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