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

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Message-ID: <6599ad830611230218w7a6c0c0el9479b497037b0be6@mail.gmail.com>
Date:	Thu, 23 Nov 2006 02:18:46 -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:
> Paul Menage wrote:
> > On 11/23/06, Pavel Emelianov <xemul@...nvz.org> wrote:
> >> 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!
> >
> > You could get round that by having a separate "irq_bc" that's never
> > valid for a task not in an interrupt.
>
> No no no. This is not what is needed. You see, we do have to
> set exec_bc as temporary (and atomic) context. Having temporary
> context is 1. flexible 2. needed by beancounters' network accountig.

I don't see why having an irq_bc wouldn't solve this. At the start of
the interrupt handler, set current->exec_bc to &irq_bc; at the end set
it to current->real_bc; use the cmpxchg() that I suggested to ensure
that you never update task->exec_bc from another task if it's not
equal to task->real_bc; use RCU to ensure that a beancounter is never
freed while someone might be accessing it.

>
> Maybe we can make smth similar to wait_task_inactive and change
> it's beancounter before unlocking the runqueue?

That could work too.

Paul
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