linux-kernel - Re: Please, put 64-bit counter per task and incr.by.one each ctxt switch.

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Date:	Tue, 26 Feb 2008 16:41:28 +0300
From:	"Alexey Dobriyan" <adobriyan@...il.com>
To:	"J.C. Pizarro" <jcpiza@...il.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Please, put 64-bit counter per task and incr.by.one each ctxt switch.

On 2/26/08, J.C. Pizarro <jcpiza@...il.com> wrote:
> On 2008/2/25, Andrew Morton <akpm@...ux-foundation.org> wrote:
> > On Sun, 24 Feb 2008 14:12:47 +0100 "J.C. Pizarro" <jcpiza@...il.com>
> wrote:
> >
> > > It's statistic, yes, but it's a very important parameter for the
> CPU-scheduler.
> > > The CPU-scheduler will know the number of context switches of each task
> > > before of to take a blind decision into infinitum!.
> >
> >
> > We already have these:
> >
> > unsigned long nvcsw, nivcsw; /* context switch counts */
> >
> > in the task_struct.
>
> 1. They use "unsigned long" instead "unsigned long long".
> 2. They use "= 0;" instead of "= 0ULL";

Very funny.

> 3. They don't use ++ (incr. by one per ctxt-switch).

No they do, read schedule() already.

> 4. I don't like the separation of voluntary and involuntary ctxt-switches,
> and i don't understand the utility of this separation.

Ah, that's why you don't like it.

> The tsk->nvcsw & tsk->nivcsw mean different to i had proposed.
>
> It's simple, when calling to function kernel/sched.c:context_switch(..)
> to do ++, but they don't do it.
>
> I propose you
> 1. unsigned long long tsk->ncsw = 0ULL; and tsk->ncsw++;
> 2. unsigned long long tsk->last_registered_ncsw = tsk->ncsw; when it's
> polling.
> 3. long tsk->vcsw = ( tsk->ncsw - tsk->last_registered_ncsw ) / ( t2 - t1 )
> /* velocity of task (ctxt-switches per second), (t1 != t2 in seconds
> for no zerodiv)
> 4. long tsk->last_registered_vcsw = tsk->vcsw;
> 5. long tsk->normalized_vcsw =
> (1 - alpha)*tsk->last_registered_vcsw + alpha*tsk->vcsw; /* 0<alpha<1
> */

   6. Profit.

As I understood the idea of CFS, all interactivity heuristics were bitbucketed,
so you'll add them back (you won't, of course, because you can't be arsed
to send a patch)

So best course of action it to describe workload and setup (distro, relevant
.config items and so on.) on which CFS behaves poorly.
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