linux-kernel - Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

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Date:	Sun, 15 Apr 2007 20:06:04 +0200
From:	Willy Tarreau <w@....eu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jiri Slaby <jirislaby@...il.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

Hi Ingo,

On Sun, Apr 15, 2007 at 07:55:55PM +0200, Ingo Molnar wrote:
> 
> * Willy Tarreau <w@....eu> wrote:
> 
> > Well, since I merged the fair-fork patch, I cannot reproduce (in fact, 
> > bash forks 1000 processes, then progressively execs scheddos, but it 
> > takes some time). So I'm rebuilding right now. But I think that Linus 
> > has an interesting clue about GPM and notification before switching 
> > the terminal. I think it was enabled in console mode. I don't know how 
> > that translates to frozen xterms, but let's attack the problems one at 
> > a time.
> 
> to debug this, could you try to apply this add-on as well:
> 
>   http://redhat.com/~mingo/cfs-scheduler/sched-fair-print.patch
> 
> with this patch applied you should have a /proc/sched_debug file that 
> prints all runnable tasks and other interesting info from the runqueue. 

I don't know if you have seen my mail from yesterday evening (here). I
found that changing keventd prio fixed the problem. You may be interested
in the description. I sent it at 21:01 (+200).

> [ i've refreshed all the patches on the CFS webpage, so if this doesnt 
>   apply cleanly to your current tree then you'll probably have to 
>   refresh one of the patches.]

Fine, I'll have a look. I already had to rediff the sched-fair-fork
patch last time.

> The output should look like this:
> 
>  Sched Debug Version: v0.01
>  now at 226761724575 nsecs
> 
>  cpu: 0
>    .nr_running            : 3
>    .raw_weighted_load     : 384
>    .nr_switches           : 13666
>    .nr_uninterruptible    : 0
>    .next_balance          : 4294947416
>    .curr->pid             : 2179
>    .rq_clock              : 241337421233
>    .fair_clock            : 7503791206
>    .wait_runtime          : 2269918379
> 
>  runnable tasks:
>             task | PID | tree-key |   -delta |  waiting | switches
>  -----------------------------------------------------------------
>  +            cat  2179 7501930066   -1861140    1861140         2
>       loop_silent  2149 7503010354    -780852          0       911
>       loop_silent  2148 7503510048    -281158     280753       918

Nice.

> now for your workload the list should be considerably larger. If there's 
> starvation going on then the 'switches' field (number of context 
> switches) of one of the tasks would never increase while you have this 
> 'cannot switch consoles' problem.
> 
> maybe you'll have to unapply the fair-fork patch to make it trigger 
> again. (fair-fork does not fix anything, so it probably just hides a 
> real bug.)
> 
> (i'm meanwhile busy running your scheddos utilities to reproduce it 
> locally as well :)

I discovered I had the frame-buffer enabled (I did not notice it first
because I do not have the logo and the resolution is the same as text).
It's matroxfb with a G400, if that can help. It may be possible that
it needs some CPU that it cannot get to clear the display before
switching, I don't know.

However I won't try this right now, I'm deep in userland at the moment.

Regards,
Willy

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