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

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Message-ID: <m1wt0elrzk.fsf@ebiederm.dsl.xmission.com>
Date:	Sat, 14 Apr 2007 12:40:15 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Willy Tarreau <w@....eu>
Cc:	Ingo Molnar <mingo@...e.hu>, 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]

Willy Tarreau <w@....eu> writes:

> On Sat, Apr 14, 2007 at 07:54:33PM +0200, Ingo Molnar wrote:
>> 
>> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> 
>> > > Thinking about it, I don't know if there are calls to schedule() 
>> > > while switching from tty1 to tty2. Alt-F2 had no effect anymore, and 
>> > > "chvt 2" simply blocked. It would have been possible that a 
>> > > schedule() call somewhere got starved due to the load, I don't know.
>> > 
>> > It looks like there is a call to schedule_work.
>> 
>> so this goes over keventd, right?
>> 
>> > There are two pieces of the path. If you are switching in and out of a 
>> > tty controlled by something like X.  User space has to grant 
>> > permission before the operation happens.  Where there isn't a gate 
>> > keeper I know it is cheaper but I don't know by how much, I suspect 
>> > there is still a schedule happening in there.
>> 
>> Could keventd perhaps be starved? Willy, to exclude this possibility, 
>> could you perhaps chrt keventd to RT priority? If events/0 is PID 5 then 
>> the command to set it to SCHED_FIFO:50 would be:
>> 
>>   chrt -f -p 50 5
>> 
>> but ... events/0 is reniced to -5 by default, so it should definitely 
>> not be starved.
>
> 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.

I think it is a good clue.  However the intention of the mechanism is
that only processes that change the video mode on a VT are supposed to
use it.  So I really don't think gpm is the culprit.  However it easily could
be something else that has similar characteristics.

I just realized we do have proof that schedule_work is actually working
because SAK works, and we can't sanely do SAK from interrupt context
so we call schedule work.

Eric
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