linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Date:	Sat, 21 Apr 2007 18:57:02 +0200
From:	Willy Tarreau <w@....eu>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44

On Sat, Apr 21, 2007 at 06:53:47PM +0200, Ingo Molnar wrote:
> 
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> > It would be even better to simply have the rule:
> >  - child gets almost no points at startup
> >  - but when a parent does a "waitpid()" call and blocks, it will spread 
> >    out its points to the childred (the "vfork()" blocking is another case 
> >    that is really the same).
> > 
> > This is a very special kind of "priority inversion" logic: you give 
> > higher priority to the things you wait for. Not because of holding any 
> > locks, but simply because a blockign waitpid really is a damn big hint 
> > that "ok, the child now works for the parent".
> 
> yeah. One problem i can see with the implementation of this though is 
> that shells typically do nonspecific waits - for example bash does this 
> on a simple 'ls' command:
> 
>   21310 clone(child_stack=0,  ...) = 21399
>   ...
>   21399 execve("/bin/ls", 
>   ...
>   21310 waitpid(-1, <unfinished ...>
> 
> the PID is -1 so we dont actually know which task we are waiting for. We 
> could use the first entry from the p->children list, but that looks too 
> specific of a hack to me. It should catch most of the 
> synchronous-helper-task cases though.

The last one should be more appropriate IMHO. If you waitpid(), it's very
likely that you're waiting for the result of the very last fork().

Willy

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