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

Willy Tarreau
2007-04-21T16:59:38Z

On Sat, Apr 21, 2007 at 06:53:47PM +0200, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@linux-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