Re: dynamic-hz

[Andrea Arcangeli <andrea@suse.de>]

On Tue, Dec 14, 2004 at 08:54:29AM -0800, Nish Aravamudan wrote:
> Hmm, schedule_timeout(0) working that way is interesting. There is
> also the option to use schedule_timeout(MAX_SCHEDULE_TIMEOUT) which
> should sleep indefinitely (depending of course on the conditions of
> the state). Oh but I think I understand what you're saying... the
> driver needs to sleep indefinitely in total (potentially), but needs
> to be able to return quite often (like yield() used to) so they could
> check a condition...
> 
> Thanks for the input!

what do you mean like yield() used to? yield() is still there in latest
2.6, just call yield() and you'll get the same effect of sched_yield in
userspace. yields in the kernel are a bad thing though (they usually
mean code is not well written, code should be event driven not polled
driven).

Note that __set_current_state(..); schedule_timeout(0) is not like
yield. yield will return immediatly if it's the only task running. A
yielding loop will consume all available cpu, while the
schedule_timeout(0) will wait less than 1/HZ sec. But really
schedule_timeout(0) makes little sense, either use schedule_timeout(1)
and explicitly wait 1msec, or use yield. schedule_timeout(0) just
happens to work because the timer code has to approximate for excess and
it will wait for the next timer irq for timeouts <= 0 and it will wait
for two ticks for timeouts == 1 etc...

I guess we could change schedule_timeout() to WARN_ON if 0 is being
passed to it.