RE: [PATCH] scheduler rate controller

[George Dunlap <george.dunlap@citrix.com>]

On Fri, 2011-10-28 at 11:09 +0100, Dario Faggioli wrote:
> Not sure yet, I can imagine it's tricky and I need to dig a bit more in
> the code, but I'll let know if I found a way of doing that...

There are lots of reasons why the SCHEDULE_SOFTIRQ gets raised.  But I
think we want to focus on the scheduler itself raising it as a result of
the .wake() callback.  Whether the .wake() happens as a result of a HW
interrupt or something else, I don't think really matters.

Dario and Hui,  neither of you have commented on my idea, which is
simply don't preempt a VM if it has run for less than some amount of
time (say, 500us or 1ms).  If a higher-priority VM is woken up, see how
long the current VM has run.  If it's less than 1ms, set a 1ms timer and
call schedule() then.

> > > More generally speaking, I see how this feature can be useful, and I
> > > also think it could live in the generic schedule.c code, but (as George
> > > was saying) the algorithm by which rate-limiting is happening needs to
> > > be well known, documented and exposed to the user (more than by means
> > > of a couple of perf-counters).
> > > 
> > 
> > One question is that, what is the right palace to document such information? I'd like to make it as clear as possible to the users.
> > 
> Well, don't know, maybe a WARN (a WARN_ONCE alike thing would probably
> be better), or in general something that leave a footstep in the logs,
> so that one can find out by means of `xl dmesg' or related. Obviously,
> I'm not suggesting of printk-ing each suppressed schedule invocation, or
> the overhead would get even worse... :-P
> 
> I'm thinking of something that happens the very first time the limiting
> fires, or maybe oncee some period/number of suppressions, just to remind
> the user that he's getting weird behaviour because _he_enabled_
> rate-limiting. Hopefully, that might also be useful for the user itself
> to fine tune the limiting parameters, although I think the perf-counters
> are already quite well suited for this.

As much as possible, we want the system to Just Work.  Under normal
circumstances it wouldn't be too unusual for a VM to have a several-ms
delay between receiving a physical interrupt and being scheduled; I
think that if the 1ms delay works, having it on all the time would
probably be the best solution.  That's another reason I'm in favor of
trying it -- it's simple and easy to understand, and doesn't require
detecting when to "turn it on".

 -George