To David Miller: David don't apply yet - this patch needs change.

Patrick, read below:

On Tue, Dec 09, 2008 at 02:20:00PM +0100, Patrick McHardy wrote:

Jarek Poplawski wrote:

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On Tue, Dec 09, 2008 at 01:25:15PM +0100, Patrick McHardy wrote:

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Jarek Poplawski wrote:

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The algorithm is we want to "continue on the next jiffie". We know
we've lost here a lot of time (~2 jiffies), and this will be added
later. Since these jiffies are not precise enough wrt. psched ticks
or ktime, and we will add around 2000 (for HZ 1000) psched ticks
anyway this +1 here simply doesn't matter and can mean "a bit after
q->now".

This might as well return q->now, no?

Yes, but IMHO it looks worse, considering the problem here (we want to
avoid scheduling in the past).

I guess its a matter of taste.

Exactly. (And could be changed.)

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The elapsed time is added
on top later anyways. But anyways, I think both the approach and
the patch are wrong.

	/* charge used jiffies */
	start_at = jiffies - start_at;
	if (start_at > 0)
		next_event += start_at * PSCHED_TICKS_PER_SEC / HZ;

What relationship does the duration it ran for has to the time it
should run at again?

The scheduling times won't be in the past mostly and hrtimers won't
trigger too soon, but approximately around we really need and can
afford a new try without stopping everything else.

Sure. But it also won't be in the past if we simply add .. lets say
the current uptime in ms. My point was that there's absolutely no
relationship between those two times and combining them just to
get a value thats not in the past is wrong. Especially considering
*why* we want a value in the future and what we'll get from that
calculation.

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The focus on jiffies is wrong IMO, the reason why we get high
load is because the CPU can't keep up, delaying things even
longer is not going to help get the work done. The only reason to
look at jiffies is because its a cheap indication that it has
ran for too long and we should give other tasks a change to run
as well, but it should continue immediately after it did that.
So all it should do is make sure that the watchdog is scheduled
with a very small positive delay.

This needs additional psched_get_time(), and as I've written before
there is no apparent advantage in problematic cases, but this would
add more overhead for common cases.

htb_do_events() exceeding two jiffies is fortunately not a common
case. You (incorrectly) made the calculation somewhat of a common
case by also adding to the delay if the inner classes simply throttled
and already returned the exact delay they want.

I see! You are right and this needs fixing.

Much better (again considering what we want to achieve here) would
be to not use the hrtimer watchdog at all. We want to give lower
priority tasks a chance to run, so ideally we'd use a low priority
task for wakeup.

I'm not sure I get ot right: for precise scheduling hrtimers look
useful. Do you really mean "at all"?
 

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As for the implementation: the increase in delay (the snippet
above) is also done in the case that no packets were available
for other reasons (throttling), in which case we might needlessly
delay for an extra jiffie if jiffies wrapped while it tried to
dequeue.

But in another similar cases there could be no change in jiffies, but
almost a jiffie used for counting, so wrong schedule time as well.

Its not "wrong". We don't want to delay. Its a courtesy to the
remaining system.

In this case it's probably self-courtesy too: this ksoftirqd takes
most of the time and it's useless.

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Approximatly this all should be fine, and it still can be tuned later.
IMHO, this all should not affect "common" cases, which are expected to
use less then jiffie here.

Jiffies might wrap even if it only took only a few nanoseconds.
And its not fine, in the case of throttled classes there's no
reason to add extra delay *at all*.

Yes, you are right with this. I can try too fix this tomorrow, unless
you prefer to send your version of this patch.

Thanks,
Jarek P.