On Mon, 2021-05-03 at 11:33 -0500, Scott Wood wrote:

On Sun, 2021-05-02 at 05:25 +0200, Mike Galbraith wrote:

quoted

On Sat, 2021-05-01 at 17:03 -0500, Scott Wood wrote:

quoted

On Thu, 2021-04-29 at 09:12 +0200, Vincent Guittot wrote:

quoted

Hi Scott,

On Thu, 29 Apr 2021 at 01:28, Scott Wood [off-list ref] wrote:

quoted

These patches mitigate latency caused by newidle_balance() on large
systems when PREEMPT_RT is enabled, by enabling interrupts when the
lock
is dropped, and exiting early at various points if an RT task is
runnable
on the current CPU.

On a system with 128 CPUs, these patches dropped latency (as
measured by
a 12 hour rteval run) from 1045us to 317us (when applied to
5.12.0-rc3-rt3).

The patch below has been queued for v5.13 and removed the update of
blocked load what seemed to be the major reason for long preempt/irq
off during newly idle balance:
https://lore.kernel.org/lkml/20210224133007.28644-1-vincent.guittot@linaro.org/ (local)

I would be curious to see how it impacts your cases

I still get 1000+ ms latencies with those patches applied.

If NEWIDLE balancing migrates one task, how does that manage to consume
a full *millisecond*, and why would that only be a problem for RT?

	-Mike

(rt tasks don't play !rt balancer here, if CPU goes idle, tough titty)

Determining which task to pull is apparently taking that long (again, this
is on a 128-cpu system).  RT is singled out because that is the config that
makes significant tradeoffs to keep latencies down (I expect this would be
far from the only possible 1ms+ latency on a non-RT kernel), and there was
concern about the overhead of a double context switch when pulling a task to
a newidle cpu.

What I think has be going on is that you're running a synchronized RT
load, many CPUs go idle as a thundering herd, and meet at focal point
busiest.  What I was alluding to was that preventing such size scale
pile-ups would be way better than poking holes in it for RT to try to
sneak through.  If pile-up it is, while not particularly likely, the
same should happen with normal tasks, wasting cycles generating heat.

The main issue I see with these patches is that the resulting number is
still so gawd awful as to mean "nope, not rt ready", making the whole
exercise look a bit like a noop.

	-Mike