On Mon, 2021-05-03 at 11:33 -0500, Scott Wood wrote: > On Sun, 2021-05-02 at 05:25 +0200, Mike Galbraith wrote: > > On Sat, 2021-05-01 at 17:03 -0500, Scott Wood wrote: > > > On Thu, 2021-04-29 at 09:12 +0200, Vincent Guittot wrote: > > > > Hi Scott, > > > > > > > > On Thu, 29 Apr 2021 at 01:28, Scott Wood <swood@xxxxxxxxxx> wrote: > > > > > 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@xxxxxxxxxx/ > > > > > > > > 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
