Re: High latency of a system based on 5.19 rt

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On Mon, 2023-10-02 at 12:05 +0200, Sebastian Andrzej Siewior wrote:
> On 2023-09-26 15:15:46 [+0200], Mike Galbraith wrote:
> > On Tue, 2023-09-26 at 12:30 +0000, Clark Williams wrote:
> > > On Tue, Sep 26, 2023 at 7:05 AM Mike Galbraith <efault@xxxxxx> wrote:
> > > > On Mon, 2023-09-25 at 18:30 +0200, g.medini@xxxxxxxxxxx wrote:
> > > > > # tracer: wakeup_rt
> > > > > #
> > > > > # wakeup_rt latency trace v1.1.5 on 5.19.0-rt10
> > > > > # --------------------------------------------------------------------
> > > > > # latency: 357 us, #401/401, CPU#0 | (M:preempt_rt VP:0, KP:0, SP:0 HP:0 #P:2)
> > > > > #    -----------------
> > > > > #    | task: ktimers/0-15 (uid:0 nice:0 policy:1 rt_prio:1)
> > > > > #    -----------------
> > > >
> > > > The first thing that pokes me in the eye is that priority. I'd bump
> > > > that a lot.  As it sits, anything high priority ktimers may wake when
> > > > it finally gets the CPU gets to enjoy all the latency ktimers is eating
> > > > in this trace due to it having been deemed relatively unimportant.
> > >
> > > Hmmm, IRQs are running at FIFO:50 by default. Do we want the ktimer
> > > running above the IRQ service thread? 
> >
> > I think so yeah, quick like bunny wakeup resource should punch through.
>
> Why not perform all wakes from hardirq then?

Sounds good to me iff we're talking about a dinky irq width delta.

Threads bundling up what are otherwise irq context cycles is loaded
with goodness, but static priority leaves you holding a bill and paying
context switch fees on top.  Pick your poison carefully applies I
suppose.  A tiny swig of hemlock can't do _too_ much harm, right ;-)

	-Mike





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