Re: [PATCH 11/23] sched,spi: Convert to sched_set_fifo*()

[Doug Anderson]

Hi,

On Wed, Apr 22, 2020 at 8:48 AM Guenter Roeck <groeck@xxxxxxxxxx> wrote:
>
> On Wed, Apr 22, 2020 at 7:35 AM Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
> >
> > Hi,
> >
> > On Wed, Apr 22, 2020 at 6:56 AM Mark Brown <broonie@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Apr 22, 2020 at 01:27:30PM +0200, Peter Zijlstra wrote:
> > > > Because SCHED_FIFO is a broken scheduler model (see previous patches)
> > > > take away the priority field, the kernel can't possibly make an
> > > > informed decision.
> > > >
> > > > No effective change.
> > >
> > > Copying Doug who did this change and Guenter who reviewed it.  This
> > > looks fine to me but I've no particular involvement with the code or
> > > platforms that are affected here.
> >
> > Thanks!  Probably the maintainers of cros_ec_spi.c (Benson and Enric)
> > should be aware of it, too.  CCing them.
> >
> > From my point of view, my response is pretty much identical to the one
> > I wrote when the priority was reduced from "MAX_RT_PRIO - 1" to
> > "MAX_RT_PRIO / 2" [1].  Basically, any priority that keeps us from
> > being preempted by tasks that are only high priority for performance
> > reasons (like dm crypt and loopback did when I last analyzed) is fine.
> > Our priority needs to be high not for performance reasons but for
> > correctness reasons (the other side will drop our data if we don't
> > respond in time).
> >
> The crypto engine ends up running at the same priority level, so I am
> a bit concerned that this patch series will re-introduce the problem
> that Doug's initial patch tried to solve.

Do you have a pointer to the code you're looking at?  Digging through
my old investigation for dm-crypt showed the problem to be the code
touched by commit a1b89132dc4f ("dm crypt: use
WQ_HIGHPRI for the IO and crypt workqueues").  Interestingly enough,
that's been reverted in commit f612b2132db5 ("Revert "dm crypt: use
WQ_HIGHPRI for the IO and crypt workqueues"").

Even if something is using WQ_HIGHPRI, last I checked WQ_HIGHPRI was
the highest non-relatime priority.  Looking quickly I see
"HIGHPRI_NICE_LEVEL" which is MIN_NICE.  I don't think that implies
realtime, but I assume sched_fifo() still does.

-Doug