Re: INFO: rcu detected stall in do_idle

[Juri Lelli <juri.lelli@redhat.com>]

On 18/10/18 11:48, Peter Zijlstra wrote:
> On Thu, Oct 18, 2018 at 10:28:38AM +0200, Juri Lelli wrote:
> 
> > Another side problem seems also to be that with such tiny parameters we
> > spend lot of time in the while (dl_se->runtime <= 0) loop of replenish_dl_
> > entity() (actually uselessly, as deadline is most probably going to
> > still be in the past when eventually runtime becomes positive again), as
> > delta_exec is huge w.r.t. runtime and runtime has to keep up with tiny
> > increments of dl_runtime. I guess we could ameliorate things here by
> > limiting the number of time we execute the loop before bailing out.
> 
> That's the "DL replenish lagged too much" case, right? Yeah, there is
> only so much we can recover from.

Right.

> Funny that GCC actually emits that loop; sometimes we've had to fight
> GCC not to turn that into a division.
> 
> But yes, I suppose we can put a limit on how many periods we can lag
> before just giving up.

OK.

> > So, I tend to think that we might want to play safe and put some higher
> > minimum value for dl_runtime (it's currently at 1ULL << DL_SCALE).
> > Guess the problem is to pick a reasonable value, though. Maybe link it
> > someway to HZ? Then we might add a sysctl (or similar) thing with which
> > knowledgeable users can do whatever they think their platform/config can
> > support?
> 
> Yes, a HZ related limit sounds like something we'd want. But if we're
> going to do a minimum sysctl, we should also consider adding a maximum,
> if you set a massive period/deadline, you can, even with a relatively
> low u, incur significant delays.
> 
> And do we want to put the limit on runtime or on period ?
> 
> That is, something like:
> 
>   TICK_NSEC/2 < period < 10*TICK_NSEC
> 
> and/or
> 
>   TICK_NSEC/2 < runtime < 10*TICK_NSEC
> 
> Hmm, for HZ=1000 that ends up with a max period of 10ms, that's far too
> low, 24Hz needs ~41ms. We can of course also limit the runtime by
> capping u for users (as we should anyway).

I also thought of TICK_NSEC/2 as a reasonably safe lower limit, that
will implicitly limit period as well since

   runtime <= deadline <= period

Not sure about the upper limit, though. Lower limit is something related
to the inherent granularity of the platform/config, upper limit is more
to do with highest prio stuff with huge period delaying everything else;
doesn't seem to be related to HZ?

Maybe we could just pick something that seems reasonably big to handle
SCHED_DEADLINE users needs and not too big to jeopardize everyone else,
say 0.5s?