Re: [PATCH V4 2/2] rcu: Update jiffies in rcu_cpu_stall_reset()

[Huacai Chen]

Hi, Joel,

On Tue, Aug 29, 2023 at 4:47 AM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
>
> Hi Huacai,
>
> On Mon, Aug 28, 2023 at 11:13 AM Huacai Chen <chenhuacai@xxxxxxxxxx> wrote:
> >
> [...]
> > >
> > > > [Huacai]
> > > > I also think the original patch should be OK, but I have another
> > > > question: what will happen if the current GP ends before
> > > > nr_fqs_jiffies_stall reaches zero?
> > >
> > > Nothing should happen. Stall detection only happens when a GP is in
> > > progress. If a new GP starts, it resets nr_fqs_jiffies_stall.
> > >
> > > Or can you elaborate your concern more?
> > OK, I will test your patch these days. Maybe putting
> > nr_fqs_jiffies_stall before jiffies_force_qs is better, because I
> > think putting an 'int' between two 'long' is wasting space. :)
>
> That's a good point and I'll look into that.
Another point, is it better to replace ULONG_MAX with ULONG_MAX/4 as
Paul suggested?

>
> Meanwhile I pushed the patch out to my 6.4 stable tree for testing on my fleet.
>
> Ideally, I'd like to change the stall detection test in the rcutorture
> to actually fail rcutorture if stalls don't happen in time. But at
> least I verified this manually using rcutorture.
>
> I should also add a documentation patch for stallwarn.rst to document
> the understandable sensitivity of RCU stall detection to jiffies
> updates (or lack thereof). Or if you have time, I'd appreciate support
> on such a patch (not mandatory but I thought it would not hurt to
> ask).
>
> Looking forward to how your testing goes as well!
I have tested, it works for KGDB.

Huacai
>
> thanks,
>
>  - Joel