Re: [PATCH 5.15 000/183] 5.15.134-rc1 review

[Joel Fernandes]

On Sun, Oct 8, 2023 at 9:20 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
[...]
> > > > How frequent is this function called?  We could check something for
> > > > early boot... or track down where the cpu is put online and restore idle
> > > > before that happens?
> > >
> > > Once per RCU Tasks Trace grace period per reader seen to be blocking
> > > that grace period.  Its performance is as issue, but not to anywhere
> > > near the same extent as (say) rcu_read_lock_trace().
> > >
> > > > > > It's also worth noting that the bug this fixes wasn't exposed until the
> > > > > > maple tree (added in v6.1) was used for the IRQ descriptors (added in
> > > > > > v6.5).
> > > > >
> > > > > Lots of latent bugs, to be sure, even with rcutorture.  :-/
> > > >
> > > > The Right Thing is to fix the bug all the way back to the introduction,
> > > > but what fallout makes the backport less desirable than living with the
> > > > unexposed bug?
> > >
> > > You are quite right that it is possible for the risk of a backport to
> > > exceed the risk of the original bug.
> > >
> > > I defer to Joel (CCed) on how best to resolve this in -stable.
> >
> > Maybe I am missing something but this issue should also be happening
> > in mainline right?
> >
> > Even though mainline has 897ba84dc5aa ("rcu-tasks: Handle idle tasks
> > for recently offlined CPUs") , the warning should still be happening
> > due to Liam's "kernel/sched: Modify initial boot task idle setup"
> > because the warning is just rearranged a bit but essentially the same.
> >
> > IMHO, the right thing to do then is to drop Liam's patch from 5.15 and
> > fix it in mainline (using the ideas described in this thread), then
> > backport both that new fix and Liam's patch to 5.15.
> >
> > Or is there a reason this warning does not show up on the mainline?
> >
> > My impression is that dropping Liam's patch for the stable release and
> > revisiting it later is a better approach since tiny RCU is used way
> > less in the wild than tree/tasks RCU. Thoughts?
>
> I think that this one is strange enough that we need to write down the
> situation in detail, make sure we have all the corner cases covered in
> both mainline and -stable, and decide what to do from there.
>
> Yes, I know, this email thread contains much of this information, but
> a little organizing of it would be good.
>
> Would you like to put that together, or should I?  If me, I will get
> a draft out by the end of this coming Tuesday, Pacific Time.

I apologize, I haven't been able to do any real work as I was OOO for
the most part due to dental issues. I am about 25% back now. I will
review your other email writeup and thanks for putting it together!

 - Joel