Re: [tip: core/rcu] rcu: Enable tick for nohz_full CPUs slow to provide expedited QS

From: Borislav Petkov
Date: Sat Jan 25 2020 - 12:54:54 EST


On Sat, Jan 25, 2020 at 08:10:50AM -0800, Paul E. McKenney wrote:
> How big? (Seriously, given that the fix may depend on the number of CPUs.)

[ 7.660017] smp: Brought up 2 nodes, 256 CPUs

> So the problem appears to be that some of the boot-time processing
> is looping in the kernel, which is preventing the grace period from
> completing. One could argue that such code should be fixed, but on the
> other hand, boot time is a bit special. Later in -rcu's dev branch,
> there are commits that forgive this boot-time misbehavior, but this is
> a bit late in process to dump all of those commits into -tip.

Aha.

> The RT guys might need the warning, and it was them that I was thinking
> of when adding it.

But "boot time is a bit special". Or do they care about deadlines during
boot too?

> But let's see what works for mainline first. And
> since your box was booting fine without the warning before, I bet that
> it boots just fine with that warning removed.

Yes, it does.

> So could you please try out the (untested) patch below?

Warning's gone.

> If that works, I will re-introduce the warning with proper protection
> for the merge window following this coming one.

My big box is at your service if you need stuff tested later.

Thx Paul.

--
Regards/Gruss,
Boris.

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