linux-kernel - Re: [tip: core/rcu] rcu: Enable tick for nohz

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Message-ID: <20200125175442.GA4369@zn.tnic>
Date:   Sat, 25 Jan 2020 18:54:42 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        x86 <x86@...nel.org>
Subject: Re: [tip: core/rcu] rcu: Enable tick for nohz_full CPUs slow to
 provide expedited QS

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.

https://people.kernel.org/tglx/notes-about-netiquette