Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU changes for v6.9]

[Frederic Weisbecker]

Le Tue, Mar 12, 2024 at 01:32:03PM -0700, Florian Fainelli a écrit :
> Hi Boqun,
> 
> On 3/8/24 09:15, Boqun Feng wrote:
> > Hi Linus,
> > 
> > Please pull this for the RCU changes of v6.9:
> > 
> > The following changes since commit 41bccc98fb7931d63d03f326a746ac4d429c1dd3:
> > 
> >    Linux 6.8-rc2 (2024-01-28 17:01:12 -0800)
> > 
> > are available in the Git repository at:
> > 
> >    git://git.kernel.org/pub/scm/linux/kernel/git/boqun/linux.git tags/rcu.next.v6.9
> > 
> > for you to fetch changes up to 3add00be5fe5810d7aa5ec3af8b6a245ef33144b:
> > 
> >    Merge branches 'rcu-doc.2024.02.14a', 'rcu-nocb.2024.02.14a', 'rcu-exp.2024.02.14a', 'rcu-tasks.2024.02.26a' and 'rcu-misc.2024.02.14a' into rcu.2024.02.26a (2024-02-26 17:37:25 -0800)
> > 
> > 
> > Two merge conflicts were detected by linux-next:
> > 
> > * https://lore.kernel.org/lkml/20240226135745.12ac854d@xxxxxxxxxxxxxxxx/
> > * https://lore.kernel.org/lkml/20240227125522.2bdbe6be@xxxxxxxxxxxxxxxx/
> > 
> > These conflict resolutions from linux-next look good to me, plus I made
> > my own resolutions at branch merge/rcu.2024.02.27a for your reference.
> > 
> > 
> > Some highlights of the changes:
> > 
> > * Eliminates deadlocks involving do_exit() and RCU tasks, by Paul:
> >    Instead of SRCU read side critical sections, now a percpu list is used
> >    in do_exit() for scaning yet-to-exit tasks.
> > 
> > * Fixes a deadlock due to the dependency between workqueue and RCU
> >    expedited grace period, reported by Anna-Maria Behnsen and Thomas
> >    Gleixner and fixed by Frederic: Now RCU expedited always uses its own
> >    kthread worker instead of a workqueue.
> 
> At least one device in my test farm (ARM 32-bit) has consistently shown a
> very long boot, and some others are intermittently affected. This
> consistently looks like this on most of my devices:
> 
> [    2.450351] bcmgenet f0480000.ethernet: GENET 5.0 EPHY: 0x0000
> [    2.547562] ata1: SATA link down (SStatus 0 SControl 300)
> [  162.107264] unimac-mdio unimac-mdio.0: Broadcom UniMAC MDIO bus
> 
> this gets flagged by my boot script as a boot failure since we exceeded the
> 30 seconds timeout given to boot a kernel to a prompt.
> 
> It has been somewhat difficult to get a reliable bisection going on, but
> what I am sure of is that e5a3878c947ceef7b6ab68fdc093f3848059842c~1 does
> not expose the problem for 10 consecutive boots, while I *might* see it at
> e5a3878c947ceef7b6ab68fdc093f3848059842c and beyond.
> 
> Any clues what is going on here?

I understand it doesn't reliably reproduce. But is there any chance you could
test that alone?

     0bb11a372fc8 (rcu-tasks: Maintain real-time response in rcu_tasks_postscan())

And if it reproduces there try a bisection?

git bisect bad 0bb11a372fc8d7006b4d0f42a2882939747bdbff
git bisect good 41bccc98fb7931d63d03f326a746ac4d429c1dd3

Thanks.