linux-kernel - Re: Boot warning at rcu_check_gp_start

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Date:   Tue, 21 Jan 2020 06:19:23 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     rcu@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: Boot warning at rcu_check_gp_start_stall()

On Tue, Jan 21, 2020 at 06:45:32AM -0500, Qian Cai wrote:
> 
> 
> > On Jan 21, 2020, at 12:09 AM, Paul E. McKenney <paulmck@...nel.org> wrote:
> > 
> > This is what you get when a grace period has been requested, but does
> > not start within 21 seconds or so.  The "->state: 0x1ffff" is a new one
> > on me -- that normally happens only before RCU's grace-period kthread
> > has been spawned.  But by 97 seconds after boot, it should definitely
> > already be up and running.
> > 
> > Is the system responsive at this point?
> 
> Yes, it works fine.
> 
> > Except...  Why is it taking 96 seconds for the system to get to the point
> > where it prints "Dentry cache hash table entries:"?  That happens at 0.139
> > seconds on my laptop.  And at about the same time on a much larger system.
> > 
> > I could easily imagine that all sorts of things would break when boot
> > takes that long.
> 
> I suppose the kernel has CONFIG_EFI_PGT_DUMP=y, so it takes a while to run just before rcu_check_gp_start_stall().

New one on me!

One approach would be to boot with rcupdate.rcu_cpu_stall_timeout=300,
which would allow more time.

Longer term, I could suppress this warning during boot when
CONFIG_EFI_PGT_DUMP=y, but that sounds quite specific.  Alternatively,
I could provide a Kconfig option that suppressed this during boot
that was selected by whatever long-running boot-time Kconfig option
needed it.  Yet another approach would be for long-running operations
like efi_dump_pagetable() to suppress stalls on entry and re-enable them
upon exit.

Thoughts?

							Thanx, Paul