linux-kernel - Re: [PATCH] rcu: Align stall warning 'idle=' output with documentation

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Message-ID: <CAErzpmsNu8NQ73ovB41Cam1_tc4doH52mwnt-Vw01=2K5xjiMw@mail.gmail.com>
Date: Mon, 5 Jan 2026 09:31:24 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Zqiang <qiang.zhang@...ux.dev>, Joel Fernandes <joelagnelf@...dia.com>, paulmck@...nel.org, 
	frederic@...nel.org, neeraj.upadhyay@...nel.org, rostedt@...dmis.org, 
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Donglin Peng <pengdonglin@...omi.com>
Subject: Re: [PATCH] rcu: Align stall warning 'idle=' output with documentation

On Mon, Jan 5, 2026 at 8:34 AM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Sun, Jan 4, 2026 at 7:32 PM Zqiang <qiang.zhang@...ux.dev> wrote:
> >
> > >
> > > On Sun, Jan 04, 2026 at 04:10:27PM +0800, Donglin Peng wrote:
> > >
> > > >
> > > > From: Donglin Peng <pengdonglin@...omi.com>
> > > >
> > > >  The RCU stall warning message includes an "idle=" field to indicate
> > > >  the dyntick-idle state of a CPU. According to Documentation/RCU/stallwarn.rst,
> > > >  the hexadecimal number before the first '/' represents the low-order 16
> > > >  bits of the dynticks counter. An even value denotes that the CPU is in
> > > >  dyntick-idle mode, while an odd value indicates otherwise.
> [...]
> > > >  kernel/rcu/tree_stall.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > >  diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > > >  index b67532cb8770..d25cc826d77a 100644
> > > >  --- a/kernel/rcu/tree_stall.h
> > > >  +++ b/kernel/rcu/tree_stall.h
> > > >  @@ -555,7 +555,7 @@ static void print_cpu_stall_info(int cpu)
> > > >  rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
> > > >  "!."[!delta],
> > > >  ticks_value, ticks_title,
> > > >  - ct_rcu_watching_cpu(cpu) & 0xffff,
> > > >  + (ct_rcu_watching_cpu(cpu) >> ilog2(CT_RCU_WATCHING)) & 0xffff,
> >
> >
> >
> > May be also use CT_RCU_WATCHING_START to
> > replace ilog2(CT_RCU_WATCHING) operations? (I didn't actually test it)

Thank you for the suggestion. I agree that using the CT_RCU_WATCHING_START
macro is cleaner. I will fix it in the next version.

I noted that the CT_RCU_WATCHING_START macro was introduced in
kernel version 6.19-rc1 by commit d1e6d2773898 ("rcu: Add a
small-width RCU watching counter debug option").
Therefore, if this patch needs to be backported to older kernel versions
that lack this macro, using ilog2(CT_RCU_WATCHING) would be the
simpler, more compatible approach.

>
> It should compile down to the same thing, but either is fine with me.
>
> thanks,
>
>  - Joel