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

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Message-ID: <3188cbf5bcbe5799c77a6cc870ad0ee85b58d255@linux.dev>
Date: Mon, 05 Jan 2026 00:26:54 +0000
From: "Zqiang" <qiang.zhang@...ux.dev>
To: "Joel Fernandes" <joelagnelf@...dia.com>, "Donglin Peng"
 <dolinux.peng@...il.com>
Cc: 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 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.
> >  
> >  This was accurate until commit 171476775d32 ("context_tracking: Convert state to atomic_t"),
> >  which merged the context tracking state and the dynticks counter into a
> >  single atomic value. Consequently, the value printed in the stall warning
> >  no longer directly corresponds to the documented dynticks counter bits.
> >  
> >  To restore consistency between the code's output and the documentation,
> >  right-shift the merged atomic state value to extract and display the
> >  correct low-order bits of the dynticks counter.
> >  
> >  Fixes: 171476775d32 ("context_tracking: Convert state to atomic_t")
> >  Signed-off-by: Donglin Peng <pengdonglin@...omi.com>
> > 
> Looks good to me! Also nice use of ilog2() which resolves to a single shift
> right instruction per my testing (shift right of 2).
> 
> Reviewed-by: Joel Fernandes <joelagnelf@...dia.com>
> 
> thanks,
> 
>  - Joel
> 
> > 
> > ---
> >  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)

Thanks
Zqiang


> >  ct_nesting_cpu(cpu), ct_nmi_nesting_cpu(cpu),
> >  rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
> >  data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
> >  -- 
> >  2.34.1
> >
>