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

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Message-Id: <20260104081027.189234-1-dolinux.peng@gmail.com>
Date: Sun,  4 Jan 2026 16:10:27 +0800
From: Donglin Peng <dolinux.peng@...il.com>
To: paulmck@...nel.org
Cc: frederic@...nel.org,
	neeraj.upadhyay@...nel.org,
	joelagnelf@...dia.com,
	qiang.zhang@...ux.dev,
	rostedt@...dmis.org,
	rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Donglin Peng <pengdonglin@...omi.com>
Subject: [PATCH] rcu: Align stall warning 'idle=' output with documentation

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>
---
 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,
 	       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