[PATCH v2] rcu: Align stall warning 'idle=' output with documentation

[Donglin Peng <dolinux.peng@gmail.com>]

From: Donglin Peng <pengdonglin@xiaomi.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 valid until commit 171476775d32 ("context_tracking: Convert state to atomic_t"),
which merged the context-tracking state and dynticks counter into a
single atomic variable. In the new layout, the dynticks counter occupies
the higher bits starting from CT_RCU_WATCHING_START.

However, the current stall warning code prints the value from
`ct_rcu_watching_cpu()`, which returns `atomic_read(&ct->state) &
CT_RCU_WATCHING_MASK`. This masks out (clears) the lower state bits,
resulting in a value that is always even. This obscures the CPU's true
idle state and makes the output inconsistent with the documentation.

To restore consistency between the code's output and the documentation,
shift the atomic value right by CT_RCU_WATCHING_START bits before printing.
This extracts and displays only the relevant dynticks counter portion,
allowing the parity (even/odd) to correctly reflect the CPU's dyntick-idle
state.

Fixes: 171476775d32 ("context_tracking: Convert state to atomic_t")
Signed-off-by: Donglin Peng <pengdonglin@xiaomi.com>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
---
v2:
 - Use CT_RCU_WATCHING_START to replace ilog2(CT_RCU_WATCHING) to clean
   up the code, thanks to Zqiang.
 - Clarify the commit message.
---
 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..fd9d7fd6e17c 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) >> CT_RCU_WATCHING_START) & 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