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

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

On 1/5/26 11:21, Donglin Peng wrote:
> 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,

Gentle ping on this patch. Thanks!