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

[Joel Fernandes <joelagnelf@nvidia.com>]

On Sun, Jan 04, 2026 at 04:10:27PM +0800, 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 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@xiaomi.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@nvidia.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,
>  	       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
>