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

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

[Donglin Peng]

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

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

[Peng Donglin]

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!

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

[Paul E. McKenney]

On Fri, Sep 04, 2026 at 08:24:05PM +0800, Peng Donglin wrote:
> 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!

The advantage of the current code is that the CT_STATE_KERNEL,
CT_STATE_IDLE, CT_STATE_USER, and CT_STATE_GUEST information is visible
in the lower two bits of that hex number.

Now, if you wanted to print that information symbolically and then
shift down the counter, that might well be a valuable improvement.
Given corresponding updates to stallwarn.rst, of course.

Or am I missing something subtle here?

							Thanx, Paul

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

[Donglin Peng]

On Sat, Sep 12, 2026 at 12:06 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Fri, Sep 04, 2026 at 08:24:05PM +0800, Peng Donglin wrote:
> > 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!
>
> The advantage of the current code is that the CT_STATE_KERNEL,
> CT_STATE_IDLE, CT_STATE_USER, and CT_STATE_GUEST information is visible
> in the lower two bits of that hex number.
>
> Now, if you wanted to print that information symbolically and then
> shift down the counter, that might well be a valuable improvement.
> Given corresponding updates to stallwarn.rst, of course.

Thanks, that makes sense. I'll print the state symbolically, shift down
the counter, update stallwarn.rst accordingly, and send a v3.

>
> Or am I missing something subtle here?
>
>                                                         Thanx, Paul

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

[Paul E. McKenney]

On Wed, Sep 16, 2026 at 09:37:50PM +0800, Donglin Peng wrote:
> On Sat, Sep 12, 2026 at 12:06 AM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Fri, Sep 04, 2026 at 08:24:05PM +0800, Peng Donglin wrote:
> > > 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!
> >
> > The advantage of the current code is that the CT_STATE_KERNEL,
> > CT_STATE_IDLE, CT_STATE_USER, and CT_STATE_GUEST information is visible
> > in the lower two bits of that hex number.
> >
> > Now, if you wanted to print that information symbolically and then
> > shift down the counter, that might well be a valuable improvement.
> > Given corresponding updates to stallwarn.rst, of course.
> 
> Thanks, that makes sense. I'll print the state symbolically, shift down
> the counter, update stallwarn.rst accordingly, and send a v3.

Sounds good, and looking forward to it!

							Thanx, Paul

> > Or am I missing something subtle here?
> >
> >                                                         Thanx, Paul