[PATCH rcu 0/7] Callback-offload (nocb) updates for v6.1

[PATCH rcu 0/7] Callback-offload (nocb) updates for v6.1

[Paul E. McKenney]

Hello!

This series provides NOCB-CPU updates:

1.	Choose the right rcuog/rcuop kthreads to output, courtesy
	of Zqiang.

2.	Add CPU number to CPU-{,de}offload failure messages.

3.	Simplify rcu_init_nohz() cpumask handling, courtesy of Zhen Lei.

						Thanx, Paul

------------------------------------------------------------------------

 b/kernel/rcu/tree_nocb.h |    6 +++---
 kernel/rcu/tree_nocb.h   |   36 +++++++++++-------------------------
 2 files changed, 14 insertions(+), 28 deletions(-)

[PATCH rcu 1/3] rcu/nocb: Choose the right rcuog/rcuop kthreads to output

[Paul E. McKenney]

From: Zqiang <qiang1.zhang@intel.com>

The show_rcu_nocb_gp_state() function is supposed to dump out the rcuog
kthread and the show_rcu_nocb_state() function is supposed to dump out
the rcuo[ps] kthread.  Currently, both do a mixture, which is not optimal
for debugging, even though it does not affect functionality.

This commit therefore adjusts these two functions to focus on their
respective kthreads.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_nocb.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index a8f574d8850d2..f20aec4f4394b 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1452,8 +1452,8 @@ static void show_rcu_nocb_gp_state(struct rcu_data *rdp)
 		(long)rdp->nocb_gp_seq,
 		rnp->grplo, rnp->grphi, READ_ONCE(rdp->nocb_gp_loops),
 		rdp->nocb_gp_kthread ? task_state_to_char(rdp->nocb_gp_kthread) : '.',
-		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
-		show_rcu_should_be_on_cpu(rdp->nocb_cb_kthread));
+		rdp->nocb_gp_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
+		show_rcu_should_be_on_cpu(rdp->nocb_gp_kthread));
 }
 
 /* Dump out nocb kthread state for the specified rcu_data structure. */
@@ -1497,7 +1497,7 @@ static void show_rcu_nocb_state(struct rcu_data *rdp)
 		".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
 		rcu_segcblist_n_cbs(&rdp->cblist),
 		rdp->nocb_cb_kthread ? task_state_to_char(rdp->nocb_cb_kthread) : '.',
-		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
+		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_cb_kthread) : -1,
 		show_rcu_should_be_on_cpu(rdp->nocb_cb_kthread));
 
 	/* It is OK for GP kthreads to have GP state. */
-- 
2.31.1.189.g2e36527f23

[PATCH rcu 2/3] rcu/nocb: Add CPU number to CPU-{,de}offload failure messages

[Paul E. McKenney]

Offline CPUs cannot be offloaded or deoffloaded.  Any attempt to offload
or deoffload an offline CPU causes a message to be printed on the console,
which is good, but this message does not contain the CPU number, which
is bad.  Such a CPU number can be helpful when debugging, as it gives a
clear indication that the CPU in question is in fact offline.  This commit
therefore adds the CPU number to the CPU-{,de}offload failure messages.

Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_nocb.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index f20aec4f4394b..0a5f0ef414845 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1111,7 +1111,7 @@ int rcu_nocb_cpu_deoffload(int cpu)
 			if (!ret)
 				cpumask_clear_cpu(cpu, rcu_nocb_mask);
 		} else {
-			pr_info("NOCB: Can't CB-deoffload an offline CPU\n");
+			pr_info("NOCB: Cannot CB-deoffload offline CPU %d\n", rdp->cpu);
 			ret = -EINVAL;
 		}
 	}
@@ -1196,7 +1196,7 @@ int rcu_nocb_cpu_offload(int cpu)
 			if (!ret)
 				cpumask_set_cpu(cpu, rcu_nocb_mask);
 		} else {
-			pr_info("NOCB: Can't CB-offload an offline CPU\n");
+			pr_info("NOCB: Cannot CB-offload offline CPU %d\n", rdp->cpu);
 			ret = -EINVAL;
 		}
 	}
-- 
2.31.1.189.g2e36527f23

[PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Paul E. McKenney]

From: Zhen Lei <thunder.leizhen@huawei.com>

In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
Except that kernels booted without the rcu_nocbs= will not have
allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
its need_rcu_nocb_mask and offload_all local variables to track the
rcu_nocb and nohz_full state.

But there is a much simpler approach, namely creating a cpumask pointer
to track the default and then using cpumask_available() to check the
rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
and shortening the rcu_init_nohz() function.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
 1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0a5f0ef414845..c8167be2288fa 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 void __init rcu_init_nohz(void)
 {
 	int cpu;
-	bool need_rcu_nocb_mask = false;
-	bool offload_all = false;
 	struct rcu_data *rdp;
+	const struct cpumask *cpumask = NULL;
 
 #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
-	if (!rcu_state.nocb_is_setup) {
-		need_rcu_nocb_mask = true;
-		offload_all = true;
-	}
-#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
-
-#if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
-		need_rcu_nocb_mask = true;
-		offload_all = false; /* NO_HZ_FULL has its own mask. */
-	}
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+	cpumask = cpu_possible_mask;
+#elif defined(CONFIG_NO_HZ_FULL)
+	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+		cpumask = tick_nohz_full_mask;
+#endif
 
-	if (need_rcu_nocb_mask) {
+	if (cpumask) {
 		if (!cpumask_available(rcu_nocb_mask)) {
 			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
 				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
 				return;
 			}
 		}
+
+		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
 		rcu_state.nocb_is_setup = true;
 	}
 
 	if (!rcu_state.nocb_is_setup)
 		return;
 
-#if defined(CONFIG_NO_HZ_FULL)
-	if (tick_nohz_full_running)
-		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
-
-	if (offload_all)
-		cpumask_setall(rcu_nocb_mask);
-
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
 		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
-- 
2.31.1.189.g2e36527f23

Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Frederic Weisbecker]

On Wed, Aug 31, 2022 at 11:10:44AM -0700, Paul E. McKenney wrote:
> From: Zhen Lei <thunder.leizhen@huawei.com>
> 
> In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> Except that kernels booted without the rcu_nocbs= will not have
> allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
> its need_rcu_nocb_mask and offload_all local variables to track the
> rcu_nocb and nohz_full state.
> 
> But there is a much simpler approach, namely creating a cpumask pointer
> to track the default and then using cpumask_available() to check the
> rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
> and shortening the rcu_init_nohz() function.
> 
> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
>  1 file changed, 9 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef414845..c8167be2288fa 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
> -	bool offload_all = false;
>  	struct rcu_data *rdp;
> +	const struct cpumask *cpumask = NULL;
>  
>  #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> -	if (!rcu_state.nocb_is_setup) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = true;
> -	}
> -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> -
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> -	}
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> +	cpumask = cpu_possible_mask;

You're missing the rcu_state.nocb_is_setup check, so
CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will now always override the rcu_nocbs=
parameter (should be the other way around).


> +#elif defined(CONFIG_NO_HZ_FULL)
> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> +		cpumask = tick_nohz_full_mask;
> +#endif

A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
now override nohz_full=

I don't mind, it's probably what we want in the end, but the changelog should
tell about it, or even better, this should be a separate change.

Thanks.

>  
> -	if (need_rcu_nocb_mask) {
> +	if (cpumask) {
>  		if (!cpumask_available(rcu_nocb_mask)) {
>  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
>  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
>  				return;
>  			}
>  		}
> +
> +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
>  		rcu_state.nocb_is_setup = true;
>  	}
>  
>  	if (!rcu_state.nocb_is_setup)
>  		return;
>  
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running)
> -		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> -
> -	if (offload_all)
> -		cpumask_setall(rcu_nocb_mask);
> -
>  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
>  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
>  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> -- 
> 2.31.1.189.g2e36527f23
> 

Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Paul E. McKenney]

On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
> On Wed, Aug 31, 2022 at 11:10:44AM -0700, Paul E. McKenney wrote:
> > From: Zhen Lei <thunder.leizhen@huawei.com>
> > 
> > In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> > CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> > Except that kernels booted without the rcu_nocbs= will not have
> > allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
> > its need_rcu_nocb_mask and offload_all local variables to track the
> > rcu_nocb and nohz_full state.
> > 
> > But there is a much simpler approach, namely creating a cpumask pointer
> > to track the default and then using cpumask_available() to check the
> > rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
> > and shortening the rcu_init_nohz() function.
> > 
> > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
> >  1 file changed, 9 insertions(+), 23 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef414845..c8167be2288fa 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> >  void __init rcu_init_nohz(void)
> >  {
> >  	int cpu;
> > -	bool need_rcu_nocb_mask = false;
> > -	bool offload_all = false;
> >  	struct rcu_data *rdp;
> > +	const struct cpumask *cpumask = NULL;
> >  
> >  #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> > -	if (!rcu_state.nocb_is_setup) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = true;
> > -	}
> > -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> > -
> > -#if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> > -	}
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > +	cpumask = cpu_possible_mask;
> 
> You're missing the rcu_state.nocb_is_setup check, so
> CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will now always override the rcu_nocbs=
> parameter (should be the other way around).

This is indeed a problem, so especially thank you for looking this over!

I will revert this patch.

> > +#elif defined(CONFIG_NO_HZ_FULL)
> > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > +		cpumask = tick_nohz_full_mask;
> > +#endif
> 
> A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
> now override nohz_full=
> 
> I don't mind, it's probably what we want in the end, but the changelog should
> tell about it, or even better, this should be a separate change.

Good point.  Perhaps the key point is that if there is nohz_full=,
rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
to include at least those bits set by nohz_full=.

Are there any other constraints?

Zhen Lei, please feel free to submit an updated patch.

							Thanx, Paul

> Thanks.
> 
> >  
> > -	if (need_rcu_nocb_mask) {
> > +	if (cpumask) {
> >  		if (!cpumask_available(rcu_nocb_mask)) {
> >  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> >  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> >  				return;
> >  			}
> >  		}
> > +
> > +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
> >  		rcu_state.nocb_is_setup = true;
> >  	}
> >  
> >  	if (!rcu_state.nocb_is_setup)
> >  		return;
> >  
> > -#if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running)
> > -		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > -
> > -	if (offload_all)
> > -		cpumask_setall(rcu_nocb_mask);
> > -
> >  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> >  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> >  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > -- 
> > 2.31.1.189.g2e36527f23
> > 

Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Frederic Weisbecker]

On Thu, Sep 01, 2022 at 03:25:20AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
> > > +#elif defined(CONFIG_NO_HZ_FULL)
> > > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > > +		cpumask = tick_nohz_full_mask;
> > > +#endif
> > 
> > A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
> > now override nohz_full=
> > 
> > I don't mind, it's probably what we want in the end, but the changelog should
> > tell about it, or even better, this should be a separate change.
> 
> Good point.  Perhaps the key point is that if there is nohz_full=,
> rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
> to include at least those bits set by nohz_full=.

Not sure I get what you mean. nohz_full= should in any case always force
rcu_nocbs at least on the nohz_full CPUs.

For example assuming the following combination: rcu_nocbs=6, nohz_full=7 AND
CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the result should be:

NOCB CPUs = 6,7
NOHZ_FULL CPUs = 7

(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y is overriden by rcu_nocbs=6).

Now if we have nohz_full=7 AND CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the
result is expected to be either:

NOCB CPUs = 7 (upstream behaviour)
NOHZ_FULL CPUs = 7

or 

NOCB CPUs = all
NOHZ_FULL CPUs = 7

The second makes more sense IMHO but that should be in a separate change.

Thanks.

Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Leizhen (ThunderTown)]

On 2022/9/1 19:11, Frederic Weisbecker wrote:
> On Thu, Sep 01, 2022 at 03:25:20AM -0700, Paul E. McKenney wrote:
>> On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
>>>> +#elif defined(CONFIG_NO_HZ_FULL)
>>>> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
>>>> +		cpumask = tick_nohz_full_mask;
>>>> +#endif
>>>
>>> A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
>>> now override nohz_full=
>>>
>>> I don't mind, it's probably what we want in the end, but the changelog should
>>> tell about it, or even better, this should be a separate change.
>>
>> Good point.  Perhaps the key point is that if there is nohz_full=,
>> rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
>> to include at least those bits set by nohz_full=.
> 
> Not sure I get what you mean. nohz_full= should in any case always force
> rcu_nocbs at least on the nohz_full CPUs.
> 
> For example assuming the following combination: rcu_nocbs=6, nohz_full=7 AND
> CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the result should be:
> 
> NOCB CPUs = 6,7
> NOHZ_FULL CPUs = 7
> 
> (CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y is overriden by rcu_nocbs=6).
> 
> Now if we have nohz_full=7 AND CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the
> result is expected to be either:
> 
> NOCB CPUs = 7 (upstream behaviour)
> NOHZ_FULL CPUs = 7

OK, I got it. CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes effect only when there
are no 'rcu_nocbs=' and 'nohz_full='.

> 
> or 
> 
> NOCB CPUs = all
> NOHZ_FULL CPUs = 7
> 
> The second makes more sense IMHO but that should be in a separate change.

OK, in this case, CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y takes effect only when there
is no 'rcu_nocbs='.

> 
> Thanks.
> .
> 

-- 
Regards,
  Zhen Lei

Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

[Paul E. McKenney]

On Thu, Sep 01, 2022 at 01:11:14PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 01, 2022 at 03:25:20AM -0700, Paul E. McKenney wrote:
> > On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
> > > > +#elif defined(CONFIG_NO_HZ_FULL)
> > > > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > > > +		cpumask = tick_nohz_full_mask;
> > > > +#endif
> > > 
> > > A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
> > > now override nohz_full=
> > > 
> > > I don't mind, it's probably what we want in the end, but the changelog should
> > > tell about it, or even better, this should be a separate change.
> > 
> > Good point.  Perhaps the key point is that if there is nohz_full=,
> > rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
> > to include at least those bits set by nohz_full=.
> 
> Not sure I get what you mean. nohz_full= should in any case always force
> rcu_nocbs at least on the nohz_full CPUs.
> 
> For example assuming the following combination: rcu_nocbs=6, nohz_full=7 AND
> CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the result should be:
> 
> NOCB CPUs = 6,7
> NOHZ_FULL CPUs = 7
> 
> (CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y is overriden by rcu_nocbs=6).
> 
> Now if we have nohz_full=7 AND CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the
> result is expected to be either:
> 
> NOCB CPUs = 7 (upstream behaviour)
> NOHZ_FULL CPUs = 7
> 
> or 
> 
> NOCB CPUs = all
> NOHZ_FULL CPUs = 7
> 
> The second makes more sense IMHO but that should be in a separate change.

Your examples are consistent with what I was trying to say.  ;-)

							Thanx, Paul