[PATCH v4 0/4] Extending NMI watchdog during LPM

[PATCH v4 0/4] Extending NMI watchdog during LPM

[Laurent Dufour]

When a partition is transferred, once it arrives at the destination node,
the partition is active but much of its memory must be transferred from the
start node.

It depends on the activity in the partition, but the more CPU the partition
has, the more memory to be transferred is likely to be. This causes latency
when accessing pages that need to be transferred, and often, for large
partitions, it triggers the NMI watchdog.

The NMI watchdog causes the CPU stack to dump where it appears to be
stuck. In this case, it does not bring much information since it can happen
during any memory access of the kernel.

In addition, the NMI interrupt mechanism is not secure and can generate a
dump system in the event that the interruption is taken while MSR[RI]=0.

Depending on the LPAR size and load, it may be interesting to extend the
NMI watchdog timer during the LPM.

That's configurable through sysctl with the new introduced variable
(specific to powerpc) nmi_watchdog_factor. This value represents the
percentage added to watchdog_tresh to set the NMI watchdog timeout during a
LPM.

Changes in v4 (no functional changes in this version):
 - Patch 1/4 :fix typo and add a comment in pseries_migrate_partition()
 - Patch 3/4: rename new variables and functions as Nick requested.
 - Patch 4/4: rename the called new function

v2:
https://lore.kernel.org/linuxppc-dev/121217bb-6a34-8ccb-9819-f82806d6f47c@linux.ibm.com/

Laurent Dufour (4):
  powerpc/mobility: wait for memory transfer to complete
  watchdog: export lockup_detector_reconfigure
  powerpc/watchdog: introduce a NMI watchdog's factor
  pseries/mobility: set NMI watchdog factor during LPM

 Documentation/admin-guide/sysctl/kernel.rst | 12 +++
 arch/powerpc/include/asm/nmi.h              |  2 +
 arch/powerpc/kernel/watchdog.c              | 21 ++++-
 arch/powerpc/platforms/pseries/mobility.c   | 91 ++++++++++++++++++++-
 include/linux/nmi.h                         |  2 +
 kernel/watchdog.c                           | 21 +++--
 6 files changed, 141 insertions(+), 8 deletions(-)

-- 
2.37.0

[PATCH v4 1/4] powerpc/mobility: wait for memory transfer to complete

[Laurent Dufour]

In pseries_migration_partition(), loop until the memory transfer is
complete. This way the calling drmgr process will not exit earlier,
allowing callbacks to be run only once the migration is fully completed.

If reading the VASI state is done after the hypervisor has completed the
migration, the HCALL is returning H_PARAMETER. We can safely assume that
the memory transfer is achieved if this happens.

This will also allow to manage the NMI watchdog state in the next commits.

Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 arch/powerpc/platforms/pseries/mobility.c | 48 ++++++++++++++++++++++-
 1 file changed, 46 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/mobility.c b/arch/powerpc/platforms/pseries/mobility.c
index 78f3f74c7056..6297467072e6 100644
--- a/arch/powerpc/platforms/pseries/mobility.c
+++ b/arch/powerpc/platforms/pseries/mobility.c
@@ -427,6 +427,43 @@ static int wait_for_vasi_session_suspending(u64 handle)
 	return ret;
 }
 
+static void wait_for_vasi_session_completed(u64 handle)
+{
+	unsigned long state = 0;
+	int ret;
+
+	pr_info("waiting for memory transfer to complete...\n");
+
+	/*
+	 * Wait for transition from H_VASI_RESUMED to H_VASI_COMPLETED.
+	 */
+	while (true) {
+		ret = poll_vasi_state(handle, &state);
+
+		/*
+		 * If the memory transfer is already complete and the migration
+		 * has been cleaned up by the hypervisor, H_PARAMETER is return,
+		 * which is translate in EINVAL by poll_vasi_state().
+		 */
+		if (ret == -EINVAL || (!ret && state == H_VASI_COMPLETED)) {
+			pr_info("memory transfer completed.\n");
+			break;
+		}
+
+		if (ret) {
+			pr_err("H_VASI_STATE return error (%d)\n", ret);
+			break;
+		}
+
+		if (state != H_VASI_RESUMED) {
+			pr_err("unexpected H_VASI_STATE result %lu\n", state);
+			break;
+		}
+
+		msleep(500);
+	}
+}
+
 static void prod_single(unsigned int target_cpu)
 {
 	long hvrc;
@@ -673,9 +710,16 @@ static int pseries_migrate_partition(u64 handle)
 	vas_migration_handler(VAS_SUSPEND);
 
 	ret = pseries_suspend(handle);
-	if (ret == 0)
+	if (ret == 0) {
 		post_mobility_fixup();
-	else
+		/*
+		 * Wait until the memory transfer is complete, so that the user
+		 * space process returns from the syscall after the transfer is
+		 * complete. This allows the user hooks to be executed at the
+		 * right time.
+		 */
+		wait_for_vasi_session_completed(handle);
+	} else
 		pseries_cancel_migration(handle, ret);
 
 	vas_migration_handler(VAS_RESUME);
-- 
2.37.0

[PATCH v4 2/4] watchdog: export lockup_detector_reconfigure

[Laurent Dufour]

In some circumstances it may be interesting to reconfigure the watchdog
from inside the kernel.

On PowerPC, this may helpful before and after a LPAR migration (LPM) is
initiated, because it implies some latencies, watchdog, and especially NMI
watchdog is expected to be triggered during this operation. Reconfiguring
the watchdog with a factor, would prevent it to happen too frequently
during LPM.

Rename lockup_detector_reconfigure() as __lockup_detector_reconfigure() and
create a new function lockup_detector_reconfigure() calling
__lockup_detector_reconfigure() under the protection of watchdog_mutex.

Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 include/linux/nmi.h |  2 ++
 kernel/watchdog.c   | 21 ++++++++++++++++-----
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 750c7f395ca9..f700ff2df074 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -122,6 +122,8 @@ int watchdog_nmi_probe(void);
 int watchdog_nmi_enable(unsigned int cpu);
 void watchdog_nmi_disable(unsigned int cpu);
 
+void lockup_detector_reconfigure(void);
+
 /**
  * touch_nmi_watchdog - restart NMI watchdog timeout.
  *
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 20a7a55e62b6..90e6c41d5e33 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -541,7 +541,7 @@ int lockup_detector_offline_cpu(unsigned int cpu)
 	return 0;
 }
 
-static void lockup_detector_reconfigure(void)
+static void __lockup_detector_reconfigure(void)
 {
 	cpus_read_lock();
 	watchdog_nmi_stop();
@@ -561,6 +561,13 @@ static void lockup_detector_reconfigure(void)
 	__lockup_detector_cleanup();
 }
 
+void lockup_detector_reconfigure(void)
+{
+	mutex_lock(&watchdog_mutex);
+	__lockup_detector_reconfigure();
+	mutex_unlock(&watchdog_mutex);
+}
+
 /*
  * Create the watchdog infrastructure and configure the detector(s).
  */
@@ -577,13 +584,13 @@ static __init void lockup_detector_setup(void)
 		return;
 
 	mutex_lock(&watchdog_mutex);
-	lockup_detector_reconfigure();
+	__lockup_detector_reconfigure();
 	softlockup_initialized = true;
 	mutex_unlock(&watchdog_mutex);
 }
 
 #else /* CONFIG_SOFTLOCKUP_DETECTOR */
-static void lockup_detector_reconfigure(void)
+void __lockup_detector_reconfigure(void)
 {
 	cpus_read_lock();
 	watchdog_nmi_stop();
@@ -591,9 +598,13 @@ static void lockup_detector_reconfigure(void)
 	watchdog_nmi_start();
 	cpus_read_unlock();
 }
+static inline void lockup_detector_reconfigure(void)
+{
+	__lockup_detector_reconfigure();
+}
 static inline void lockup_detector_setup(void)
 {
-	lockup_detector_reconfigure();
+	__lockup_detector_reconfigure();
 }
 #endif /* !CONFIG_SOFTLOCKUP_DETECTOR */
 
@@ -633,7 +644,7 @@ static void proc_watchdog_update(void)
 {
 	/* Remove impossible cpus to keep sysctl output clean. */
 	cpumask_and(&watchdog_cpumask, &watchdog_cpumask, cpu_possible_mask);
-	lockup_detector_reconfigure();
+	__lockup_detector_reconfigure();
 }
 
 /*
-- 
2.37.0

[PATCH v4 3/4] powerpc/watchdog: introduce a NMI watchdog's factor

[Laurent Dufour]

Introduce a factor which would apply to the NMI watchdog timeout.

This factor is a percentage added to the watchdog_tresh value. The value is
set under the watchdog_mutex protection and lockup_detector_reconfigure()
is called to recompute wd_panic_timeout_tb.

Once the factor is set, it remains until it is set back to 0, which means
no impact.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 arch/powerpc/include/asm/nmi.h |  2 ++
 arch/powerpc/kernel/watchdog.c | 21 ++++++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/nmi.h b/arch/powerpc/include/asm/nmi.h
index ea0e487f87b1..c3c7adef74de 100644
--- a/arch/powerpc/include/asm/nmi.h
+++ b/arch/powerpc/include/asm/nmi.h
@@ -5,8 +5,10 @@
 #ifdef CONFIG_PPC_WATCHDOG
 extern void arch_touch_nmi_watchdog(void);
 long soft_nmi_interrupt(struct pt_regs *regs);
+void watchdog_nmi_set_timeout_pct(u64 pct);
 #else
 static inline void arch_touch_nmi_watchdog(void) {}
+static inline void watchdog_nmi_set_timeout_pct(u64 pct) {}
 #endif
 
 #ifdef CONFIG_NMI_IPI
diff --git a/arch/powerpc/kernel/watchdog.c b/arch/powerpc/kernel/watchdog.c
index 7d28b9553654..5d903e63f932 100644
--- a/arch/powerpc/kernel/watchdog.c
+++ b/arch/powerpc/kernel/watchdog.c
@@ -91,6 +91,10 @@ static cpumask_t wd_smp_cpus_pending;
 static cpumask_t wd_smp_cpus_stuck;
 static u64 wd_smp_last_reset_tb;
 
+#ifdef CONFIG_PPC_PSERIES
+static u64 wd_timeout_pct;
+#endif
+
 /*
  * Try to take the exclusive watchdog action / NMI IPI / printing lock.
  * wd_smp_lock must be held. If this fails, we should return and wait
@@ -527,7 +531,13 @@ static int stop_watchdog_on_cpu(unsigned int cpu)
 
 static void watchdog_calc_timeouts(void)
 {
-	wd_panic_timeout_tb = watchdog_thresh * ppc_tb_freq;
+	u64 threshold = watchdog_thresh;
+
+#ifdef CONFIG_PPC_PSERIES
+	threshold += (READ_ONCE(wd_timeout_pct) * threshold) / 100;
+#endif
+
+	wd_panic_timeout_tb = threshold * ppc_tb_freq;
 
 	/* Have the SMP detector trigger a bit later */
 	wd_smp_panic_timeout_tb = wd_panic_timeout_tb * 3 / 2;
@@ -570,3 +580,12 @@ int __init watchdog_nmi_probe(void)
 	}
 	return 0;
 }
+
+#ifdef CONFIG_PPC_PSERIES
+void watchdog_nmi_set_timeout_pct(u64 pct)
+{
+	pr_info("Set the NMI watchdog timeout factor to %llu%%\n", pct);
+	WRITE_ONCE(wd_timeout_pct, pct);
+	lockup_detector_reconfigure();
+}
+#endif
-- 
2.37.0

[PATCH v4 4/4] pseries/mobility: set NMI watchdog factor during LPM

[Laurent Dufour]

During a LPM, while the memory transfer is in progress on the arrival side,
some latencies is generated when accessing not yet transferred pages on the
arrival side. Thus, the NMI watchdog may be triggered too frequently, which
increases the risk to hit a NMI interrupt in a bad place in the kernel,
leading to a kernel panic.

Disabling the Hard Lockup Watchdog until the memory transfer could be a too
strong work around, some users would want this timeout to be eventually
triggered if the system is hanging even during LPM.

Introduce a new sysctl variable nmi_watchdog_factor. It allows to apply
a factor to the NMI watchdog timeout during a LPM. Just before the CPU are
stopped for the switchover sequence, the NMI watchdog timer is set to
 watchdog_tresh + factor%

A value of 0 has no effect. The default value is 200, meaning that the NMI
watchdog is set to 30s during LPM (based on a 10s watchdog_tresh value).
Once the memory transfer is achieved, the factor is reset to 0.

Setting this value to a high number is like disabling the NMI watchdog
during a LPM.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
 Documentation/admin-guide/sysctl/kernel.rst | 12 ++++++
 arch/powerpc/platforms/pseries/mobility.c   | 43 +++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index ddccd1077462..0bb0b7f27e96 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -592,6 +592,18 @@ to the guest kernel command line (see
 Documentation/admin-guide/kernel-parameters.rst).
 
 
+nmi_watchdog_factor (PPC only)
+==================================
+
+Factor apply to to the NMI watchdog timeout (only when ``nmi_watchdog`` is
+set to 1). This factor represents the percentage added to
+``watchdog_thresh`` when calculating the NMI watchdog timeout during a
+LPM. The soft lockup timeout is not impacted.
+
+A value of 0 means no change. The default value is 200 meaning the NMI
+watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
+
+
 numa_balancing
 ==============
 
diff --git a/arch/powerpc/platforms/pseries/mobility.c b/arch/powerpc/platforms/pseries/mobility.c
index 6297467072e6..78535a0791f9 100644
--- a/arch/powerpc/platforms/pseries/mobility.c
+++ b/arch/powerpc/platforms/pseries/mobility.c
@@ -48,6 +48,39 @@ struct update_props_workarea {
 #define MIGRATION_SCOPE	(1)
 #define PRRN_SCOPE -2
 
+#ifdef CONFIG_PPC_WATCHDOG
+static unsigned int nmi_wd_factor = 200;
+
+#ifdef CONFIG_SYSCTL
+static struct ctl_table nmi_wd_factor_ctl_table[] = {
+	{
+		.procname	= "nmi_watchdog_factor",
+		.data		= &nmi_wd_factor,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec_minmax,
+	},
+	{}
+};
+static struct ctl_table nmi_wd_factor_sysctl_root[] = {
+	{
+		.procname       = "kernel",
+		.mode           = 0555,
+		.child          = nmi_wd_factor_ctl_table,
+	},
+	{}
+};
+
+static int __init register_nmi_wd_factor_sysctl(void)
+{
+	register_sysctl_table(nmi_wd_factor_sysctl_root);
+
+	return 0;
+}
+device_initcall(register_nmi_wd_factor_sysctl);
+#endif /* CONFIG_SYSCTL */
+#endif /* CONFIG_PPC_WATCHDOG */
+
 static int mobility_rtas_call(int token, char *buf, s32 scope)
 {
 	int rc;
@@ -702,13 +735,20 @@ static int pseries_suspend(u64 handle)
 static int pseries_migrate_partition(u64 handle)
 {
 	int ret;
+	unsigned int factor = 0;
 
+#ifdef CONFIG_PPC_WATCHDOG
+	factor = nmi_wd_factor;
+#endif
 	ret = wait_for_vasi_session_suspending(handle);
 	if (ret)
 		return ret;
 
 	vas_migration_handler(VAS_SUSPEND);
 
+	if (factor)
+		watchdog_nmi_set_timeout_pct(factor);
+
 	ret = pseries_suspend(handle);
 	if (ret == 0) {
 		post_mobility_fixup();
@@ -722,6 +762,9 @@ static int pseries_migrate_partition(u64 handle)
 	} else
 		pseries_cancel_migration(handle, ret);
 
+	if (factor)
+		watchdog_nmi_set_timeout_pct(0);
+
 	vas_migration_handler(VAS_RESUME);
 
 	return ret;
-- 
2.37.0

Re: [PATCH v4 4/4] pseries/mobility: set NMI watchdog factor during LPM

[Randy Dunlap]

Hi--

On 7/12/22 07:32, Laurent Dufour wrote:
> During a LPM, while the memory transfer is in progress on the arrival side,
> some latencies is generated when accessing not yet transferred pages on the

                 are

> arrival side. Thus, the NMI watchdog may be triggered too frequently, which
> increases the risk to hit a NMI interrupt in a bad place in the kernel,

                            an NMI

> leading to a kernel panic.
> 
> Disabling the Hard Lockup Watchdog until the memory transfer could be a too
> strong work around, some users would want this timeout to be eventually
> triggered if the system is hanging even during LPM.
> 
> Introduce a new sysctl variable nmi_watchdog_factor. It allows to apply
> a factor to the NMI watchdog timeout during a LPM. Just before the CPU are

                                              an LPM.            the CPU is

> stopped for the switchover sequence, the NMI watchdog timer is set to
>  watchdog_tresh + factor%

   watchdog_thresh

> 
> A value of 0 has no effect. The default value is 200, meaning that the NMI
> watchdog is set to 30s during LPM (based on a 10s watchdog_tresh value).

                                                    watchdog_thresh

> Once the memory transfer is achieved, the factor is reset to 0.
> 
> Setting this value to a high number is like disabling the NMI watchdog
> during a LPM.

         an LPM.

> 
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 12 ++++++
>  arch/powerpc/platforms/pseries/mobility.c   | 43 +++++++++++++++++++++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index ddccd1077462..0bb0b7f27e96 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -592,6 +592,18 @@ to the guest kernel command line (see
>  Documentation/admin-guide/kernel-parameters.rst).
>  

This entire block should be in kernel-parameters.txt, not .rst,
and it should be formatted like everything else in the .txt file.

>  
> +nmi_watchdog_factor (PPC only)
> +==================================
> +
> +Factor apply to to the NMI watchdog timeout (only when ``nmi_watchdog`` is

   Factor to apply to the NMI

> +set to 1). This factor represents the percentage added to
> +``watchdog_thresh`` when calculating the NMI watchdog timeout during a

                                                                 during an

> +LPM. The soft lockup timeout is not impacted.
> +
> +A value of 0 means no change. The default value is 200 meaning the NMI
> +watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
> +
> +
>  numa_balancing
>  ==============
>  


-- 
~Randy

Re: [PATCH v4 4/4] pseries/mobility: set NMI watchdog factor during LPM

[Laurent Dufour]

Le 12/07/2022 à 18:25, Randy Dunlap a écrit :
> Hi--
> 
> On 7/12/22 07:32, Laurent Dufour wrote:
>> During a LPM, while the memory transfer is in progress on the arrival side,
>> some latencies is generated when accessing not yet transferred pages on the
> 
>                  are
> 
>> arrival side. Thus, the NMI watchdog may be triggered too frequently, which
>> increases the risk to hit a NMI interrupt in a bad place in the kernel,
> 
>                             an NMI
> 
>> leading to a kernel panic.
>>
>> Disabling the Hard Lockup Watchdog until the memory transfer could be a too
>> strong work around, some users would want this timeout to be eventually
>> triggered if the system is hanging even during LPM.
>>
>> Introduce a new sysctl variable nmi_watchdog_factor. It allows to apply
>> a factor to the NMI watchdog timeout during a LPM. Just before the CPU are
> 
>                                               an LPM.            the CPU is
> 
>> stopped for the switchover sequence, the NMI watchdog timer is set to
>>  watchdog_tresh + factor%
> 
>    watchdog_thresh
> 
>>
>> A value of 0 has no effect. The default value is 200, meaning that the NMI
>> watchdog is set to 30s during LPM (based on a 10s watchdog_tresh value).
> 
>                                                     watchdog_thresh
> 
>> Once the memory transfer is achieved, the factor is reset to 0.
>>
>> Setting this value to a high number is like disabling the NMI watchdog
>> during a LPM.
> 
>          an LPM.
> 
>>
>> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
>> ---
>>  Documentation/admin-guide/sysctl/kernel.rst | 12 ++++++
>>  arch/powerpc/platforms/pseries/mobility.c   | 43 +++++++++++++++++++++
>>  2 files changed, 55 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
>> index ddccd1077462..0bb0b7f27e96 100644
>> --- a/Documentation/admin-guide/sysctl/kernel.rst
>> +++ b/Documentation/admin-guide/sysctl/kernel.rst
>> @@ -592,6 +592,18 @@ to the guest kernel command line (see
>>  Documentation/admin-guide/kernel-parameters.rst).
>>  
> 
> This entire block should be in kernel-parameters.txt, not .rst,
> and it should be formatted like everything else in the .txt file.

Thanks for reviewing this patch.

I'll apply your requests in the next version.

However, regarding the change in kernel-parameters.txt, I'm confused. The
newly introduced parameter is only exposed through sysctl. Not as a kernel
boot option. In that case, should it be mentioned in kernel-parameters.txt?

Documentation/process/4.Coding.rst says:
The file :ref:`Documentation/admin-guide/kernel-parameters.rst
<kernelparameters>` describes all of the kernel's boot-time parameters.
Any patch which adds new parameters should add the appropriate entries to
this file.

And Documentation/process/submit-checklist.rst says:
16) All new kernel boot parameters are documented in
    ``Documentation/admin-guide/kernel-parameters.rst``.

What are the rules about editing .txt or .rst files?

>>  
>> +nmi_watchdog_factor (PPC only)
>> +==================================
>> +
>> +Factor apply to to the NMI watchdog timeout (only when ``nmi_watchdog`` is
> 
>    Factor to apply to the NMI
> 
>> +set to 1). This factor represents the percentage added to
>> +``watchdog_thresh`` when calculating the NMI watchdog timeout during a
> 
>                                                                  during an
> 
>> +LPM. The soft lockup timeout is not impacted.
>> +
>> +A value of 0 means no change. The default value is 200 meaning the NMI
>> +watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
>> +
>> +
>>  numa_balancing
>>  ==============
>>  
> 
> 

Re: [PATCH v4 4/4] pseries/mobility: set NMI watchdog factor during LPM

[Randy Dunlap]

Hi,

On 7/13/22 03:56, Laurent Dufour wrote:
> Le 12/07/2022 à 18:25, Randy Dunlap a écrit :
>> Hi--
>>
>> On 7/12/22 07:32, Laurent Dufour wrote:

>>>
>>> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
>>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
>>> ---
>>>  Documentation/admin-guide/sysctl/kernel.rst | 12 ++++++
>>>  arch/powerpc/platforms/pseries/mobility.c   | 43 +++++++++++++++++++++
>>>  2 files changed, 55 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
>>> index ddccd1077462..0bb0b7f27e96 100644
>>> --- a/Documentation/admin-guide/sysctl/kernel.rst
>>> +++ b/Documentation/admin-guide/sysctl/kernel.rst
>>> @@ -592,6 +592,18 @@ to the guest kernel command line (see
>>>  Documentation/admin-guide/kernel-parameters.rst).
>>>  
>>
>> This entire block should be in kernel-parameters.txt, not .rst,
>> and it should be formatted like everything else in the .txt file.

My apologies. I misread the file name.
I don't see a problem with this part of the patch or its location.

> Thanks for reviewing this patch.
> 
> I'll apply your requests in the next version.
> 
> However, regarding the change in kernel-parameters.txt, I'm confused. The
> newly introduced parameter is only exposed through sysctl. Not as a kernel
> boot option. In that case, should it be mentioned in kernel-parameters.txt?
> 
> Documentation/process/4.Coding.rst says:
> The file :ref:`Documentation/admin-guide/kernel-parameters.rst
> <kernelparameters>` describes all of the kernel's boot-time parameters.
> Any patch which adds new parameters should add the appropriate entries to
> this file.
> 
> And Documentation/process/submit-checklist.rst says:
> 16) All new kernel boot parameters are documented in
>     ``Documentation/admin-guide/kernel-parameters.rst``.
> 
> What are the rules about editing .txt or .rst files?

Yeah, that's a little confusing.
kernel-parameters.txt in included in kernel-parameters.rst when
'make htmldocs' is run, so the produced output looks like it is from
the .rst file.

Kernel boot parameters should be added to the .txt file.
The .rst file is just intro material.

Thanks.

-- 
~Randy