Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-30 Thread Don Zickus 
On Fri, Jul 25, 2014 at 01:25:11PM +0200, Andrew Jones wrote:
  to enable hard lockup detection explicitly.
  
  I think changing the 'watchdog_thresh' while 'watchdog_running' is true 
  should
  _not_ enable hard lockup detection as a side-effect, because a user may 
  have a
  'sysctl.conf' entry such as
  
 kernel.watchdog_thresh = ...
  
  or may only want to change the 'watchdog_thresh' on the fly.
  
  I think the following flow of execution could cause such undesired 
  side-effect.
  
 proc_dowatchdog
   if (watchdog_user_enabled  watchdog_thresh) {
  
   watchdog_enable_hardlockup_detector
 hardlockup_detector_enabled = true
  
   watchdog_enable_all_cpus
 if (!watchdog_running) {
 ...
 } else if (sample_period_changed)
update_timers_all_cpus
  for_each_online_cpu
  update_timers
watchdog_nmi_disable
...
watchdog_nmi_enable
  
  watchdog_hardlockup_detector_is_enabled
return true
  
  enable perf counter for hard lockup 
  detection
  
  Regards,
  
  Uli
 
 Nice catch. Looks like this will need a v2. Paolo, do we have a
 consensus on the proc echoing? Or should that be revisited in the v2 as
 well?

As discussed privately, how about something like this to handle that case:
(applied on top of these patches)

Cheers,
Don

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 34eca29..027fb6c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -666,7 +666,12 @@ int proc_dowatchdog(struct ctl_table *table, int write,
 * watchdog_*_all_cpus() function takes care of this.
 */
if (watchdog_user_enabled  watchdog_thresh) {
-   watchdog_enable_hardlockup_detector(true);
+   /*
+* Prevent a change in watchdog_thresh accidentally overriding
+* the enablement of the hardlockup detector.
+*/
+   if (watchdog_user_enabled != old_enabled)
+   watchdog_enable_hardlockup_detector(true);
err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
} else
watchdog_disable_all_cpus();

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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-30 Thread Paolo Bonzini 
Il 30/07/2014 15:43, Don Zickus ha scritto:
  Nice catch. Looks like this will need a v2. Paolo, do we have a
  consensus on the proc echoing? Or should that be revisited in the v2 as
  well?
 As discussed privately, how about something like this to handle that case:
 (applied on top of these patches)

Don, what do you think about proc?

My opinion is still what I mentioned earlier in the thread, i.e. that if
the file says 1, writing 0 and then 1 should not constitute a
change WRT to the initial state.

Paolo
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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-30 Thread Don Zickus 
On Wed, Jul 30, 2014 at 04:16:38PM +0200, Paolo Bonzini wrote:
 Il 30/07/2014 15:43, Don Zickus ha scritto:
   Nice catch. Looks like this will need a v2. Paolo, do we have a
   consensus on the proc echoing? Or should that be revisited in the v2 as
   well?
  As discussed privately, how about something like this to handle that case:
  (applied on top of these patches)
 
 Don, what do you think about proc?
 
 My opinion is still what I mentioned earlier in the thread, i.e. that if
 the file says 1, writing 0 and then 1 should not constitute a
 change WRT to the initial state.
 

I can agree.  The problem is there are two things this proc value
controls, softlockup and hardlockup.  I have always tried to keep the both
disabled or enabled together.

This patchset tries to separate them for an edge case.  Hence the proc
value becomes slightly confusing.

I don't know the right way to solve this without introducing more proc
values.

We have /proc/sys/kernel/nmi_watchdog and /proc/sys/kernel/watchdog which
point to the same internal variable.  Do I separate them and have
'nmi_watchdog' just mean hardlockup and 'watchdog' mean softlockup?  Then
we can be clear on what the output is.  Or does 'watchdog' represent a
superset of 'nmi_watchdog'  softlockup?

That is where the confusion lies.

Cheers,
Don

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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-25 Thread Ulrich Obergfell 
 - Original Message -
 From: Andrew Jones drjo...@redhat.com
 To: linux-ker...@vger.kernel.org, kvm@vger.kernel.org
 Cc: uober...@redhat.com, dzic...@redhat.com, pbonz...@redhat.com, 
 a...@linux-foundation.org, mi...@redhat.com
 Sent: Thursday, July 24, 2014 12:13:30 PM
 Subject: [PATCH 2/3] watchdog: control hard lockup detection default

[...]

 The running kernel still has the ability to enable/disable at any
 time with /proc/sys/kernel/nmi_watchdog us usual. However even
 when the default has been overridden /proc/sys/kernel/nmi_watchdog
 will initially show '1'. To truly turn it on one must disable/enable
 it, i.e.
   echo 0  /proc/sys/kernel/nmi_watchdog
   echo 1  /proc/sys/kernel/nmi_watchdog

[...]

 @@ -626,15 +665,17 @@ int proc_dowatchdog(struct ctl_table *table, int write,
  * disabled. The 'watchdog_running' variable check in
  * watchdog_*_all_cpus() function takes care of this.
  */
 -if (watchdog_user_enabled  watchdog_thresh)
 +if (watchdog_user_enabled  watchdog_thresh) {
 +watchdog_enable_hardlockup_detector(true);
  err = watchdog_enable_all_cpus(old_thresh != 
 watchdog_thresh);
 -else
 +} else

[...]


I just realized a possible issue in the above part of the patch:

If we would want to give the user the option to override the effect of patch 3/3
via /proc, I think proc_dowatchdog() should enable hard lockup detection _only_
in case of a state transition from 'NOT watchdog_running' to 'watchdog_running'.
  |
   if (watchdog_user_enabled  watchdog_thresh) {| need to add this
   if (!watchdog_running) ---'
   watchdog_enable_hardlockup_detector(true);
   err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
   } else
   ...

The additional 'if (!watchdog_running)' would _require_ the user to perform the
sequence of commands

   echo 0  /proc/sys/kernel/nmi_watchdog
   echo 1  /proc/sys/kernel/nmi_watchdog

to enable hard lockup detection explicitly.

I think changing the 'watchdog_thresh' while 'watchdog_running' is true should
_not_ enable hard lockup detection as a side-effect, because a user may have a
'sysctl.conf' entry such as

   kernel.watchdog_thresh = ...

or may only want to change the 'watchdog_thresh' on the fly.

I think the following flow of execution could cause such undesired side-effect.

   proc_dowatchdog
 if (watchdog_user_enabled  watchdog_thresh) {

 watchdog_enable_hardlockup_detector
   hardlockup_detector_enabled = true

 watchdog_enable_all_cpus
   if (!watchdog_running) {
   ...
   } else if (sample_period_changed)
  update_timers_all_cpus
for_each_online_cpu
update_timers
  watchdog_nmi_disable
  ...
  watchdog_nmi_enable

watchdog_hardlockup_detector_is_enabled
  return true

enable perf counter for hard lockup detection

Regards,

Uli
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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-25 Thread Andrew Jones 
On Fri, Jul 25, 2014 at 04:32:55AM -0400, Ulrich Obergfell wrote:
  - Original Message -
  From: Andrew Jones drjo...@redhat.com
  To: linux-ker...@vger.kernel.org, kvm@vger.kernel.org
  Cc: uober...@redhat.com, dzic...@redhat.com, pbonz...@redhat.com, 
  a...@linux-foundation.org, mi...@redhat.com
  Sent: Thursday, July 24, 2014 12:13:30 PM
  Subject: [PATCH 2/3] watchdog: control hard lockup detection default
 
 [...]
 
  The running kernel still has the ability to enable/disable at any
  time with /proc/sys/kernel/nmi_watchdog us usual. However even
  when the default has been overridden /proc/sys/kernel/nmi_watchdog
  will initially show '1'. To truly turn it on one must disable/enable
  it, i.e.
echo 0  /proc/sys/kernel/nmi_watchdog
echo 1  /proc/sys/kernel/nmi_watchdog
 
 [...]
 
  @@ -626,15 +665,17 @@ int proc_dowatchdog(struct ctl_table *table, int 
  write,
   * disabled. The 'watchdog_running' variable check in
   * watchdog_*_all_cpus() function takes care of this.
   */
  -if (watchdog_user_enabled  watchdog_thresh)
  +if (watchdog_user_enabled  watchdog_thresh) {
  +watchdog_enable_hardlockup_detector(true);
   err = watchdog_enable_all_cpus(old_thresh != 
  watchdog_thresh);
  -else
  +} else
 
 [...]
 
 
 I just realized a possible issue in the above part of the patch:
 
 If we would want to give the user the option to override the effect of patch 
 3/3
 via /proc, I think proc_dowatchdog() should enable hard lockup detection 
 _only_
 in case of a state transition from 'NOT watchdog_running' to 
 'watchdog_running'.
   |
if (watchdog_user_enabled  watchdog_thresh) {| need to add this
if (!watchdog_running) ---'
watchdog_enable_hardlockup_detector(true);
err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
} else
...
 
 The additional 'if (!watchdog_running)' would _require_ the user to perform 
 the
 sequence of commands
 
echo 0  /proc/sys/kernel/nmi_watchdog
echo 1  /proc/sys/kernel/nmi_watchdog
 
 to enable hard lockup detection explicitly.
 
 I think changing the 'watchdog_thresh' while 'watchdog_running' is true should
 _not_ enable hard lockup detection as a side-effect, because a user may have a
 'sysctl.conf' entry such as
 
kernel.watchdog_thresh = ...
 
 or may only want to change the 'watchdog_thresh' on the fly.
 
 I think the following flow of execution could cause such undesired 
 side-effect.
 
proc_dowatchdog
  if (watchdog_user_enabled  watchdog_thresh) {
 
  watchdog_enable_hardlockup_detector
hardlockup_detector_enabled = true
 
  watchdog_enable_all_cpus
if (!watchdog_running) {
...
} else if (sample_period_changed)
   update_timers_all_cpus
 for_each_online_cpu
 update_timers
   watchdog_nmi_disable
   ...
   watchdog_nmi_enable
 
 watchdog_hardlockup_detector_is_enabled
   return true
 
 enable perf counter for hard lockup detection
 
 Regards,
 
 Uli

Nice catch. Looks like this will need a v2. Paolo, do we have a
consensus on the proc echoing? Or should that be revisited in the v2 as
well?

drew
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[PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Andrew Jones 
From: Ulrich Obergfell uober...@redhat.com

In some cases we don't want hard lockup detection enabled by default.
An example is when running as a guest. Introduce

  watchdog_enable_hardlockup_detector(bool)

allowing those cases to disable hard lockup detection. This must be
executed early by the boot processor from e.g. smp_prepare_boot_cpu,
in order to allow kernel command line arguments to override it, as
well as to avoid hard lockup detection being enabled before we've
had a chance to indicate that it's unwanted. In summary,

  initial boot: default=enabled
  smp_prepare_boot_cpu
watchdog_enable_hardlockup_detector(false): default=disabled
  cmdline has 'nmi_watchdog=1': default=enabled

The running kernel still has the ability to enable/disable at any
time with /proc/sys/kernel/nmi_watchdog us usual. However even
when the default has been overridden /proc/sys/kernel/nmi_watchdog
will initially show '1'. To truly turn it on one must disable/enable
it, i.e.
  echo 0  /proc/sys/kernel/nmi_watchdog
  echo 1  /proc/sys/kernel/nmi_watchdog

This patch will be immediately useful for KVM with the next patch
of this series. Other hypervisor guest types may find it useful as
well.

Signed-off-by: Ulrich Obergfell uober...@redhat.com
Signed-off-by: Andrew Jones drjo...@redhat.com
---
 include/linux/nmi.h |  9 +
 kernel/watchdog.c   | 45 +++--
 2 files changed, 52 insertions(+), 2 deletions(-)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 447775ee2c4b0..72aacf4e3d539 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -17,11 +17,20 @@
 #if defined(CONFIG_HAVE_NMI_WATCHDOG) || defined(CONFIG_HARDLOCKUP_DETECTOR)
 #include asm/nmi.h
 extern void touch_nmi_watchdog(void);
+extern void watchdog_enable_hardlockup_detector(bool val);
+extern bool watchdog_hardlockup_detector_is_enabled(void);
 #else
 static inline void touch_nmi_watchdog(void)
 {
touch_softlockup_watchdog();
 }
+static inline void watchdog_enable_hardlockup_detector(bool)
+{
+}
+static inline bool watchdog_hardlockup_detector_is_enabled(void)
+{
+   return true;
+}
 #endif
 
 /*
diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index c985a21926545..34eca29e28a4c 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -63,6 +63,25 @@ static unsigned long soft_lockup_nmi_warn;
 static int hardlockup_panic =
CONFIG_BOOTPARAM_HARDLOCKUP_PANIC_VALUE;
 
+static bool hardlockup_detector_enabled = true;
+/*
+ * We may not want to enable hard lockup detection by default in all cases,
+ * for example when running the kernel as a guest on a hypervisor. In these
+ * cases this function can be called to disable hard lockup detection. This
+ * function should only be executed once by the boot processor before the
+ * kernel command line parameters are parsed, because otherwise it is not
+ * possible to override this in hardlockup_panic_setup().
+ */
+void watchdog_enable_hardlockup_detector(bool val)
+{
+   hardlockup_detector_enabled = val;
+}
+
+bool watchdog_hardlockup_detector_is_enabled(void)
+{
+   return hardlockup_detector_enabled;
+}
+
 static int __init hardlockup_panic_setup(char *str)
 {
if (!strncmp(str, panic, 5))
@@ -71,6 +90,14 @@ static int __init hardlockup_panic_setup(char *str)
hardlockup_panic = 0;
else if (!strncmp(str, 0, 1))
watchdog_user_enabled = 0;
+   else if (!strncmp(str, 1, 1) || !strncmp(str, 2, 1)) {
+   /*
+* Setting 'nmi_watchdog=1' or 'nmi_watchdog=2' (legacy option)
+* has the same effect.
+*/
+   watchdog_user_enabled = 1;
+   watchdog_enable_hardlockup_detector(true);
+   }
return 1;
 }
 __setup(nmi_watchdog=, hardlockup_panic_setup);
@@ -451,6 +478,15 @@ static int watchdog_nmi_enable(unsigned int cpu)
struct perf_event_attr *wd_attr;
struct perf_event *event = per_cpu(watchdog_ev, cpu);
 
+   /*
+* Some kernels need to default hard lockup detection to
+* 'disabled', for example a guest on a hypervisor.
+*/
+   if (!watchdog_hardlockup_detector_is_enabled()) {
+   event = ERR_PTR(-ENOENT);
+   goto handle_err;
+   }
+
/* is it already setup and enabled? */
if (event  event-state  PERF_EVENT_STATE_OFF)
goto out;
@@ -465,6 +501,7 @@ static int watchdog_nmi_enable(unsigned int cpu)
/* Try to register using hardware perf events */
event = perf_event_create_kernel_counter(wd_attr, cpu, NULL, 
watchdog_overflow_callback, NULL);
 
+handle_err:
/* save cpu0 error for future comparision */
if (cpu == 0  IS_ERR(event))
cpu0_err = PTR_ERR(event);
@@ -610,11 +647,13 @@ int proc_dowatchdog(struct ctl_table *table, int write,
void __user *buffer,

Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Paolo Bonzini 
Il 24/07/2014 12:13, Andrew Jones ha scritto:
 
 The running kernel still has the ability to enable/disable at any
 time with /proc/sys/kernel/nmi_watchdog us usual. However even
 when the default has been overridden /proc/sys/kernel/nmi_watchdog
 will initially show '1'. To truly turn it on one must disable/enable
 it, i.e.
   echo 0  /proc/sys/kernel/nmi_watchdog
   echo 1  /proc/sys/kernel/nmi_watchdog

Why is it hard to make this show the right value? :)

Paolo

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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Ulrich Obergfell 
 - Original Message -
 From: Paolo Bonzini pbonz...@redhat.com
 To: Andrew Jones drjo...@redhat.com, linux-ker...@vger.kernel.org, 
 kvm@vger.kernel.org
 Cc: uober...@redhat.com, dzic...@redhat.com, a...@linux-foundation.org, 
 mi...@redhat.com
 Sent: Thursday, July 24, 2014 12:46:11 PM
 Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

Il 24/07/2014 12:13, Andrew Jones ha scritto:
 
 The running kernel still has the ability to enable/disable at any
 time with /proc/sys/kernel/nmi_watchdog us usual. However even
 when the default has been overridden /proc/sys/kernel/nmi_watchdog
 will initially show '1'. To truly turn it on one must disable/enable
 it, i.e.
   echo 0  /proc/sys/kernel/nmi_watchdog
   echo 1  /proc/sys/kernel/nmi_watchdog

 Why is it hard to make this show the right value? :)

 Paolo

'echo 1  /proc/sys/kernel/nmi_watchdog' enables both - hard lockup and
soft lockup detection. watchdog_enable_all_cpus() starts a 'watchdog/N'
thread for each CPU. If the kernel runs on a bare metal system where the
processor does not have a PMU, or when perf_event_create_kernel_counter()
returns failure to watchdog_nmi_enable(), or when the kernel runs as a
guest on a hypervisor that does not emulate a PMU, then the 'watchdog/N'
threads are still active for soft lockup detection. Patch 2/3 essentially
makes watchdog_nmi_enable() behave in the same way as if -ENOENT would
have been returned by perf_event_create_kernel_counter(). This is then
reported via a console message.

  NMI watchdog: disabled (cpu0): hardware events not enabled

It's hard say what _is_ 'the right value' (because lockup detection is
then enabled 'partially'), regardless of whether patch 2/3 is applied
or not.

Regards,

Uli
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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Ulrich Obergfell 
- Original Message -
From: Paolo Bonzini pbonz...@redhat.com
To: Ulrich Obergfell uober...@redhat.com
Cc: Andrew Jones drjo...@redhat.com, linux-ker...@vger.kernel.org, 
kvm@vger.kernel.org, dzic...@redhat.com, a...@linux-foundation.org, 
mi...@redhat.com
Sent: Thursday, July 24, 2014 1:26:40 PM
Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

Il 24/07/2014 13:18, Ulrich Obergfell ha scritto:
  The running kernel still has the ability to enable/disable at any
  time with /proc/sys/kernel/nmi_watchdog us usual. However even
  when the default has been overridden /proc/sys/kernel/nmi_watchdog
  will initially show '1'. To truly turn it on one must disable/enable
  it, i.e.
echo 0  /proc/sys/kernel/nmi_watchdog
echo 1  /proc/sys/kernel/nmi_watchdog
 
  Why is it hard to make this show the right value? :)
 
  Paolo
 'echo 1  /proc/sys/kernel/nmi_watchdog' enables both - hard lockup and
 soft lockup detection. watchdog_enable_all_cpus() starts a 'watchdog/N'
 thread for each CPU. If the kernel runs on a bare metal system where the
 processor does not have a PMU, or when perf_event_create_kernel_counter()
 returns failure to watchdog_nmi_enable(), or when the kernel runs as a
 guest on a hypervisor that does not emulate a PMU, then the 'watchdog/N'
 threads are still active for soft lockup detection. Patch 2/3 essentially
 makes watchdog_nmi_enable() behave in the same way as if -ENOENT would
 have been returned by perf_event_create_kernel_counter(). This is then
 reported via a console message.
 
   NMI watchdog: disabled (cpu0): hardware events not enabled
 
 It's hard say what _is_ 'the right value' (because lockup detection is
 then enabled 'partially'), regardless of whether patch 2/3 is applied
 or not.

 But this means that it is not possible to re-enable softlockup detection
 only.  I think that should be the effect of echo 0 + echo 1, if
 hardlockup detection was disabled by either the command line or patch 3.

 Paolo

The idea was to give the user two options to override the effect of patch 3/3.
Either via the kernel command line ('nmi_watchdog=') at boot time or via /proc
('echo 0' + 'echo 1') when the system is up and running.

Regards,

Uli
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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Paolo Bonzini 
Il 24/07/2014 13:44, Ulrich Obergfell ha scritto:
  But this means that it is not possible to re-enable softlockup detection
  only.  I think that should be the effect of echo 0 + echo 1, if
  hardlockup detection was disabled by either the command line or patch 3.

 The idea was to give the user two options to override the effect of patch 3/3.
 Either via the kernel command line ('nmi_watchdog=') at boot time or via /proc
 ('echo 0' + 'echo 1') when the system is up and running.

I think the kernel command line is enough; another alternative is to
split the nmi_watchdog /proc entry in two.

Paolo
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Re: [PATCH 2/3] watchdog: control hard lockup detection default

2014-07-24 Thread Ulrich Obergfell 
- Original Message -
From: Paolo Bonzini pbonz...@redhat.com
To: Ulrich Obergfell uober...@redhat.com
Cc: Andrew Jones drjo...@redhat.com, linux-ker...@vger.kernel.org, 
kvm@vger.kernel.org, dzic...@redhat.com, a...@linux-foundation.org, 
mi...@redhat.com
Sent: Thursday, July 24, 2014 1:45:47 PM
Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

Il 24/07/2014 13:44, Ulrich Obergfell ha scritto:
  But this means that it is not possible to re-enable softlockup detection
  only.  I think that should be the effect of echo 0 + echo 1, if
  hardlockup detection was disabled by either the command line or patch 3.

 The idea was to give the user two options to override the effect of patch 
 3/3.
 Either via the kernel command line ('nmi_watchdog=') at boot time or via 
 /proc
 ('echo 0' + 'echo 1') when the system is up and running.

 I think the kernel command line is enough; another alternative is to
 split the nmi_watchdog /proc entry in two.

 Paolo

The current behaviour (without the patch) already allows a user to disable
NMI watchdog at boot time ('nmi_watchdog=0') and enable it explicitly when
the system is up and running ('echo 0' + 'echo 1'). I think it would be
more consistent with this behaviour and more intuitive if we would give
the user the option to override the effect of patch 3/3 via /proc. By
'intuitive' I mean that the user says: 'I _want_ this to be enabled'.

Regards,

Uli

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