powerpc/perf: Fix for core/nest imc call trace on cpuhotplug

Message ID 1505151134-26128-1-git-send-email-anju@linux.vnet.ibm.com
State Changes Requested
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Series
  • powerpc/perf: Fix for core/nest imc call trace on cpuhotplug
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Commit Message

Anju T Sudhakar Sept. 11, 2017, 5:32 p.m.
Nest/core pmu units are enabled only when it is used. A reference count is
maintained for the events which uses the nest/core pmu units. Currently in
*_imc_counters_release function a WARN() is used for notification of any
underflow of ref count. Replace WARN() with a pr_info since it is an overkill.

Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Acked-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
---
 arch/powerpc/perf/imc-pmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Michael Ellerman Sept. 21, 2017, 4:34 a.m. | #1
Anju T Sudhakar <anju@linux.vnet.ibm.com> writes:

> Nest/core pmu units are enabled only when it is used. A reference count is
> maintained for the events which uses the nest/core pmu units. Currently in
> *_imc_counters_release function a WARN() is used for notification of any
> underflow of ref count. Replace WARN() with a pr_info since it is an overkill.

As discussed elsewhere this is not the right solution.

If it's OK for the reference count to be negative, then we shouldn't
print anything when it is.

But I don't understand how it can be OK for the refcount to be negative.
That means someone has a negative number of references to something?

cheers
Anju T Sudhakar Sept. 25, 2017, 11:52 a.m. | #2
Hi mpe,


On Thursday 21 September 2017 10:04 AM, Michael Ellerman wrote:
> Anju T Sudhakar <anju@linux.vnet.ibm.com> writes:
>
>> Nest/core pmu units are enabled only when it is used. A reference count is
>> maintained for the events which uses the nest/core pmu units. Currently in
>> *_imc_counters_release function a WARN() is used for notification of any
>> underflow of ref count. Replace WARN() with a pr_info since it is an overkill.
> As discussed elsewhere this is not the right solution.
>
> If it's OK for the reference count to be negative, then we shouldn't
> print anything when it is.
>
> But I don't understand how it can be OK for the refcount to be negative.
> That means someone has a negative number of references to something?
>
> cheers
>

Scenario where this happens is in a stress test where perf session is 
started, followed by offlining of all cpus in a given core. And finally 
terminate the perf session.

So, in cpuhotplug offline path(ppc_core_imc_cpu_offline), function set 
the ref->count to zero, if the current cpu which is about to offline is 
the last cpu in a given core and make an OPAL call to disable the engine 
in that core.
And on perf session termination, perf->destory 
(core_imc_counters_release) will first decrement the ref->count for this 
core and based on the ref->count value an opal call is made to disable 
the core-imc engine.

Now, since cpuhotplug path already clears the ref->count for core and 
disabled the engine, perf->destroy() decrementing again at event 
termination make it negative which in turn fires the WARN_ON.

So we do prefer to remove the message as this wont happen in normal 
operation and the core counters are working as expected.
I will send out a patch by removing the message asap.


Thanks,
Anju

Patch

diff --git a/arch/powerpc/perf/imc-pmu.c b/arch/powerpc/perf/imc-pmu.c
index 9ccac86..4d523d0 100644
--- a/arch/powerpc/perf/imc-pmu.c
+++ b/arch/powerpc/perf/imc-pmu.c
@@ -409,7 +409,7 @@  static void nest_imc_counters_release(struct perf_event *event)
 			return;
 		}
 	} else if (ref->refc < 0) {
-		WARN(1, "nest-imc: Invalid event reference count\n");
+		pr_info("nest-imc: Invalid event reference count\n");
 		ref->refc = 0;
 	}
 	mutex_unlock(&ref->lock);
@@ -656,7 +656,7 @@  static void core_imc_counters_release(struct perf_event *event)
 			return;
 		}
 	} else if (ref->refc < 0) {
-		WARN(1, "core-imc: Invalid event reference count\n");
+		pr_info("core-imc: Invalid event reference count\n");
 		ref->refc = 0;
 	}
 	mutex_unlock(&ref->lock);