[PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[Kanaka Juvva]

Added lock in event reader function. The cqm_pick_event_reader() function
accesses cqm_cpumask and it is critical section between this and
cqm_stable().

This situation is true when a CPU is hotplugged. Mutex is used to protect
the critical section.

Signed-off-by: Kanaka Juvva <kanaka.d.juvva@linux.intel.com>
---
 arch/x86/kernel/cpu/perf_event_intel_cqm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
index 1880761..e17e37f 100644
--- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
+++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
@@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
 	int phys_id = topology_physical_package_id(cpu);
 	int i;
 
+	mutex_lock(&cache_mutex);
 	for_each_cpu(i, &cqm_cpumask) {
 		if (phys_id == topology_physical_package_id(i))
-			return;	/* already got reader for this socket */
+			goto out; /* already got reader for this socket */
 	}
 
 	cpumask_set_cpu(cpu, &cqm_cpumask);
+out:
+	mutex_unlock(&cache_mutex);
 }
 
 static void intel_cqm_cpu_prepare(unsigned int cpu)
-- 
2.1.0

Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[Thomas Gleixner]

On Thu, 18 Jun 2015, Kanaka Juvva wrote:

> Added lock in event reader function. The cqm_pick_event_reader() function
> accesses cqm_cpumask and it is critical section between this and
> cqm_stable().
> 
> This situation is true when a CPU is hotplugged. Mutex is used to protect
> the critical section.
> 
> Signed-off-by: Kanaka Juvva <kanaka.d.juvva@linux.intel.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_cqm.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> index 1880761..e17e37f 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
> @@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
>  	int phys_id = topology_physical_package_id(cpu);
>  	int i;
>  
> +	mutex_lock(&cache_mutex);

I already explained it to Vikas. You CANNOT take a mutex in that code
path as it runs with interrupts disabled on a CPU which cannot
schedule.

Sigh.

	tglx

Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[Dave Hansen]

On 06/18/2015 12:47 PM, Thomas Gleixner wrote:
>> > @@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
>> >  	int phys_id = topology_physical_package_id(cpu);
>> >  	int i;
>> >  
>> > +	mutex_lock(&cache_mutex);
> I already explained it to Vikas. You CANNOT take a mutex in that code
> path as it runs with interrupts disabled on a CPU which cannot
> schedule.

How did lockdep not blow up and scream about this?

Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[Thomas Gleixner]

On Thu, 18 Jun 2015, Dave Hansen wrote:
> On 06/18/2015 12:47 PM, Thomas Gleixner wrote:
> >> > @@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
> >> >  	int phys_id = topology_physical_package_id(cpu);
> >> >  	int i;
> >> >  
> >> > +	mutex_lock(&cache_mutex);
> > I already explained it to Vikas. You CANNOT take a mutex in that code
> > path as it runs with interrupts disabled on a CPU which cannot
> > schedule.
> 
> How did lockdep not blow up and scream about this?

Dunno. I did not test that stuff.

The might_sleep() in mutex_lock() should catch it as well, IF that
code was ever executed AFTER boot on a running system.

Thanks,

	tglx

Re: [PATCH v1 2/2] x86, perf,cqm: handle CPU hotplug

[Vikas Shivappa]

On Thu, 18 Jun 2015, Thomas Gleixner wrote:

> On Thu, 18 Jun 2015, Kanaka Juvva wrote:
>
>> Added lock in event reader function. The cqm_pick_event_reader() function
>> accesses cqm_cpumask and it is critical section between this and
>> cqm_stable().
>>
>> This situation is true when a CPU is hotplugged. Mutex is used to protect
>> the critical section.
>>
>> Signed-off-by: Kanaka Juvva <kanaka.d.juvva@linux.intel.com>
>> ---
>>  arch/x86/kernel/cpu/perf_event_intel_cqm.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/cpu/perf_event_intel_cqm.c b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
>> index 1880761..e17e37f 100644
>> --- a/arch/x86/kernel/cpu/perf_event_intel_cqm.c
>> +++ b/arch/x86/kernel/cpu/perf_event_intel_cqm.c
>> @@ -1239,12 +1239,15 @@ static inline void cqm_pick_event_reader(int cpu)
>>  	int phys_id = topology_physical_package_id(cpu);
>>  	int i;
>>
>> +	mutex_lock(&cache_mutex);
>
> I already explained it to Vikas. You CANNOT take a mutex in that code
> path as it runs with interrupts disabled on a CPU which cannot
> schedule.

This patch also needs to be merged with the new package mask changes that 
was added. 
Will merge and send the fix.

Thanks,
Vikas

>
> Sigh.
>
> 	tglx
>