Re: [Intel-gfx] [PATCH v2 2/2] drm/i915/pmu: Fix CPU hotplug with multiple GPUs

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 20/10/2020 17:19, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2020-10-20 17:11:44)
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Since we keep a driver global mask of online CPUs and base the decision
>> whether PMU needs to be migrated upon it, we need to make sure the
>> migration is done for all registered PMUs (so GPUs).
>>
>> To do this we need to track the current CPU for each PMU and base the
>> decision on whether to migrate on a comparison between global and local
>> state.
>>
>> At the same time, since dynamic CPU hotplug notification slots are a
>> scarce resource and given how we already register the multi instance type
>> state, we can and should add multiple instance of the i915 PMU to this
>> same state and not allocate a new one for every GPU.
>>
>> v2:
>>   * Use pr_notice. (Chris)
>>
>> v3:
>>   * Handle a nasty interaction where unregistration which triggers a false
>>     CPU offline event. (Chris)
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Suggested-by: Daniel Vetter <daniel.vetter@intel.com> # dynamic slot optimisation
>> Cc: Chris Wilson <chris@chris-wilson.co.uk>
>> ---
>>   drivers/gpu/drm/i915/i915_pci.c |  7 +++-
>>   drivers/gpu/drm/i915/i915_pmu.c | 57 +++++++++++++++++++++------------
>>   drivers/gpu/drm/i915/i915_pmu.h |  6 +++-
>>   3 files changed, 48 insertions(+), 22 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/i915_pci.c b/drivers/gpu/drm/i915/i915_pci.c
>> index 27964ac0638a..a384f51c91c1 100644
>> --- a/drivers/gpu/drm/i915/i915_pci.c
>> +++ b/drivers/gpu/drm/i915/i915_pci.c
>> @@ -1150,9 +1150,13 @@ static int __init i915_init(void)
>>                  return 0;
>>          }
>>   
>> +       i915_pmu_init();
>> +
>>          err = pci_register_driver(&i915_pci_driver);
>> -       if (err)
>> +       if (err) {
>> +               i915_pmu_exit();
>>                  return err;
> 
> We could do an onion bhaji and call i915_globals_exit() as well.
> 
>> +       }
>>   
>>          i915_perf_sysctl_register();
>>          return 0;
>> @@ -1166,6 +1170,7 @@ static void __exit i915_exit(void)
>>          i915_perf_sysctl_unregister();
>>          pci_unregister_driver(&i915_pci_driver);
>>          i915_globals_exit();
>> +       i915_pmu_exit();
> 
> pmu_exit then globals_exit to pair with i915_init?

Maybe later, sorry.

>>   }
>>   
>>   module_init(i915_init);
>> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
>> index 51ed7d0efcdc..cd786ad12be7 100644
>> --- a/drivers/gpu/drm/i915/i915_pmu.c
>> +++ b/drivers/gpu/drm/i915/i915_pmu.c
>> @@ -30,6 +30,7 @@
>>   #define ENGINE_SAMPLE_BITS (1 << I915_PMU_SAMPLE_BITS)
>>   
>>   static cpumask_t i915_pmu_cpumask;
>> +static unsigned int i915_pmu_target_cpu = -1;
>>   
>>   static u8 engine_config_sample(u64 config)
>>   {
>> @@ -1049,25 +1050,39 @@ static int i915_pmu_cpu_online(unsigned int cpu, struct hlist_node *node)
>>   static int i915_pmu_cpu_offline(unsigned int cpu, struct hlist_node *node)
>>   {
>>          struct i915_pmu *pmu = hlist_entry_safe(node, typeof(*pmu), cpuhp.node);
>> -       unsigned int target;
>> +       unsigned int target = i915_pmu_target_cpu;
>>   
>>          GEM_BUG_ON(!pmu->base.event_init);
>>   
>> +       /*
>> +        * Unregistering an instance generates a CPU offline event which we must
>> +        * ignore to avoid incorrectly modifying the shared i915_pmu_cpumask.
>> +        */
>> +       if (pmu->closed)
>> +               return 0;
>> +
>>          if (cpumask_test_and_clear_cpu(cpu, &i915_pmu_cpumask)) {
>>                  target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
>> +
>>                  /* Migrate events if there is a valid target */
>>                  if (target < nr_cpu_ids) {
>>                          cpumask_set_cpu(target, &i915_pmu_cpumask);
>> -                       perf_pmu_migrate_context(&pmu->base, cpu, target);
>> +                       i915_pmu_target_cpu = target;
>>                  }
>>          }
>>   
>> +       if (target < nr_cpu_ids && target != pmu->cpuhp.cpu) {
>> +               perf_pmu_migrate_context(&pmu->base, cpu, target);
>> +               pmu->cpuhp.cpu = target;
>> +       }
>> +
>>          return 0;
>>   }
>>   
>> -static int i915_pmu_register_cpuhp_state(struct i915_pmu *pmu)
>> +static enum cpuhp_state cpuhp_slot = CPUHP_INVALID;
>> +
>> +void i915_pmu_init(void)
>>   {
>> -       enum cpuhp_state slot;
>>          int ret;
>>   
>>          ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN,
>> @@ -1075,27 +1090,29 @@ static int i915_pmu_register_cpuhp_state(struct i915_pmu *pmu)
>>                                        i915_pmu_cpu_online,
>>                                        i915_pmu_cpu_offline);
>>          if (ret < 0)
>> -               return ret;
>> +               pr_notice("Failed to setup cpuhp state for i915 PMU! (%d)\n",
>> +                         ret);
>> +       else
>> +               cpuhp_slot = ret;
>> +}
>>   
>> -       slot = ret;
>> -       ret = cpuhp_state_add_instance(slot, &pmu->cpuhp.node);
>> -       if (ret) {
>> -               cpuhp_remove_multi_state(slot);
>> -               return ret;
>> -       }
>> +void i915_pmu_exit(void)
>> +{
>> +       if (cpuhp_slot != CPUHP_INVALID)
>> +               cpuhp_remove_multi_state(cpuhp_slot);
>> +}
>>   
>> -       pmu->cpuhp.slot = slot;
>> -       return 0;
>> +static int i915_pmu_register_cpuhp_state(struct i915_pmu *pmu)
>> +{
>> +       if (cpuhp_slot == CPUHP_INVALID)
>> +               return -EINVAL;
>> +
>> +       return cpuhp_state_add_instance(cpuhp_slot, &pmu->cpuhp.node);
>>   }
>>   
>>   static void i915_pmu_unregister_cpuhp_state(struct i915_pmu *pmu)
>>   {
>> -       struct drm_i915_private *i915 = container_of(pmu, typeof(*i915), pmu);
>> -
>> -       drm_WARN_ON(&i915->drm, pmu->cpuhp.slot == CPUHP_INVALID);
>> -       drm_WARN_ON(&i915->drm, cpuhp_state_remove_instance(pmu->cpuhp.slot, &pmu->cpuhp.node));
>> -       cpuhp_remove_multi_state(pmu->cpuhp.slot);
>> -       pmu->cpuhp.slot = CPUHP_INVALID;
>> +       cpuhp_state_remove_instance(cpuhp_slot, &pmu->cpuhp.node);
>>   }
>>   
>>   static bool is_igp(struct drm_i915_private *i915)
>> @@ -1129,7 +1146,7 @@ void i915_pmu_register(struct drm_i915_private *i915)
>>          spin_lock_init(&pmu->lock);
>>          hrtimer_init(&pmu->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
>>          pmu->timer.function = i915_sample;
>> -       pmu->cpuhp.slot = CPUHP_INVALID;
>> +       pmu->cpuhp.cpu = -1;
> 
> To the best of my limited understanding of cpuhp,
> Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

Pushed these two, having declared shard runs flip-flops are unrelated. 
Thanks!

Regards,

Tvrtko
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