Re: [RFC 4/5] drm/i915: Expose per-engine client busyness

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

On 15/02/2018 09:44, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-02-15 09:41:53)
>>
>> On 14/02/2018 19:17, Chris Wilson wrote:
>>> Quoting Tvrtko Ursulin (2018-02-14 18:50:34)
>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>
>>>> Expose per-client and per-engine busyness under the previously added sysfs
>>>> client root.
>>>>
>>>> The new files are one per-engine instance and located under the 'busy'
>>>> directory.
>>>>
>>>> Each contains a monotonically increasing nano-second resolution times each
>>>> client's jobs were executing on the GPU.
>>>>
>>>> $ cat /sys/class/drm/card0/clients/5/busy/rcs0
>>>> 32516602
>>>>
>>>> This data can serve as an interface to implement a top like utility for
>>>> GPU jobs. For instance I have prototyped a tool in IGT which produces
>>>> periodic output like:
>>>>
>>>> neverball[  6011]:  rcs0:  41.01%  bcs0:   0.00%  vcs0:   0.00%  vecs0:   0.00%
>>>>        Xorg[  5664]:  rcs0:  31.16%  bcs0:   0.00%  vcs0:   0.00%  vecs0:   0.00%
>>>>       xfwm4[  5727]:  rcs0:   0.00%  bcs0:   0.00%  vcs0:   0.00%  vecs0:   0.00%
>>>>
>>>> This tools can also be extended to use the i915 PMU and show overall engine
>>>> busyness, and engine loads using the queue depth metric.
>>>>
>>>> v2: Use intel_context_engine_get_busy_time.
>>>> v3: New directory structure.
>>>>
>>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>> ---
>>>>    drivers/gpu/drm/i915/i915_drv.h |  8 ++++
>>>>    drivers/gpu/drm/i915/i915_gem.c | 86 +++++++++++++++++++++++++++++++++++++++--
>>>>    2 files changed, 91 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
>>>> index 372d13cb2472..d6b2883b42fe 100644
>>>> --- a/drivers/gpu/drm/i915/i915_drv.h
>>>> +++ b/drivers/gpu/drm/i915/i915_drv.h
>>>> @@ -315,6 +315,12 @@ struct drm_i915_private;
>>>>    struct i915_mm_struct;
>>>>    struct i915_mmu_object;
>>>>    
>>>> +struct i915_engine_busy_attribute {
>>>> +       struct device_attribute attr;
>>>> +       struct drm_i915_file_private *file_priv;
>>>> +       struct intel_engine_cs *engine;
>>>> +};
>>>> +
>>>>    struct drm_i915_file_private {
>>>>           struct drm_i915_private *dev_priv;
>>>>           struct drm_file *file;
>>>> @@ -350,10 +356,12 @@ struct drm_i915_file_private {
>>>>           unsigned int client_pid;
>>>>           char *client_name;
>>>>           struct kobject *client_root;
>>>> +       struct kobject *busy_root;
>>>>    
>>>>           struct {
>>>>                   struct device_attribute pid;
>>>>                   struct device_attribute name;
>>>> +               struct i915_engine_busy_attribute busy[I915_NUM_ENGINES];
>>>>           } attr;
>>>>    };
>>>>    
>>>> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
>>>> index 46ac7b3ca348..01298d924524 100644
>>>> --- a/drivers/gpu/drm/i915/i915_gem.c
>>>> +++ b/drivers/gpu/drm/i915/i915_gem.c
>>>> @@ -5631,6 +5631,45 @@ show_client_pid(struct device *kdev, struct device_attribute *attr, char *buf)
>>>>           return snprintf(buf, PAGE_SIZE, "%u", file_priv->client_pid);
>>>>    }
>>>>    
>>>> +struct busy_ctx {
>>>> +       struct intel_engine_cs *engine;
>>>> +       u64 total;
>>>> +};
>>>> +
>>>> +static int busy_add(int _id, void *p, void *data)
>>>> +{
>>>> +       struct i915_gem_context *ctx = p;
>>>> +       struct busy_ctx *bc = data;
>>>> +
>>>> +       bc->total +=
>>>> +               ktime_to_ns(intel_context_engine_get_busy_time(ctx,
>>>> +                                                              bc->engine));
>>>> +
>>>> +       return 0;
>>>> +}
>>>> +
>>>> +static ssize_t
>>>> +show_client_busy(struct device *kdev, struct device_attribute *attr, char *buf)
>>>> +{
>>>> +       struct i915_engine_busy_attribute *i915_attr =
>>>> +               container_of(attr, typeof(*i915_attr), attr);
>>>> +       struct drm_i915_file_private *file_priv = i915_attr->file_priv;
>>>> +       struct intel_engine_cs *engine = i915_attr->engine;
>>>> +       struct drm_i915_private *i915 = engine->i915;
>>>> +       struct busy_ctx bc = { .engine = engine };
>>>> +       int ret;
>>>> +
>>>> +       ret = i915_mutex_lock_interruptible(&i915->drm);
>>>> +       if (ret)
>>>> +               return ret;
>>>> +
>>>
>>> Doesn't need struct_mutex, just rcu_read_lock() will suffice.
>>>
>>> Neither the context nor idr will be freed too soon, and the data is
>>> involatile when the context is unreffed (and contexts don't have the
>>> nasty zombie/undead status of requests). So the busy-time will be
>>> stable.
>>
>> Are you sure? What holds a reference to contexts while userspace might
>> by in sysfs reading the stat? It would be super nice if we could avoid
>> struct mutex here.. I just don't understand at the moment why it would
>> be safe.
> 
> RCU keeps the pointer alive, and in this case the context is not reused
> before being freed (unlike requests). So given that it's unref, it is
> dead and the stats will be stable, just not yet freed.

Somehow I missed the suggestion to replace with rcu_read_lock. Cool, 
this is then even more light-weight than I imagined.

Regards,

Tvrtko
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