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

Tvrtko Ursulin
2018-02-15T09:41:56Z

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.

Regards,

Tvrtko