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

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 14/02/2018 19:17, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2018-02-14 18:50:34)
> > From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >
> > 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@xxxxxxxxx>
> > ---
> >   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
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