Re: [PATCH v9 7/7] drm/i915: add a sysfs entry to let users set sseu configs

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On 30/05/2018 15:33, Lionel Landwerlin wrote:
There are concerns about denial of service around the per context sseu
configuration capability. In a previous commit introducing the
capability we allowed it only for capable users. This changes adds a
new debugfs entry to let any user configure its own context
powergating setup.

As far as I understood it, Joonas' concerns here are:

1) That in the containers use case individual containers wouldn't be able to turn on the sysfs toggle for them.

2) That also in the containers use case if box admin turns on the feature, some containers would potentially start negatively affecting the others (via the accumulated cost of slice re-configuration on context switching).

I am not familiar with typical container setups to be authoritative here, but intuitively I find it reasonable that a low-level hardware switch like this would be under the control of a master domain administrator. ("If you are installing our product in the container environment, make sure your system administrator enables this hardware feature.", "Note to system administrators: Enabling this features may negatively affect the performance of other containers.")

Alternative proposal is for the i915 to apply an "or" filter on all requested masks and in that way ensure dynamic re-configuration doesn't happen on context switches, but driven from userspace via ioctls.

In other words, should _all_ userspace agree between themselves that they want to turn off a slice, they would then need to send out a concerted ioctl storm, where number of needed ioctls equals the number of currently active contexts. (This may have its own performance consequences caused by the barriers needed to modify all context images.)

This was deemed acceptable the the media use case, but my concern is the approach is not elegant and will tie us with the "or" policy in the ABI. (Performance concerns I haven't evaluated yet, but they also may be significant.)

If we go back thinking about the containers use case, then it transpires that even though the "or" policy does prevent one container from affecting the other from one angle, it also prevents one container from exercising the feature unless all containers co-operate.

As such, we can view the original problem statement where we have an issue if not everyone co-operates, as conceptually the same just from an opposite angle. (Rather than one container incurring the increased cost of context switches to the rest, we would have one container preventing the optimized slice configuration to the other.)

From this follows that both proposals require complete co-operation from all running userspace to avoid complete control of the feature.

Since the balance between the benefit of optimized slice configuration (or penalty of suboptimal one), versus the penalty of increased context switch times, cannot be know by the driver (barring venturing into the heuristics territory), that is another reason why I find the "or" policy in the driver questionable.

We can also ask a question of - If we go with the "or" policy, why require N per-context ioctls to modify the global GPU configuration and not instead add a global driver ioctl to modify the state?

If a future hardware requires, or enables, the per-context behaviour in a more efficient way, we could then revisit the problem space.

In the mean time I see the "or" policy solution as adding some ABI which doesn't do anything for many use cases without any way for the sysadmin to enable it. At the same time master sysfs knob at least enables the sysadmin to make a decision. Here I am thinking about a random client environment where not all userspace co-operates, but for instance user is running the feature aware media stack, and non-feature aware OpenCL/3d stack.

I guess the complete story boils down to - is the master sysfs knob really a problem in container use cases.

Regards,

Tvrtko


v2: Rename sysfs entry (Tvrtko)
     Lock interruptible the device in sysfs (Tvrtko)
     Fix dropped error code in getting dynamic sseu value (Tvrtko)
     s/dev_priv/i915/ (Tvrtko)

v3: Drop locked function suffix (Tvrtko)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@xxxxxxxxx>
---
  drivers/gpu/drm/i915/i915_drv.h         | 16 +++++++---
  drivers/gpu/drm/i915/i915_gem_context.c | 38 +++++++++++++++++++++++
  drivers/gpu/drm/i915/i915_sysfs.c       | 40 +++++++++++++++++++++++++
  3 files changed, 90 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index b2386c37437d..5aa96a6650b4 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1842,6 +1842,8 @@ struct drm_i915_private {
  		struct ida hw_ida;
  #define MAX_CONTEXT_HW_ID (1<<21) /* exclusive */
  #define GEN11_MAX_CONTEXT_HW_ID (1<<11) /* exclusive */
+
+		bool dynamic_sseu;
  	} contexts;
u32 fdi_rx_config;
@@ -3259,6 +3261,10 @@ i915_gem_context_lookup(struct drm_i915_file_private *file_priv, u32 id)
  	return ctx;
  }
+int i915_gem_contexts_set_dynamic_sseu(struct drm_i915_private *i915,
+				       bool allowed);
+bool i915_gem_contexts_get_dynamic_sseu(struct drm_i915_private *i915);
+
  int i915_perf_open_ioctl(struct drm_device *dev, void *data,
  			 struct drm_file *file);
  int i915_perf_add_config_ioctl(struct drm_device *dev, void *data,
@@ -3859,11 +3865,13 @@ intel_engine_prepare_sseu(struct intel_engine_cs *engine,
  	struct drm_i915_private *i915 = engine->i915;
/*
-	 * If i915/perf is active, we want a stable powergating configuration
-	 * on the system. The most natural configuration to take in that case
-	 * is the default (i.e maximum the hardware can do).
+	 * If i915/perf is active or dynamic sseu configuration is not allowed
+	 * (through sysfs), we want a stable powergating configuration on the
+	 * system. The most natural configuration to take in that case is the
+	 * default (i.e maximum the hardware can do).
  	 */
-	return i915->perf.oa.exclusive_stream ?
+	return (i915->perf.oa.exclusive_stream ||
+		!i915->contexts.dynamic_sseu) ?
  		intel_device_default_sseu(i915) : sseu;
  }
diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 79029815c21f..453bdc976be3 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -1118,6 +1118,44 @@ int i915_gem_context_reset_stats_ioctl(struct drm_device *dev,
  	return ret;
  }
+int i915_gem_contexts_set_dynamic_sseu(struct drm_i915_private *i915,
+				       bool allowed)
+{
+	struct intel_engine_cs *engine = i915->engine[RCS];
+	struct i915_gem_context *ctx;
+	int ret = 0;
+
+	lockdep_assert_held(&i915->drm.struct_mutex);
+
+	if (!engine->emit_rpcs_config)
+		return -ENODEV;
+
+	if (allowed == i915->contexts.dynamic_sseu)
+		return 0;
+
+	i915->contexts.dynamic_sseu = allowed;
+
+	list_for_each_entry(ctx, &i915->contexts.list, link) {
+		struct intel_context *ce = to_intel_context(ctx, engine);
+
+		ret = i915_gem_context_reconfigure_sseu(ctx, engine, ce->sseu);
+		if (ret)
+			break;
+	}
+
+	return ret;
+}
+
+bool i915_gem_contexts_get_dynamic_sseu(struct drm_i915_private *i915)
+{
+	struct intel_engine_cs *engine = i915->engine[RCS];
+
+	if (!engine->emit_rpcs_config)
+		return false;
+
+	return i915->contexts.dynamic_sseu;
+}
+
  #if IS_ENABLED(CONFIG_DRM_I915_SELFTEST)
  #include "selftests/mock_context.c"
  #include "selftests/i915_gem_context.c"
diff --git a/drivers/gpu/drm/i915/i915_sysfs.c b/drivers/gpu/drm/i915/i915_sysfs.c
index e5e6f6bb2b05..d895054d245e 100644
--- a/drivers/gpu/drm/i915/i915_sysfs.c
+++ b/drivers/gpu/drm/i915/i915_sysfs.c
@@ -483,6 +483,44 @@ static ssize_t gt_rp_mhz_show(struct device *kdev, struct device_attribute *attr
  	return snprintf(buf, PAGE_SIZE, "%d\n", val);
  }
+static ssize_t allow_dynamic_sseu_show(struct device *kdev,
+				       struct device_attribute *attr,
+				       char *buf)
+{
+	struct drm_i915_private *i915 = kdev_minor_to_i915(kdev);
+	bool value = i915_gem_contexts_get_dynamic_sseu(i915);
+
+	return snprintf(buf, PAGE_SIZE, "%d\n", value);
+}
+
+static ssize_t allow_dynamic_sseu_store(struct device *kdev,
+					struct device_attribute *attr,
+					const char *buf, size_t count)
+{
+	struct drm_i915_private *i915 = kdev_minor_to_i915(kdev);
+	ssize_t ret;
+	u32 val;
+
+	ret = kstrtou32(buf, 0, &val);
+	if (ret)
+		return ret;
+
+	if (val != 0 && val != 1)
+		return -EINVAL;
+
+	ret = mutex_lock_interruptible(&i915->drm.struct_mutex);
+	if (ret)
+		return ret;
+
+	ret = i915_gem_contexts_set_dynamic_sseu(i915, val != 0);
+
+	mutex_unlock(&i915->drm.struct_mutex);
+
+	return ret ?: count;
+}
+
+static DEVICE_ATTR_RW(allow_dynamic_sseu);
+
  static const struct attribute *gen6_attrs[] = {
  	&dev_attr_gt_act_freq_mhz.attr,
  	&dev_attr_gt_cur_freq_mhz.attr,
@@ -492,6 +530,7 @@ static const struct attribute *gen6_attrs[] = {
  	&dev_attr_gt_RP0_freq_mhz.attr,
  	&dev_attr_gt_RP1_freq_mhz.attr,
  	&dev_attr_gt_RPn_freq_mhz.attr,
+	&dev_attr_allow_dynamic_sseu.attr,
  	NULL,
  };
@@ -505,6 +544,7 @@ static const struct attribute *vlv_attrs[] = {
  	&dev_attr_gt_RP1_freq_mhz.attr,
  	&dev_attr_gt_RPn_freq_mhz.attr,
  	&dev_attr_vlv_rpe_freq_mhz.attr,
+	&dev_attr_allow_dynamic_sseu.attr,
  	NULL,
  };
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