On 17/04/2024 01:53, Maíra Canal wrote:
In V3D, the conclusion of a job is indicated by a IRQ. When a job
finishes, then we update the local and the global GPU stats of that
queue. But, while the GPU stats are being updated, a user might be
reading the stats from sysfs or fdinfo.
For example, on `gpu_stats_show()`, we could think about a scenario where
`v3d->queue[queue].start_ns != 0`, then an interruption happens, we update
interrupt
the value of `v3d->queue[queue].start_ns` to 0, we come back to
`gpu_stats_show()` to calculate `active_runtime` and now,
`active_runtime = timestamp`.
In this simple example, the user would see a spike in the queue usage,
that didn't matches reality.
match
In order to address this issue properly, use a seqcount to protect read
and write sections of the code.
Fixes: 09a93cc4f7d1 ("drm/v3d: Implement show_fdinfo() callback for GPU usage stats")
Reported-by: Tvrtko Ursulin <tursulin@xxxxxxxxxx>
Signed-off-by: Maíra Canal <mcanal@xxxxxxxxxx>
---
drivers/gpu/drm/v3d/v3d_drv.c | 10 ++++++----
drivers/gpu/drm/v3d/v3d_drv.h | 21 +++++++++++++++++++++
drivers/gpu/drm/v3d/v3d_gem.c | 7 +++++--
drivers/gpu/drm/v3d/v3d_sched.c | 7 +++++++
drivers/gpu/drm/v3d/v3d_sysfs.c | 11 +++--------
5 files changed, 42 insertions(+), 14 deletions(-)
diff --git a/drivers/gpu/drm/v3d/v3d_drv.c b/drivers/gpu/drm/v3d/v3d_drv.c
index 52e3ba9df46f..cf15fa142968 100644
--- a/drivers/gpu/drm/v3d/v3d_drv.c
+++ b/drivers/gpu/drm/v3d/v3d_drv.c
@@ -121,6 +121,7 @@ v3d_open(struct drm_device *dev, struct drm_file *file)
1, NULL);
memset(&v3d_priv->stats[i], 0, sizeof(v3d_priv->stats[i]));
+ seqcount_init(&v3d_priv->stats[i].lock);
}
v3d_perfmon_open_file(v3d_priv);
@@ -150,20 +151,21 @@ static void v3d_show_fdinfo(struct drm_printer *p, struct drm_file *file)
for (queue = 0; queue < V3D_MAX_QUEUES; queue++) {
struct v3d_stats *stats = &file_priv->stats[queue];
+ u64 active_runtime, jobs_completed;
+
+ v3d_get_stats(stats, timestamp, &active_runtime, &jobs_completed);
/* Note that, in case of a GPU reset, the time spent during an
* attempt of executing the job is not computed in the runtime.
*/
drm_printf(p, "drm-engine-%s: \t%llu ns\n",
- v3d_queue_to_string(queue),
- stats->start_ns ? stats->enabled_ns + timestamp - stats->start_ns
- : stats->enabled_ns);
+ v3d_queue_to_string(queue), active_runtime);
/* Note that we only count jobs that completed. Therefore, jobs
* that were resubmitted due to a GPU reset are not computed.
*/
drm_printf(p, "v3d-jobs-%s: \t%llu jobs\n",
- v3d_queue_to_string(queue), stats->jobs_completed);
+ v3d_queue_to_string(queue), jobs_completed);
}
}
diff --git a/drivers/gpu/drm/v3d/v3d_drv.h b/drivers/gpu/drm/v3d/v3d_drv.h
index 5a198924d568..5211df7c7317 100644
--- a/drivers/gpu/drm/v3d/v3d_drv.h
+++ b/drivers/gpu/drm/v3d/v3d_drv.h
@@ -40,8 +40,29 @@ struct v3d_stats {
u64 start_ns;
u64 enabled_ns;
u64 jobs_completed;
+
+ /*
+ * This seqcount is used to protect the access to the GPU stats
+ * variables. It must be used as, while we are reading the stats,
+ * IRQs can happen and the stats can be updated.
+ */
+ seqcount_t lock;
};
+static inline void v3d_get_stats(const struct v3d_stats *stats, u64 timestamp,
+ u64 *active_runtime, u64 *jobs_completed)
+{
+ unsigned int seq;
+
+ do {
+ seq = read_seqcount_begin(&stats->lock);
+ *active_runtime = stats->enabled_ns;
+ if (stats->start_ns)
+ *active_runtime += timestamp - stats->start_ns;
+ *jobs_completed = stats->jobs_completed;
+ } while (read_seqcount_retry(&stats->lock, seq));
+}
Patch reads clean and obviously correct to me.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx>
The only possible discussion point I see is whether v3d_get_stats could
have been introduced first to avoid mixing pure refactors with
functionality, and whether it deserves to be in a header, or could be a
function call in v3d_drv.c just as well. No strong opinion from me,
since it is your driver your preference.
Regards,
Tvrtko
+
struct v3d_queue_state {
struct drm_gpu_scheduler sched;
diff --git a/drivers/gpu/drm/v3d/v3d_gem.c b/drivers/gpu/drm/v3d/v3d_gem.c
index d14589d3ae6c..da8faf3b9011 100644
--- a/drivers/gpu/drm/v3d/v3d_gem.c
+++ b/drivers/gpu/drm/v3d/v3d_gem.c
@@ -247,8 +247,11 @@ v3d_gem_init(struct drm_device *dev)
int ret, i;
for (i = 0; i < V3D_MAX_QUEUES; i++) {
- v3d->queue[i].fence_context = dma_fence_context_alloc(1);
- memset(&v3d->queue[i].stats, 0, sizeof(v3d->queue[i].stats));
+ struct v3d_queue_state *queue = &v3d->queue[i];
+
+ queue->fence_context = dma_fence_context_alloc(1);
+ memset(&queue->stats, 0, sizeof(queue->stats));
+ seqcount_init(&queue->stats.lock);
}
spin_lock_init(&v3d->mm_lock);
diff --git a/drivers/gpu/drm/v3d/v3d_sched.c b/drivers/gpu/drm/v3d/v3d_sched.c
index b9614944931c..7cd8c335cd9b 100644
--- a/drivers/gpu/drm/v3d/v3d_sched.c
+++ b/drivers/gpu/drm/v3d/v3d_sched.c
@@ -114,16 +114,23 @@ v3d_job_start_stats(struct v3d_job *job, enum v3d_queue queue)
struct v3d_stats *local_stats = &file->stats[queue];
u64 now = local_clock();
+ write_seqcount_begin(&local_stats->lock);
local_stats->start_ns = now;
+ write_seqcount_end(&local_stats->lock);
+
+ write_seqcount_begin(&global_stats->lock);
global_stats->start_ns = now;
+ write_seqcount_end(&global_stats->lock);
}
static void
v3d_stats_update(struct v3d_stats *stats, u64 now)
{
+ write_seqcount_begin(&stats->lock);
stats->enabled_ns += now - stats->start_ns;
stats->jobs_completed++;
stats->start_ns = 0;
+ write_seqcount_end(&stats->lock);
}
void
diff --git a/drivers/gpu/drm/v3d/v3d_sysfs.c b/drivers/gpu/drm/v3d/v3d_sysfs.c
index 6a8e7acc8b82..d610e355964f 100644
--- a/drivers/gpu/drm/v3d/v3d_sysfs.c
+++ b/drivers/gpu/drm/v3d/v3d_sysfs.c
@@ -15,18 +15,15 @@ gpu_stats_show(struct device *dev, struct device_attribute *attr, char *buf)
struct v3d_dev *v3d = to_v3d_dev(drm);
enum v3d_queue queue;
u64 timestamp = local_clock();
- u64 active_runtime;
ssize_t len = 0;
len += sysfs_emit(buf, "queue\ttimestamp\tjobs\truntime\n");
for (queue = 0; queue < V3D_MAX_QUEUES; queue++) {
struct v3d_stats *stats = &v3d->queue[queue].stats;
+ u64 active_runtime, jobs_completed;
- if (stats->start_ns)
- active_runtime = timestamp - stats->start_ns;
- else
- active_runtime = 0;
+ v3d_get_stats(stats, timestamp, &active_runtime, &jobs_completed);
/* Each line will display the queue name, timestamp, the number
* of jobs sent to that queue and the runtime, as can be seem here:
@@ -40,9 +37,7 @@ gpu_stats_show(struct device *dev, struct device_attribute *attr, char *buf)
*/
len += sysfs_emit_at(buf, len, "%s\t%llu\t%llu\t%llu\n",
v3d_queue_to_string(queue),
- timestamp,
- stats->jobs_completed,
- stats->enabled_ns + active_runtime);
+ timestamp, jobs_completed, active_runtime);
}
return len;
