[PATCH v2 4/4] drm/v3d: Fix race-condition between sysfs/fdinfo and interrupt handler
Tvrtko Ursulin
tvrtko.ursulin at igalia.com
Wed Apr 17 08:19:09 UTC 2024
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 at igalia.com>
> Signed-off-by: Maíra Canal <mcanal at igalia.com>
> ---
> 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 at igalia.com>
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;
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