[RFC PATCH 2/2] ALSA: virtio: prioritize staged capture period work

[Joonho Lee <mynameisjune111@gmail.com>]

CPU contention can delay staged capture period notifications while RX
completions continue filling the unread ring. In a two-vCPU VZ guest with
16 nice-0 CPU workers, the ACK-fixed baseline stopped with ring_capacity
and EPIPE in three runs, after 11.74, 30.77 and 30.33 seconds of audio.

Route staged capture period work through system_highpri_wq with a common
helper at all ten producers, including ACK errors and self-requeue. Keep
playback and non-staged streams on the normal workqueue. Initialization,
cancellation, buffer geometry and capture accounting remain unchanged.

With the same kernel, input, geometry and userspace priority, the candidate
completed three 70-second captures and immediate reopens. Its maximum
matched queue-to-start delay was 20.366 ms, versus 108.112 ms in the first
baseline failure. The other baseline failures had pending work without
matching execution, so matched-only statistics do not bound that delay.

Reader scheduling delays were also present. Both arms passed a separate
nice -10 reader control, and host load varied between runs. These results
support the change for the tested workload without isolating a kernel-only
cause or guaranteeing lossless capture under arbitrary CPU load.
KASAN/lockdep QEMU regressions passed with the unchanged candidate source.

This follows the staged ACK fix and pending VZ capture support. The
introducing staged-capture patch has no upstream Fixes commit ID yet.

Link: https://lore.kernel.org/all/rfc-v2-vz-capture-compat-1.b9ab4c1540dc4691@gmail.com/
Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@gmail.com>
---
 sound/virtio/virtio_pcm.c     |  4 ++--
 sound/virtio/virtio_pcm.h     | 12 ++++++++++++
 sound/virtio/virtio_pcm_msg.c |  6 +++---
 sound/virtio/virtio_pcm_ops.c | 10 +++++-----
 4 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/sound/virtio/virtio_pcm.c b/sound/virtio/virtio_pcm.c
index a3478fc8848db..d8515b2144a14 100644
--- a/sound/virtio/virtio_pcm.c
+++ b/sound/virtio/virtio_pcm.c
@@ -397,7 +397,7 @@ static void virtsnd_pcm_period_elapsed(struct work_struct *work)
 	}
 
 	if (requeue)
-		schedule_work(&vss->elapsed_period);
+		virtsnd_pcm_schedule_period_elapsed(vss);
 }
 
 /**
@@ -606,7 +606,7 @@ void virtsnd_pcm_event(struct virtio_snd *snd, struct virtio_snd_event *event)
 			if (vss->xfer_enabled) {
 				vss->xfer_xrun = true;
 				if (vss->capture_staging)
-					schedule_work(&vss->elapsed_period);
+					virtsnd_pcm_schedule_period_elapsed(vss);
 			}
 		}
 		break;
diff --git a/sound/virtio/virtio_pcm.h b/sound/virtio/virtio_pcm.h
index e143a15592f05..5d1d88f2823b2 100644
--- a/sound/virtio/virtio_pcm.h
+++ b/sound/virtio/virtio_pcm.h
@@ -8,6 +8,7 @@
 
 #include <linux/atomic.h>
 #include <linux/virtio_config.h>
+#include <linux/workqueue.h>
 #include <sound/pcm.h>
 #include <sound/pcm-indirect.h>
 
@@ -114,6 +115,17 @@ struct virtio_pcm_substream {
 	wait_queue_head_t msg_empty;
 };
 
+/* Keep non-staged PCM notifications on schedule_work()'s normal queue. */
+static inline bool
+virtsnd_pcm_schedule_period_elapsed(struct virtio_pcm_substream *vss)
+{
+	if (vss->direction == SNDRV_PCM_STREAM_CAPTURE &&
+	    vss->capture_staging)
+		return queue_work(system_highpri_wq, &vss->elapsed_period);
+
+	return schedule_work(&vss->elapsed_period);
+}
+
 /*
  * ALSA may observe a subperiod tail before its period callback. Keep actual
  * unread occupancy separate from pointer observations and notification state.
diff --git a/sound/virtio/virtio_pcm_msg.c b/sound/virtio/virtio_pcm_msg.c
index 795e52e09564b..663f7695b5be8 100644
--- a/sound/virtio/virtio_pcm_msg.c
+++ b/sound/virtio/virtio_pcm_msg.c
@@ -627,7 +627,7 @@ static void virtsnd_pcm_capture_xrun_locked(struct virtio_pcm_substream *vss)
 {
 	if (!vss->xfer_xrun) {
 		vss->xfer_xrun = true;
-		schedule_work(&vss->elapsed_period);
+		virtsnd_pcm_schedule_period_elapsed(vss);
 	}
 }
 
@@ -720,7 +720,7 @@ static void virtsnd_pcm_capture_complete(struct virtio_pcm_msg *msg,
 			virtsnd_pcm_capture_xrun_locked(vss);
 		}
 		if (got_audio && !vss->xfer_xrun)
-			schedule_work(&vss->elapsed_period);
+			virtsnd_pcm_schedule_period_elapsed(vss);
 	} else if (!vss->msg_count) {
 		wake_up_all(&vss->msg_empty);
 	}
@@ -858,7 +858,7 @@ static void virtsnd_pcm_msg_complete(struct virtio_pcm_msg *msg,
 		runtime->delay =
 			bytes_to_frames(runtime,
 					le32_to_cpu(msg->status.latency_bytes));
-		schedule_work(&vss->elapsed_period);
+		virtsnd_pcm_schedule_period_elapsed(vss);
 	} else if (!vss->msg_count) {
 		wake_up_all(&vss->msg_empty);
 	}
diff --git a/sound/virtio/virtio_pcm_ops.c b/sound/virtio/virtio_pcm_ops.c
index 85e7576903d56..d50924b792f65 100644
--- a/sound/virtio/virtio_pcm_ops.c
+++ b/sound/virtio/virtio_pcm_ops.c
@@ -232,7 +232,7 @@ virtsnd_pcm_pause_restart_fail(struct virtio_pcm_substream *vss,
 				VIRTSND_PCM_PAUSE_RESTART_RESET_REQUIRED;
 	}
 	if (notify_xrun)
-		schedule_work(&vss->elapsed_period);
+		virtsnd_pcm_schedule_period_elapsed(vss);
 	vss->substream->runtime->stop_operating = true;
 }
 
@@ -1253,7 +1253,7 @@ static int virtsnd_pcm_capture_staged_ack(struct virtio_pcm_substream *vss,
 			return -EPIPE;
 		if (!virtsnd_pcm_capture_counters_valid(vss)) {
 			vss->xfer_xrun = true;
-			schedule_work(&vss->elapsed_period);
+			virtsnd_pcm_schedule_period_elapsed(vss);
 			return -EPIPE;
 		}
 
@@ -1264,7 +1264,7 @@ static int virtsnd_pcm_capture_staged_ack(struct virtio_pcm_substream *vss,
 		if (diff < 0 ||
 		    (snd_pcm_uframes_t)diff > runtime->buffer_size) {
 			vss->xfer_xrun = true;
-			schedule_work(&vss->elapsed_period);
+			virtsnd_pcm_schedule_period_elapsed(vss);
 			return -EPIPE;
 		}
 
@@ -1275,7 +1275,7 @@ static int virtsnd_pcm_capture_staged_ack(struct virtio_pcm_substream *vss,
 				       &next_acked_total) ||
 		    next_acked_total > vss->capture_copied_total) {
 			vss->xfer_xrun = true;
-			schedule_work(&vss->elapsed_period);
+			virtsnd_pcm_schedule_period_elapsed(vss);
 			return -EPIPE;
 		}
 
@@ -1292,7 +1292,7 @@ static int virtsnd_pcm_capture_staged_ack(struct virtio_pcm_substream *vss,
 			rc = virtsnd_pcm_capture_send(vss);
 			if (rc) {
 				vss->xfer_xrun = true;
-				schedule_work(&vss->elapsed_period);
+				virtsnd_pcm_schedule_period_elapsed(vss);
 			}
 		}
 	}