[RFC PATCH 2/2] ALSA: virtio: prioritize staged capture period work
From: Joonho Lee
Date: Sat Oct 17 2026 - 02:01:12 EST
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@xxxxxxxxx/
Assisted-by: LLM
Signed-off-by: Joonho Lee <mynameisjune111@xxxxxxxxx>
---
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);
}
}
}