Intel HD Audio Linux Perf Story
While waiting for a Webex video meeting to begin, the fans on my laptop started
spinning, so fired up top to see how much CPU these kind of browser based
video meetings are burning:
Tasks: 373 total, 1 running, 372 sleeping, 0 stopped, 0 zombie
%Cpu(s): 10.2 us, 2.6 sy, 0.0 ni, 85.4 id, 0.1 wa, 1.4 hi, 0.3 si, 0.0 st
MiB Mem : 23966.1 total, 9074.0 free, 3827.5 used, 11064.7 buff/cache
MiB Swap: 8032.0 total, 8032.0 free, 0.0 used. 19462.6 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
7165 tommi 20 0 9278320 202256 101308 S 84.4 0.8 4:10.03 chrome
2665 tommi 20 0 2240432 15968 11432 S 14.0 0.1 8:17.29 pulseaudio
7221 tommi 20 0 1025080 58908 48300 S 4.7 0.2 0:18.00 chrome
26477 tommi 20 0 1433436 510428 360048 S 3.0 2.1 3:16.77 chrome
26540 tommi 20 0 790292 288040 113884 S 1.3 1.2 3:51.86 chrome
2315 tommi 20 0 197452 70788 51336 S 0.7 0.3 15:48.60 Xorg
Quite high CPU usage of chrome and pulseaudio, considering that the meeting
is still fully idle and all microphones muted!
chrome
It’s the Google Chrome browser, version 80.
Let’s look at chrome (PID 7165) per-thread CPU usages, 10 second average:
$ pidstat -t -p 7165 1 10
Linux 5.5.8-100.fc30.x86_64 03/15/20 _x86_64_ (8 CPU)
[...]
Average: UID TGID TID %usr %system %CPU CPU Command
Average: 1000 7165 - 71.70 12.30 84.00 - chrome
Average: 1000 - 7165 4.80 0.80 5.60 - |__ chrome
Average: 1000 - 7166 0.90 0.50 1.40 - |__ ThreadPoolServi
Average: 1000 - 7167 1.90 0.30 2.20 - |__ ThreadPoolForeg
Average: 1000 - 7168 0.60 0.30 0.90 - |__ Chrome_ChildIOT
Average: 1000 - 7171 3.10 0.60 3.70 - |__ Compositor
[...]
Average: 1000 - 7274 0.30 0.30 0.60 - |__ PacerThread
Average: 1000 - 7275 38.60 0.30 38.90 - |__ AudioInputDevic
Average: 1000 - 7276 0.40 0.20 0.60 - |__ ModuleProcessTh
Average: 1000 - 7280 4.80 0.50 5.30 - |__ AudioOutputDevi
Average: 1000 - 7300 0.00 0.00 0.00 - |__ MemoryInfra
So almost half of the time is spent in audio processing. Can we get anything interesting out of the AudioInput thread? Without symbols not much:
$ sudo perf top -t 7275 --percent-limit 1 --stdio
PerfTop: 1720 irqs/sec kernel: 2.2% exact: 100.0% lost: 0/0 drop: 0/0 [4000Hz cycles], (target_tid: 7275)
--------------------------------------------------------------------------------------------------------------
11.67% chrome [.] 0x0000000005a9a3f5
3.53% chrome [.] 0x0000000005ac6084
2.91% chrome [.] 0x0000000005a99fb6
...
pulseaudio
Let’s study pulseaudio (PID 2665) next – start by checking 10 second average
per-thread CPU usages:
$ pidstat -t -p $(pgrep -n pulseaudio) 1 10
Linux 5.5.8-100.fc30.x86_64 03/15/20 _x86_64_ (8 CPU)
Average: UID TGID TID %usr %system %CPU CPU Command
Average: 1000 2665 - 5.39 7.39 12.79 - pulseaudio
Average: 1000 - 2665 1.90 3.40 5.29 - |__ pulseaudio
Average: 1000 - 2729 0.00 0.00 0.00 - |__ alsa-sink-HDMI
Average: 1000 - 2732 1.90 1.50 3.40 - |__ alsa-sink-CX207
Average: 1000 - 2736 1.60 2.70 4.30 - |__ alsa-source-CX2
We see that the pulseaudio process has multiple threads, presumably one “main”
thread (TID 2665), and then one thread per input and output device.
Can we now get anything interesting with perf top out of these threads?
pulseaudio, TID 2665
$ sudo perf top -t 2665 --percent-limit 1 --stdio
[...]
PerfTop: 591 irqs/sec kernel:70.2% exact: 100.0% lost: 0/0 drop: 0/0 [4000Hz cycles], (target_tid: 2665)
--------------------------------------------------------------------------------------------------------------
5.84% [kernel] [k] __fget
4.97% [kernel] [k] do_syscall_64
4.41% [kernel] [k] sock_poll
3.82% [kernel] [k] _raw_spin_lock_irqsave
3.44% [unknown] [.] 0000000000000000
3.39% [kernel] [k] do_sys_poll
2.89% [kernel] [k] psi_task_change
2.70% libpulse.so.0.20.3 [.] pa_mainloop_dispatch
2.48% [kernel] [k] fput_many
2.39% [kernel] [k] __pollwait
2.09% [kernel] [k] eventfd_poll
1.95% [kernel] [k] _raw_spin_unlock_irqrestore
1.69% [kernel] [k] entry_SYSCALL_64
1.53% [kernel] [k] syscall_return_via_sysret
1.35% [kernel] [k] unix_poll
$ sudo perf trace -t 2665 -s -- sleep 10
Summary of events:
pulseaudio (2665), 66755 events, 100.0%
syscall calls errors total min avg max stddev
(msec) (msec) (msec) (msec) (%)
----------- -------- ------ -------- --------- --------- --------- ------
ppoll 6624 0 9107.751 0.000 1.375 5.108 1.41%
read 17180 6609 143.715 0.001 0.008 0.235 0.56%
write 9033 0 122.016 0.001 0.014 0.252 0.59%
futex 462 4 59.509 0.003 0.129 0.396 2.93%
uname 42 0 0.366 0.001 0.009 0.020 7.61%
getuid 41 0 0.311 0.001 0.008 0.024 9.26%
So the thread is doing thousands of syscall per second, and most of it’s CPU time is spent in the kernel. We ought to generate a flame graph to better understand what’s happening here, but for now let’s move on.
[One curious detail in the perf trace output is that about ⅓ of read
calls fail. Also I have not earlier noticed that “errors” column – turns out it was
added recently in
v5.5-rc1.]
alsa-sink, TID 2732
$ sudo perf top -t 2732 --percent-limit 1 --stdio
PerfTop: 228 irqs/sec kernel:47.4% exact: 100.0% lost: 0/0 drop: 0/0 [4000Hz cycles], (target_tid: 2732)
--------------------------------------------------------------------------------------------------------------
17.30% [snd_hda_intel] [k] azx_get_pos_skl
4.13% [unknown] [.] 0000000000000000
2.85% [kernel] [k] do_syscall_64
2.81% libspeexdsp.so.1.5.0 [.] 0x000000000000a255
1.83% [snd_hda_codec] [k] azx_get_pos_lpib
1.49% [kernel] [k] psi_task_change
alsa-source, TID 2736
$ sudo perf top -t 2736 --percent-limit 1 --stdio
PerfTop: 205 irqs/sec kernel:59.0% exact: 100.0% lost: 0/0 drop: 0/0 [4000Hz cycles], (target_tid: 2736)
--------------------------------------------------------------------------------------------------------------
28.89% [kernel] [k] delay_tsc
16.30% [snd_hda_intel] [k] azx_get_pos_skl
5.26% [unknown] [.] 0000000000000000
2.48% libspeexdsp.so.1.5.0 [.] 0x000000000000a255
1.69% [kernel] [k] do_syscall_64
1.18% [snd_hda_codec] [k] azx_get_pos_lpib
Now these look more interesting! For both threads we have high overhead kernel
symbol azx_get_pos_skl. And what’s up with that delay_tsc?
azx_get_pos_skl
We can try to understand what’s happening by studying the
azx_get_pos_skl function in the v5.5 kernel
sources.
Turns out it’s a short function:
/* get the current DMA position with correction on SKL+ chips */
static unsigned int azx_get_pos_skl(struct azx *chip, struct azx_dev *azx_dev)
{
/* DPIB register gives a more accurate position for playback */
if (azx_dev->core.substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
return azx_skl_get_dpib_pos(chip, azx_dev);
/* For capture, we need to read posbuf, but it requires a delay
* for the possible boundary overlap; the read of DPIB fetches the
* actual posbuf
*/
udelay(20);
azx_skl_get_dpib_pos(chip, azx_dev);
return azx_get_pos_posbuf(chip, azx_dev);
}This provides some explanation to the perf top results: when in playback mode
(alsa-sink thread), “position” can be returned directly from “DPIB register”
(whatever that is).
But when capturing audio (alsa-source thread), the kernel has to
udelay(20), ie. twiddle thumbs for 20 µseconds before being able to
proceed. As it’s a “delay” and not a “sleep”, it can cause the CPU to to be
effectively “stuck”, not being able to execute other kernel code (except
interrupts) or user space programs…? While sleeping, the CPU would be
explicitly free to execute other code.
Exact details probably depend on how udelay is implemented for given
architecture, and how the kernel is configured. I’ll leave that investigation
to another blog post.
[Based on the code comments, this function is used on SKL+: Skylake
(microarchitecture) and later. And what’s my CPU? lscpu says Intel Core
i7-6820HQ. Quick internet search confirms it’s indeed
Skylake.]
ftrace
We can study what’s happening in the kernel with
ftrace.
Let’s try to use it to see how azx_get_pos_skl gets called.
We can run it simply with sudo perf ftrace -a, which starts the trace and
opens a pager, allowing to scroll the captured trace (in a busy machine, there
will be a ton of output from all CPUs in the system). We can look for the
function by typing /azx_get_pos_skl.
Bingo, the process (alsa-source) is doing ioctl syscall on CPU7, entering
the kernel, and ends up calling azx_get_pos_skl:
7) | do_syscall_64() {
7) | __x64_sys_ioctl() {
7) | ksys_ioctl() {
7) | __fdget() {
7) | __fget_light() {
7) 0.491 us | __fget();
7) 1.389 us | }
7) 2.079 us | }
7) 0.375 us | security_file_ioctl();
7) | do_vfs_ioctl() {
7) | snd_pcm_ioctl [snd_pcm]() {
7) | snd_pcm_common_ioctl [snd_pcm]() {
7) 0.376 us | snd_power_wait [snd]();
7) | snd_pcm_hwsync [snd_pcm]() {
7) | snd_pcm_stream_lock_irq [snd_pcm]() {
7) 0.344 us | _raw_spin_lock_irq();
7) 1.007 us | }
7) | do_pcm_hwsync [snd_pcm]() {
7) | snd_pcm_update_hw_ptr [snd_pcm]() {
7) | snd_pcm_update_hw_ptr0 [snd_pcm]() {
7) | azx_pcm_pointer [snd_hda_codec]() {
7) | azx_get_position [snd_hda_codec]() {
7) | azx_get_pos_skl [snd_hda_intel]() {
7) | __const_udelay() {
7) + 20.311 us | delay_tsc();
7) + 21.024 us | }
7) + 11.420 us | azx_get_pos_posbuf [snd_hda_codec]();
7) + 33.425 us | }
7) | azx_get_delay_from_lpib [snd_hda_intel]() {
7) 1.126 us | azx_get_pos_lpib [snd_hda_codec]();
7) 1.844 us | }
7) + 36.604 us | }
7) + 37.308 us | }
7) 0.410 us | ktime_get_ts64();
7) | update_audio_tstamp.isra.0 [snd_pcm]() {
7) 0.340 us | ns_to_timespec();
7) 0.410 us | ktime_get_ts64();
7) 1.901 us | }
7) 0.408 us | snd_pcm_update_state [snd_pcm]();
7) + 42.144 us | }
7) + 42.821 us | } /* snd_pcm_update_hw_ptr [snd_pcm] */
7) + 43.483 us | }
7) 0.348 us | snd_pcm_stream_unlock_irq [snd_pcm]();
7) + 46.204 us | }
7) + 47.711 us | }
7) + 48.395 us | }
7) + 49.281 us | }
7) | fput() {
7) 0.354 us | fput_many();
7) 0.986 us | }
7) + 54.551 us | }
7) + 55.298 us | }
7) + 57.050 us | }
So we see that in the C code we have udelay(20), and based on the ftrace
output this results to __const_udelay() function call, which then calls
delay_tsc(). According to the trace, delay_tsc call took 20.311 us, so it
was really sleeping just a little over 20 µseconds.
Digging Kernel Git History
Let’s dig the kernel git history, to find clues why the udelay call was
added. The function was added in 2017 in commit ALSA: hda - Improved
position reporting on
SKL+:
ALSA: hda - Improved position reporting on SKL+
Apply the same methods to obtain the current stream position as ASoC
Intel SKL driver uses. It reads the position from DPIB for a playback
stream while it still reads from the position buffer for a capture
stream. For a capture stream, some ugly workaround is needed to
settle down the inconsistent position.
So even the patch author does not like how the recording is done, calling it an ugly workaround!
The commit message mentions ASoC Intel SKL driver, which we find in
sound/soc/intel/skylake/, and in skl-pcm.c we find a more extensive
comment, explaining why the delay is 20
µseconds:
/*
* Use DPIB for Playback stream as the periodic DMA Position-in-
* Buffer Writes may be scheduled at the same time or later than
* the MSI and does not guarantee to reflect the Position of the
* last buffer that was transferred. Whereas DPIB register in
* HAD space reflects the actual data that is transferred.
* Use the position buffer for capture, as DPIB write gets
* completed earlier than the actual data written to the DDR.
*
* For capture stream following workaround is required to fix the
* incorrect position reporting.
*
* 1. Wait for 20us before reading the DMA position in buffer once
* the interrupt is generated for stream completion as update happens
* on the HDA frame boundary i.e. 20.833uSec.
* 2. Read DPIB register to flush the DMA position value. This dummy
* read is required to flush DMA position value.
* 3. Read the DMA Position-in-Buffer. This value now will be equal to
* or greater than period boundary.
*/
pulseaudio utils
I want to learn little bit more about Pulseaudio. It would be nice to be able to record audio from the command line without involving the web browser. As Pulseaudio is established open source software, I’d expect that it has some reasonable tooling available, for example for audio playback and recording, and for runtime inspecting the pulseaudio daemon.
Quick rpm query shows some promising binaries in the pulseaudio-utils
package:
$ rpm -qa "*pulseaudio*" | sort
[...]
pulseaudio-12.2-9.fc30.x86_64
pulseaudio-libs-12.2-9.fc30.x86_64
pulseaudio-libs-glib2-12.2-9.fc30.x86_64
pulseaudio-module-bluetooth-12.2-9.fc30.x86_64
pulseaudio-module-gconf-12.2-9.fc30.x86_64
pulseaudio-module-x11-12.2-9.fc30.x86_64
pulseaudio-utils-12.2-9.fc30.x86_64
$ rpm -ql pulseaudio | grep /bin
/usr/bin/pulseaudio
$ rpm -ql pulseaudio-utils | grep /bin
/usr/bin/pacat
/usr/bin/pacmd
/usr/bin/pactl
/usr/bin/padsp
/usr/bin/padsp-32
/usr/bin/pamon
/usr/bin/paplay
/usr/bin/parec
/usr/bin/parecord
/usr/bin/pasuspender
/usr/bin/pax11publish
With some experimentation and after reading the manual pages for these
binaries, found what I was looking for: pactl to inspect, and parec to
record.
parec
Let’s try parec to record some audio, and see if we get similar CPU% numbers
compared to the browser based video conference:
Executed pidstat from second terminal, and CPU% is much smaller than
previously, but why?
$ pidstat -t -p $(pgrep -n pulseaudio) 1 10
[...]
Average: UID TGID TID %usr %system %CPU CPU Command
Average: 1000 2665 - 1.30 0.60 1.90 - pulseaudio
Average: 1000 - 2665 0.20 0.30 0.50 - |__ pulseaudio
Average: 1000 - 2732 0.00 0.00 0.00 - |__ alsa-sink-CX207
Average: 1000 - 2736 1.00 0.30 1.30 - |__ alsa-source-CX2
Looking again at the parec command output, it’s reporting latency, and the
numbers are quite high, near one second. That would be way too high for a
conference call!
From pactl list command output we see that the “configured” latency
is even higher, 2000000 usec ie. two seconds:
$ pactl list | less
Source #2
State: RUNNING
Name: alsa_input.pci-0000_00_1f.3.analog-stereo
Latency: 835610 usec, configured 2000000 usec
Checking parec manual page, we discover that it has latency control options,
and some explanation for the high latency we see with the default settings:
--latency-msec=MSEC
Explicitly configure the latency, with a time specified in
milliseconds. If left out the server will pick the latency,
usually relatively high for power saving reasons.
Quick experiment with the --latency-msec flag shows that indeed there’s large
effect on CPU usages. Here’s a summary of 10 second average CPU%
for the pulseaudio process, and the alsa-source thread:
parec --latency-msec |
alsa-source thread CPU % | total process CPU % |
|---|---|---|
| 1000ms | 2.80 | 4.10 |
| 100ms | 3.60 | 5.20 |
| 10ms | 8.10 | 15.90 |
usleep_range
Based on “timers-howto” kernel
documentation
(written ~2010) udelay is generally good for “a few” µseconds – 10
µseconds or more could use usleep_range.
Let’s experiment what happens, if we modify the kernel and replace the
udelay() call with usleep_range(). The required kernel code change is
simple, and in theory should result to the process sleeping for 20-30
µseconds:
diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
index 92a042e34d3e5..aa6d2285be1c7 100644
--- a/sound/pci/hda/hda_intel.c
+++ b/sound/pci/hda/hda_intel.c
@@ -913,7 +913,7 @@ static unsigned int azx_get_pos_skl(struct azx *chip,
* for the possible boundary overlap; the read of DPIB fetches the
* actual posbuf
*/
- udelay(20);
+ usleep_range(20, 30);
azx_skl_get_dpib_pos(chip, azx_dev);
return azx_get_pos_posbuf(chip, azx_dev);
}After compiling and installing the patched kernel, and booting into it, let’s
first confirm that the change is in effect. With ftrace we can see that
usleep_range is called, and results to the calling thread being scheduled out
for the duration of the sleep:
3) | do_syscall_64() {
3) | __x64_sys_ioctl() {
3) | ksys_ioctl() {
3) | snd_pcm_ioctl() {
3) | snd_pcm_common_ioctl() {
3) 0.520 us | snd_power_wait();
3) | snd_pcm_hwsync() {
3) | do_pcm_hwsync() {
3) | snd_pcm_update_hw_ptr() {
3) | snd_pcm_update_hw_ptr0() {
3) | azx_pcm_pointer() {
3) | azx_get_position() {
3) | azx_get_pos_skl() {
3) | usleep_range() {
3) 0.685 us | ktime_get();
3) | schedule_hrtimeout_range() {
3) | schedule_hrtimeout_range_clock() {
3) | schedule() {
------------------------------------------
3) alsa-so-2416 => <idle>-0
------------------------------------------
3) 0.720 us | switch_mm_irqs_off();
------------------------------------------
3) <idle>-0 => alsa-so-2416
------------------------------------------
3) 0.558 us | finish_task_switch();
3) + 63.434 us | }
3) | hrtimer_try_to_cancel() {
3) 0.522 us | hrtimer_active();
3) 1.587 us | }
3) + 80.930 us | }
3) + 81.975 us | }
3) + 84.207 us | }
3) + 11.549 us | azx_get_pos_posbuf();
3) + 97.325 us | }
3) | azx_get_delay_from_lpib() {
3) 1.616 us | azx_get_pos_lpib();
3) 2.708 us | }
3) ! 102.308 us | }
3) ! 103.326 us | }
3) ! 110.055 us | }
3) ! 111.011 us | }
3) ! 112.061 us | }
3) ! 116.099 us | }
3) ! 118.118 us | }
3) ! 119.086 us | }
3) ! 127.386 us | }
3) ! 128.386 us | }
3) 0.554 us | switch_fpu_return();
3) ! 131.661 us | } /* do_syscall_64 */
In the trace output we see that the alsa-source thread got scheduled out for
63 µseconds – more than we want, but ftrace probably also adds some
extra overhead to the numbers.
The funcslower BCC tool reports that the usleep_range calls are taking
about 40 µseconds to complete, and funclatency produces a nice
histogram, showing that the sleeps are in most cases completing in under 64
µseconds:
# /usr/share/bcc/tools/funcslower -u 10 -p $(pgrep -n pulseaudio) usleep_range
COMM PID LAT(us) RVAL FUNC
alsa-source-CX 2371 38.12 0 usleep_range
alsa-source-CX 2371 42.80 0 usleep_range
alsa-source-CX 2371 40.62 0 usleep_range
alsa-source-CX 2371 45.67 0 usleep_range
alsa-source-CX 2371 40.48 0 usleep_range
alsa-source-CX 2371 42.16 0 usleep_range
alsa-source-CX 2371 40.29 0 usleep_range
alsa-source-CX 2371 38.36 0 usleep_range
[...]
# /usr/share/bcc/tools/funclatency -Fu -p $(pgrep -n pulseaudio) -d30 usleep_range
Tracing 1 functions for "usleep_range"... Hit Ctrl-C to end.
Function = b'usleep_range'
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 5 | |
32 -> 63 : 19086 |****************************************|
64 -> 127 : 15 | |
For reference, with the Fedora 5.5.8-100.fc30.x86_64 kernel
delay_tsc mostly completes in less than 32 µseconds:
# /usr/share/bcc/tools/funclatency -Fu -p $(pgrep -n pulseaudio) -d30 delay_tsc
Tracing 1 functions for "delay_tsc"... Hit Ctrl-C to end.
Function = b'delay_tsc'
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 19157 |****************************************|
32 -> 63 : 6 | |
What about CPU usage? Comparing pulseaudio pidstat results on patched and
unpatched 5.6-rc5, there’s actually very little change. [But interestingly
CPU% is smaller in the kernel I compiled myself vs. the Fedora kernel.
Perhaps the Fedora kernel gets compiled with additional compiler hardenings, or
in my kernel some config option change is affecting performance.]
It’s an interesting experiment, but perhaps 20 µseconds is too short time to run other meaningful work on the CPU. The trace data shows that the original 20 µsecond delay doubles into 40 µsecond sleeps – the additional latency could cause other issues.
USB Headset
We can sidestep the laptop builtin audio by using a USB headset. Quick comparison of the pulseaudio CPU% [but it’s not totally apples-to-apples comparison – the laptop audio records stereo, the USB headset records mono]:
parec --latency-msec (builtin audio) |
alsa-source thread CPU % | total CPU % |
|---|---|---|
| 1000ms | 2.80 | 4.10 |
| 100ms | 3.60 | 5.20 |
| 10ms | 8.10 | 15.90 |
parec --latency-msec (USB headset) |
alsa-source thread CPU % | total CPU % |
|---|---|---|
| 1000ms | 0.70 | 1.50 |
| 100ms | 2.10 | 5.50 |
| 10ms | 3.20 | 9.40 |
Firefox
Testing the Webex conference with Firefox (version 74), interestingly the CPU usages of Firefox and Pulseaudio are lower than what I saw with Google Chrome (again idle meeting and microphone muted):
top - 17:06:38 up 1 day, 3:45, 7 users, load average: 0,94, 0,53, 0,73
Tasks: 377 total, 1 running, 376 sleeping, 0 stopped, 0 zombie
%Cpu(s): 5,7 us, 3,9 sy, 0,0 ni, 88,7 id, 0,1 wa, 1,1 hi, 0,5 si, 0,0 st
MiB Mem : 23966,1 total, 8509,3 free, 4169,2 used, 11287,7 buff/cache
MiB Swap: 8032,0 total, 8032,0 free, 0,0 used. 19106,5 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
9601 tommi 20 0 3024256 291772 148348 S 44,7 1,2 0:25.15 Web Content
9330 tommi 20 0 3330140 330436 154532 S 17,5 1,3 0:25.20 firefox
2665 tommi 20 0 2240048 15724 11184 S 7,3 0,1 15:38.73 pulseaudio
Turns out that Firefox will stop the audio recording when the microphone is muted in the meeting, and will start it again after unmuting the microphone.
Summary
We started from top command output, dug deeper into the kernel, and
discovered an “ugly workaround” in Intel HD Audio drivers that applies to
recent (Skylake+) systems. We also learned little bit about Pulseaudio, and got
a chance to experimentally patch the kernel, and use various Linux tracing and
performance analysis tools.
Update May 2021
Comment from user L29Ah:
The following modprobe.d workaround seems to fix the insane busy waiting on my X210:
options snd_hda_intel position_fix=1
—
Indeed the driver offers various parameters that may be helpful:
$ modinfo snd_hda_intel
description: Intel HDA driver
[...]
parm: index:Index value for Intel HD audio interface. (array of int)
parm: id:ID string for Intel HD audio interface. (array of charp)
parm: enable:Enable Intel HD audio interface. (array of bool)
parm: model:Use the given board model. (array of charp)
parm: position_fix:DMA pointer read method. (-1 = system default, 0 = auto,
1 = LPIB, 2 = POSBUF, 3 = VIACOMBO, 4 = COMBO, 5 = SKL+, 6 = FIFO).
parm: bdl_pos_adj:BDL position adjustment offset. (array of int)
parm: probe_mask:Bitmask to probe codecs (default = -1). (array of int)
parm: probe_only:Only probing and no codec initialization. (array of int)
parm: jackpoll_ms:Ms between polling for jack events (default = 0,
using unsol events only) (array of int)
parm: single_cmd:Use single command to communicate with codecs
(for debugging only). (bint)
parm: enable_msi:Enable Message Signaled Interrupt (MSI) (bint)
[...]