Re: slow snd_rawmidi_drain() for VirMidi devcies

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>From 6b97042bc91ebee267cbb05e6b937cbab5502f61 Mon Sep 17 00:00:00 2001
From: Stefan Sauer <ensonic@xxxxxxxxxx>
Date: Wed, 5 Jan 2022 20:44:57 +0100
Subject: [PATCH] ALSA: seq: virmidi: Add a drain operation

If a driver does not supply a drain operation for outputs, a default code
path will execute msleep(50). Especially for a virtual midi device
this severely limits the throughput.

This implementation for the virtual midi driver simply flushes the output
workqueue.
---
 sound/core/seq/seq_virmidi.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/sound/core/seq/seq_virmidi.c b/sound/core/seq/seq_virmidi.c
index 4abc38c70cae..1d708dc20443 100644
--- a/sound/core/seq/seq_virmidi.c
+++ b/sound/core/seq/seq_virmidi.c
@@ -262,6 +262,16 @@ static int snd_virmidi_output_close(struct
snd_rawmidi_substream *substream)
  return 0;
 }

+/*
+ * drain output work queue
+ */
+static void snd_virmidi_output_drain(struct snd_rawmidi_substream
*substream)
+{
+ struct snd_virmidi *vmidi = substream->runtime->private_data;
+
+ flush_work(&vmidi->output_work);
+}
+
 /*
  * subscribe callback - allow output to rawmidi device
  */
@@ -336,6 +346,7 @@ static const struct snd_rawmidi_ops
snd_virmidi_output_ops = {
  .open = snd_virmidi_output_open,
  .close = snd_virmidi_output_close,
  .trigger = snd_virmidi_output_trigger,
+ .drain = snd_virmidi_output_drain,
 };

 /*
-- 
2.34.1


Am Mi., 5. Jan. 2022 um 15:13 Uhr schrieb Takashi Iwai <tiwai@xxxxxxx>:
>
> On Wed, 05 Jan 2022 14:39:05 +0100,
> Stefan Sauer wrote:
> >
> > -   28,78%     0,00%  rawmidi_alsa  [snd_rawmidi]         [k]
> > snd_rawmidi_drain_output
> >    - snd_rawmidi_drain_output
> >       - 26,59% msleep
> >            schedule_timeout
> >            schedule
> >          + __schedule
> >       + 2,13% schedule_timeout
> >
> > Seems to be because of:
> > https://github.com/torvalds/linux/blob/master/sound/core/rawmidi.c#L244
> > if (substream->ops->drain)
> >   substream->ops->drain(substream);
> > else
> >   msleep(50);
> >
> > It see what fileops have no drain impl:
> > https://github.com/torvalds/linux/blob/master/sound/core/rawmidi.c#L1708
> > and I see
> >
https://github.com/torvalds/linux/blob/master/sound/core/seq/seq_virmidi.c#L329
> > that define no drain op. Not which ones are actually used here :/
> >
> > The docs confirm teh 50 ms wait though:
> >
https://github.com/torvalds/linux/blob/master/Documentation/sound/kernel-api/writing-an-alsa-driver.rst#drain-callback
> >
> > would it make sense to have a dummy drain function for the seq_virmidi
output
> > ops?
>
> Yes, a proper drain implementation is definitely better.
> I guess just calling flush_work(&vmidi->output_work) there would
> suffice.
>
>
> thanks,
>
> Takashi
>
> >
> > Stefan
> >
> > Am Di., 4. Jan. 2022 um 16:54 Uhr schrieb Takashi Iwai <tiwai@xxxxxxx>:
> >
> >     On Sat, 01 Jan 2022 12:49:13 +0100,
> >     Stefan Sauer wrote:
> >     >
> >     > hi,
> >     >
> >     > I've tried to link BitwigStudio to the webapp cables.gl over
virmidi.
> >     > Unfortunately Bitwig Studio only supports rawmidi. What I
discovered is
> >     > that there is a strange slowness when sending data to virmidi
caused
> >     > by snd_rawmidi_drain().
> >     >
> >     > I've posted two tiny, self-contained c apps to:
> >     > https://gist.github.com/ensonic/c7588b87fa6c1fa94a8f753b1e0aa394
> >     > See some examples below. 2 observations:
> >     > * snd_rawmidi_type() is *not* reporting virmidi as VIRTUAL
> >     > * snd_rawmidi_drain() takes about 60ms! on virtual vs. less that
0.1 ms
> >     on
> >     > usb midi (I checked all my hw midi and the worst was avg=1ms on
physical
> >     > midi image unitor8)
> >     >
> >     > When comparing the implementations:
> >     >
> >
https://github.com/alsa-project/alsa-lib/blob/master/src/rawmidi/rawmidi_virt.c#L173
> >     >
> >
https://github.com/alsa-project/alsa-lib/blob/master/src/rawmidi/rawmidi_hw.c#L164
> >     > I see that the hw one results in an IOCTL which I can see when
striking
> >     the
> >     > code and I wonder if this is the root cause? Why is
rawmidi_virt.c not
> >     used
> >     > for virmidi?
> >     > >From poking at snd_rawmidi_open_conf() I have not yet figured
where
> >     this is
> >     > decided ....
> >     >
> >     > Stefan
> >     >
> >     > > amidi -l
> >     > Dir Device    Name
> >     > IO  hw:0,0,0  Scarlett 18i20 USB MIDI 1
> >     > IO  hw:3,0,0  nanoKEY2 nanoKEY2 _ KEYBOARD
> >     > IO  hw:5,0,0  nanoKONTROL nanoKONTROL _ SLIDE
> >     > IO  hw:10,0    Virtual Raw MIDI (16 subdevices)
> >     > IO  hw:11,0    Virtual Raw MIDI (16 subdevices)
> >     >
> >     > # using direct i/o to virmidi - all good
> >     > > ./rawmidi_oss /dev/midi11 0
> >     > Using device '/dev/midi11' without draining
> >     > write took min=  0.0015 ms, avg=  0.0016 ms, max=  0.0110 ms
> >     > > ./rawmidi_oss /dev/midi11 1
> >     > Using device '/dev/midi11' with draining
> >     > write took min=  0.0015 ms, avg=  0.0017 ms, max=  0.0101 ms
> >     > drain took min=  0.0001 ms, avg=  0.0001 ms, max=  0.0008 ms
> >     >
> >     > # using snd_rawmidi to virmidi - slow drain operations
> >     > > ./rawmidi_alsa hw:11,0 0
> >     > Using device 'hw:11,0' without draining
> >     > SND_RAWMIDI_TYPE_HW
> >     > write took min=  0.0010 ms, avg=  0.0011 ms, max=  0.0056 ms
> >     > > ./rawmidi_alsa hw:11,0 1
> >     > Using device 'hw:11,0' with draining
> >     > SND_RAWMIDI_TYPE_HW
> >     > write took min=  0.0016 ms, avg=  0.0040 ms, max=  0.0077 ms
> >     > drain took min= 55.9951 ms, avg= 60.4330 ms, max= 64.0653 ms
> >     >
> >     > # using snd_rawmidi to usb hw - all good
> >     > > ./rawmidi_alsa hw:3,0 0
> >     > Using device 'hw:3,0' without draining
> >     > SND_RAWMIDI_TYPE_HW
> >     > write took min=  0.0012 ms, avg=  0.0015 ms, max=  0.0121 ms
> >     > > ./rawmidi_alsa hw:3,0 1
> >     > Using device 'hw:3,0' with draining
> >     > SND_RAWMIDI_TYPE_HW
> >     > write took min=  0.0024 ms, avg=  0.0032 ms, max=  0.0110 ms
> >     > drain took min=  0.0293 ms, avg=  0.0636 ms, max=  0.2277 ms
> >
> >     This kind of thing needs profiling.  You can try perf or whatever
> >     available, and identify which call takes long.  My wild guess is
> >     something about snd_seq_sync_output_queue(), maybe poll syscall
takes
> >     unexpected long.
> >
> >     thanks,
> >
> >     Takashi
> >
> >



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