Kernel in a Nutshell answers my question about preempt voluntary in chapter 11:


Voluntary kernel preemption (desktop)

This option reduces the latency of the kernel by adding more “explicit preemption points” to the kernel code. These new preemption points have been selected to reduce the maximum latency of rescheduling, which provides faster response to applications at the cost of slightly lower throughput.

This option speeds up reaction to interactive events by allowing a low-priority process to voluntarily preempt itself even if it is in kernel mode executing a system call. This allows applications to appear to run more smoothly even when the system is under load.

Select this if you are building a kernel for a desktop system.


So, for audio we want voluntary "y". I have it "not set" in my kernel from Ubuntu. But I've had no problems. No idea what the default is for an otherwise preemptible kernel.


On 2/19/23 12:19 PM, Ted Felix wrote:
  Will, can you go through this first draft series of steps on the problematic box and let me know if it finds the issue?  Thanks.

Boot into the problematic kernel.

Confirm that it is a preemptible kernel.

     $ grep -i config_preempt= /boot/config-$(uname -r)

If you don't see "CONFIG_PREEMPT=y" then you need to install a preemptible kernel.

Check the grub configuration to make sure nothing is specified on the kernel command line related to preemption.

     $ grep -i preempt /etc/default/grub

You should either see nothing or "preempt=full" along with possibly other options.  Anything else might mean trouble and requires a change to your grub config.

Check for voluntary preemption.  (Not sure how this fits into everything, but it might cause problems and need some further investigation.)

     $ grep -i preempt_voluntary /boot/config-$(uname -r)


On 2/19/23 11:48 AM, Ted Felix wrote:
   On second thought...  We've been here before.  From my Linux MIDI Guide:

Note: There are variations on PREEMPT that are less than ideal. "DYNAMIC_PREEMPT" allows PREEMPT to be turned on and off in the kernel command line options (preempt=full). You'll need to check your GRUB configuration to make sure this is either set to "preempt=full" or not set at all (the default is "full") in "GRUB_CMDLINE_LINUX_DEFAULT=". You can check the setting after boot with "sudo dmesg | grep preempt". There is also PREEMPT_VOLUNTARY which is somewhere between "NONE" and "PREEMPT". You'll want to avoid this as audio requires the best possible real-time performance.

   I should probably make that a bit more prominent now that we are running into it for real.  Like move it up to the "things you'll need" section at the very top.


On 2/19/23 8:52 AM, Ted Felix wrote:
   We rely completely on ALSA for timer stuff.  The key routine for setting the timer in RG is:


   Unless there is a bug in there (which there may very well be, the ALSA documentation is incomplete and in many cases incorrect), the problems tend to be due to bugs in ALSA or the kernel.  They usually get sorted over time by the ALSA folks.  If they notice them.  This one seems like an interesting combination of requirements to reproduce and thus might fly under their radar.

   I recommend building the latest kernel and seeing if that fixes the issue.  My three-year-old Debian/Ubuntu oriented notes are here if you need:

   In general, it's really easy to do.

   If it's still there, a bug report for the ALSA folks seems like the way to go.  They can help find the issue in ALSA, or even in Rosegarden if that's where the problem is.


On 2/11/23 4:42 AM, Will Godfrey wrote:
Recently I found Rosegarden was losing the timer source, and the transport was freezing - sometimes for nearly a second after which it would send a bunch of
notes, then carry on as normal.

It was some time before I realised it only happened on my Ryzen 5 machine (the one I use for music) and not on my office Intel one. I hadn't made any changes to the system and got sidetracked into thinking maybe I'd somehow managed to get infected with some malware. However I eventually remembered I'd done a normal devuan update. The only significant change was the Linux kernel V5.10 to
V 6.1. Rolling that back cured the problem.

Asking some questions on-line identified the problem.

First off, I use the RT-Preemt kernel - the 'normal' one doesn't have the issue
at all.

Secondly, the Ryzen has the AMD equivalent of hyper-threading, my older Intel

Finally, since Linux V5.12 the scheduler has been modified to allow dynamic preemption. Disabling this in the BIOS cures the problem, albeit with a very
small loss of performance.

I wonder if there should be another look at how Rosegarden does it's timing. I did try to set the timing source to the audio hardware. It seemed to recognise it, but actually totally ignored it. Saving and reloading showed no time source
at all, yet still ran as if it was on System Timer.

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