Re: [PATCH v1 2/2] serial: 8250_dma: add parameter to queue work on specific cpu

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Oct 21, 2025 at 09:05:13PM +0800, Xin Zhao wrote:
> On Tue, 21 Oct 2025 13:28:46 +0200 Greg KH <gregkh@linuxfoundation.org> wrote:
> 
> > Meta-comment, this is a v2, and was not threaded with patch 1/1.  Please
> > use a tool like b4 or git send-email to send patches out so that we can
> > properly review/apply them.
> 
> I did use git send-email to send the patches. I have successfully used the
> git send-email tool to send a series of patches before, and that series was
> later merged into 6.18-rc1. Therefore, I feel that my usage of git send-email
> should not be an issue. The only difference of my current submission is that
> after sending each patch with git send-email, I waited for the patches to
> appear on lkml.org before sending the next one. I had encountered issues before
> where sending multiple patches in a row caused only some of them to appear on
> lkml.org, with others taking a long time to show up. That's why I specifically
> decided to wait a bit after each patch this time. I'm really not sure why the
> system didn't group my three patches together; it's quite puzzling to me.

I do not know, but it was not threaded :(

Try sending it to yourself and see if the delay maybe caused a problem?
there should not be an issue sending just 3 messages at once.

> > Are you using the RT kernel?  It seems odd that this is just coming up
> > now, given our normal latency issues in the tty layer.  What changed to
> > cause this?
> 
> > > In our system, the processing interval for each frame of IMU data
> > > transmitted via UART can experience significant jitter due to this issue.
> > > Instead of the expected 10 to 15 ms frame processing interval, we see
> > > spikes up to 30 to 35 ms. Moreover, in just one or two hours, there can
> > > be 2 to 3 occurrences of such high jitter, which is quite frequent. This
> > > jitter exceeds the software's tolerable limit of 20 ms.
> > 
> > Again, are you using the RT kernel?  If not, can you try that?
> 
> Our system is indeed RT-Linux, version 6.1.146-rt53. Our project has been
> ongoing for about 2 years, and we hadn't paid close attention to these jitter
> issues before. It is only recently that we needed to focus on them specifically.

6.1 is very old, please try the latest 6.17.y release, or 6.18-rc1.
Lots and lots of RT stuff has been fixed since 6.1.

> > This is not the 1990's, we don't add new module parameters :)
> > 
> > This will not work for systems with multiple tty devices/drivers, please
> > use a correct api for this if you really really need it.
> > 
> > But again, I think this might not be needed if you use the RT kernel
> > issue.
> 
> If user-configurable CPUs are really needed for queueing, do you have any
> recommendations on how to implement this? Would it be appropriate to create
> a new node under /sys/devices/platform/serial8250/tty/ttyS*?
> As mentioned earlier, our system is an RT-Linux system. As you say that
> using an RT kernel may not encounter this issue, are you implying that RT-Linux
> has priority inheritance logic?

I'm saying that the RT selection should cause you not to have those
larger variants in delays.  Try a newer kernel and see.

> However, the logic within work items isn't
> always protected by locks. Even if everything is under lock protection, when a
> work item starts executing and hasn't yet entered the lock, it can still be
> preempted by real-time tasks. In an RT-Linux system, such preemption of kworker
> by real-time tasks is more common compared to a standard kernel.

True, which is something that the rt changes should have addressed.
This sounds like an overall rt issue, not a serial port issue.  Have you
asked about this on the rt mailing list?

> > So you just bound ALL tty devices to the same cpu?  That feels very
> > wrong, how will that work with multiple devices on different interrupts
> > coming in on different cpus?
> > 
> > Why not just queue this up on the cpu that the irq happened on instead?
> 
> If possible, I will later add a node to separately configure each tty/ttyS*
> device. The approach I intend to use is to add this tty_flip_cpu node under
> /sys/devices/platform/serial8250/tty/ttyS*. Is it OK?
> Based on our project, the DMA RX complete events for tty are almost evenly
> distributed across each core.

This should come from a hardware definition somewhere in your DT, not as
a user-selectable option.  And again, why not just tie it to the cpu
where the irq came from automatically?

> I believe it is necessary to increase the
> flexibility of the queue_work to be located on a specific CPU, as it is difficult
> to predict whether a core is running many long-duration real-time tasks based on
> the driver code or low-level logic strategy.

That's a system design issue, not a kernel issue :)

thanks,

greg k-h