Fwd: Custom Linux Kernel Scheduler issue

[Greg KH]

On Thu, Nov 24, 2016 at 10:31:18AM -0500, Kenneth Adam Miller wrote:
> On Nov 24, 2016 2:18 AM, "Greg KH" <greg at kroah.com> wrote:
> >
> > On Thu, Nov 24, 2016 at 02:01:41AM -0500, Kenneth Adam Miller wrote:
> > > Hello,
> > >
> > >
> > > I have a scheduler issue in two different respects:
> > >
> > > 1) I have a process that is supposed to tight loop, and it is being
> > > given very very little time on the system. I don't want that - I want
> > > those who would use the processor to be given the resources to run as
> > > fast as they each can.
> >
> > What is causing it to give up its timeslice?  Is it waiting for I/O?
> > Doing something else to sleep?
> 
> It's multithreaded, so it reads in a loop in one thread and writes in
> another thread. What I saw when I ran strace on it is each process
> would run for too long- the program is designed to try and stay out of
> the kernel on each side, so it checks some shared variables before it
> ever goes.

So locking/cpu contention for those "shared variables" perhaps?

> > > 2) I am seeing with perf that the maximum overhead at each section
> > > does not sum up to be more than 15 percent. Total, probably something
> > > like 18% of cpu time is used, and my binary has rocketed in slowness
> > > from about 2 seconds or less total to several minutes.
> >
> > What changed to make things slower?  Did you change kernel versions or
> > did you change something in your userspace program?
> >
> 
> The kernel versions specifically couldnt have anything to do with it
> but it was different kernels. The test runs in less that 2 seconds on
> my host. When I copy it to our custom linux, it takes minutes for it
> to run. I think it's some extra setting that we're missing while
> building the kernel, and I don't know what that is. I got a huge
> improvement when I changed the multicore scheduling to allow
> preemption "(desktop)" but there's still a problem as I've described
> with one of the processes not using the core as it should.

What do you mean by "custom linux"?  Is this the exact same hardware as
your machine?  Or different?  If so, what is different?  What is
different between the different kernel versions you are using?  Does the
perf output look different from running on the two different machines?
If so, where?

Have you changed the priority levels of your application at all?  Have
you thought about just forcing your app to a specific CPU and getting
the kernel off of that CPU in order so that the kernel isn't even an
option here at all (Linux allows you to do this, details are somewhere
in the documentation, sorry, can't remember off the top of my head...)

But really, you should track down what the differences are between your
two machines/environments, as something is different that is causing the
slow down.

You haven't even said what kernel version you are using, and if you have
any of your own kernel patches in those kernels.

> > > I think that
> > > the linux scheduler isn't scheduling it, because this process is just
> > > some unit tests that double as benchmarks in that they shm_open a file
> > > and write into it with memcpy's.
> >
> > Are you sure that I/O isn't happening here like through swap or
> > something else?
> >
> 
> Well, we're using tmpfs and don't have a disk in the machine, but I
> will say this process is using all lot of the address space. One
> problem here is that the kernel has more ram than it thinks it does,

What do you mean, is this a hardware issue?

> but what I want to emphasize is that I haven't changed the program to
> allocate any more than it was previously. I'm not sure if that's a
> kernel change or some setting, but it went from 85% to 98%.

What exactly went up by 17%?

> The reason
> why is that there is a large latency even without that big program in
> there; I can't run my standalone tests in qemu without it also taking
> minutes. I understand qemu has to emulate, and that's its not just a
> VM, but I'm going from host CPU to guest, and the settings are the
> same.

That doesn't really make much sense, why is qemu even in the picture
here?  And no, qemu doesn't always emulate things, that depends on the
hardware you are running it on, and what type of image you are running
on it.

> > What does perf say is taking all of your time?
> 
> When I ran perf what it appeared to indicate is that the largest
> consumer of time was my library, which should be right in either
> scenario because it should use stay out of the kernel as I've designed
> it. In addition, the work takes place there anyway, so that's right.
> What's not right is the fact that the largest percent of time used is
> around 15%, and all the others combined don't add up to anything near
> 100.

So perhaps you have other processes running on the machine that you are
not noticing that is taking up the time slices?  Are you _sure_ nothing
else is running?

Basically, you have a bunch of variables, and haven't been very specific
with what really is changing, or even being used here, so there's not
much specific that I can think of at the moment.

thanks,

greg k-h