Fwd: Custom Linux Kernel Scheduler issue

Greg KH greg at kroah.com
Thu Nov 24 13:46:52 EST 2016

On Thu, Nov 24, 2016 at 11:33:04AM -0500, Kenneth Adam Miller wrote:
> On Thu, Nov 24, 2016 at 11:13 AM, Greg KH <greg at kroah.com> wrote:
> > 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?
> I don't think that could possibly be it, because the shared variables
> are controlled by atomics. It's just some memory operation to check to
> see if it needs to go to the kernel, as in is there more data in the
> shm region for me to read? If not, I'll go wait on this OS semaphore.
> It's lightening fast on my host machine.

Ah, but your "host" and your "test" machines are two totally different
things, as you say below.  So how do you know that memory accesses and
atomic writes/reads are the same?

> >> > > 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?
> I am building with buildroot a linux that is meant to be really
> stripped down and only have the things we want. In my case, the what
> the bzImage sees is either what QEMU gives it or what it sees in our
> dedicated hardware, with is just off the shelf i7 and other stuff you
> get a market - nothing custom in the sense you are thinking. Custom as
> in, roll your own linux.
> The kernel versions between my host and the target are 3.13.x and
> 3.14.5x; they don't change so much, and certainly don't affect
> performance on their own. I'm missing some setting or something with
> how I'm configuring or building linux.

Those are really old and obsolete kernel versions, not much we can do
with them here :)

> I haven't had a chance to run perf on my host. I can't find what
> ubuntu package it is just yet, but I will search for it in a minute. I
> have to go somewhere and will be right back immediately.
> >
> > 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...)
> >
> No, that may be it or help though. I thought that binding an
> application to a particular cpu had something to do with affinity and
> that there was some C api for it or something. That would work for our
> particular scenario, and we've even talked about it, I just don't know
> how to do it yet.
> > 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.
> True - the kernel configuration is most suspect based on everything I
> know. The hardware differences between my host to the target we're
> building for is each modern, and well supported by linux. I'm thinking
> it absolutely must have something to do with the way I've built linux.
> >
> > You haven't even said what kernel version you are using, and if you have
> > any of your own kernel patches in those kernels.
> >
> For the target hardware is 3.14.5x, and there aren't any kernel
> patches at this time; I've disabled grsec while in the process of
> narrowing down what the problem is.

Woah, grsec does a _lot_ of different things, you have to just not use
it if you wish to try to compare anything.

> >> > > 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?
> I don't think it's hardware; we're using this proprietary software
> beneath the linux kernel, but it's still ram of course. I can't say
> too too much, but what I can say is that while how much linux thinks
> it has could be affecting how it behaves, on our end we have the
> resources and can just change the configuration to make sure that
> linux sees and has enough ram. So that we can test on our end, and
> indeed we will.

Ah, this crazy thing.

You are running two totally different hardware platforms here, with
memory accesses working totally differently between them.  Of course
performance is going to be different, why would you expect it not to be?

So try to compare apples to apples, not apples to "the smell of apples".

Oh, and rip out grsec when doing benchmarks of anything, if you want to
have a chance of comparing kernels.

best of luck,

greg k-h

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