RE: [RFC] sched/topology: NUMA topology limitations

[Song Bao Hua (Barry Song)]

> Subject: RE: [RFC] sched/topology: NUMA topology limitations
> 
> > Subject: [RFC] sched/topology: NUMA topology limitations
> >
> > (For those of you who already got this one: sorry! I messed up LKML and
> > Vincent's addresses)
> >
> > Hi,
> >
> > Some of you may have noticed me struggling to plug some topology holes in
> > [1]. While digging in there I realized there are some non-obvious limitations
> > wrt supported NUMA topologies; IMO if we don't feel like they should be
> > "fixed", they should at the very least be documented somewhere.
> >
> > I should get to ramble about this at this year's (v)LPC during the scheduler
> > µconf, but this isn't really trivial so I figured an email wouldn't hurt. I tried to
> > package it into reasonable segments - this is still a pretty big wall of text, so
> > use of coffee/tea/beer is recommended.
> >
> > Spoiler alert: I don't have solutions for these yet.
> >
> > Diameter issue
> > ==============
> >
> > Definition
> > ++++++++++
> >
> > Each unique NUMA distance gets an index in
> sched_domains_numa_distance[],
> > and as the big wall of text atop topology.c::build_balance_mask() mentions
> > this can be seen as an array of hops. i.e. in the following table:
> >
> >       0  1
> >   0: 10 20
> >   1: 20 10
> >
> >   sched_domains_numa_distance = { 10, 20 };
> >
> > distance 10 is 0 hops (i.e. identity distance), distance 20 is 1 hop. This isn't
> > strictly speaking always true, as e.g. two consecutive distance values could
> > represent the same number of hops (with one path slightly "longer" than the
> > other), but it makes explaining that mess a bit easier.
> >
> > I'm using 'diameter' in the NoC meaning of the term, i.e. the longest shortest
> > path between any two nodes, in # of hops.
> >
> > Diameter <= 2
> > +++++++++++++
> >
> > AFAIU, one of the issues I'm encountering in [1] is that the scheduler
> topology
> > code cannot cope with a NUMA diameter > 2. I've looked into what's already
> > out there, but haven't found any (x86) machine that violates this rule. Below
> > are some examples.
> >
> > AMD Epyc
> > --------
> >
> > node   0   1   2   3   4   5   6   7
> >   0:  10  16  16  16  32  32  32  32
> >   1:  16  10  16  16  32  32  32  32
> >   2:  16  16  10  16  32  32  32  32
> >   3:  16  16  16  10  32  32  32  32
> >   4:  32  32  32  32  10  16  16  16
> >   5:  32  32  32  32  16  10  16  16
> >   6:  32  32  32  32  16  16  10  16
> >   7:  32  32  32  32  16  16  16  10
> >
> > Here, diameter=2
> >
> > Opteron 6174
> > ------------
> >
> > This is the 4-node one, which looks like this:
> >
> > node  0  1  2  3
> >   0: 10 20 20 20
> >   1: 20 10 20 20
> >   2: 20 20 10 20
> >   3: 20 20 20 10
> >
> >   0 ----- 3
> >   | \   / |
> >   |   X   |
> >   | /   \ |
> >   1 ----- 2
> >
> > Clearly this is diameter=1
> >
> > AMD Bulldozer
> > -------------
> >
> > I'm using the topology described in decade of wasted cores [2]; I'm not going
> > to bother reproducing the distance table, the topology looks like:
> >
> >    _______________________
> >   /        _____________  \
> >   |       /              \|
> >   0 ----- 4 ----- 5 ----- 1
> >   | \   / | \   / | \   / |
> >   |   X   |   X   |   X   |
> >   | /   \ | /   \ | /   \ |
> >   6 ----- 2 ----- 3 ----- 7
> >   |       \______________/|
> >   \_______________________/
> >
> > Which is diameter=2 (assuming equal edge weights)
> >
> > Diameter > 2
> > ++++++++++++
> >
> > I haven't found other NUMA systems out there with a diameter > 2 other
> than
> > the one I'm rambling about in [1]. Nevertheless, I think there are some
> > theoretical setups that aren't too insane (at least less than [1]).
> >
> > One of them is a 6-node mesh, which is diameter=3 and would look like:
> >
> >   0 ----- 5
> >   |       |
> >   1 ----- 4
> >   |       |
> >   2 ----- 3
> >
> > A more convoluted one would be a 12-node "spidergon"; it's a ring with an
> > even number of nodes N, each node is connected to both of its neighbours
> and
> > there is an interlink between node X and node (X + N/2) % N. I won't pretend
> I
> > can draw it in ASCII, but that again gives us a diameter of 3.
> >
> > Case study w/ QEMU
> > ++++++++++++++++++
> >
> > Setup
> > -----
> >
> > The topology I'll describe below is a simplified version of the one in [1], as it
> is
> > the smallest reproducer of the issue. Morten has been calling this a
> > "segmented bus".
> >
> >    1 ----- 0
> >    |
> >    |
> >    |
> >    2 ----- 3
> >
> > node   0   1   2   3
> >   0:  10  20  30  40
> >   1:  20  10  20  30
> >   2:  30  20  10  20
> >   3:  40  30  20  10
> >
> > Diameter here is 3. I can recreate this with the following qemu
> > incantation:
> >
> >   $ qemu-system-aarch64 -kernel Image -initrd rootfs.cpio -append \
> >                       'console=ttyAMA0 earlycon=pl011,0x9000000
> > root=/dev/vda debug earlyprintk=serial sched_debug' \
> >                       -smp cores=4 -nographic -m 512 -cpu cortex-a53
> > -machine virt \
> >                       -numa node,cpus=0,nodeid=0 -numa
> > node,cpus=1,nodeid=1, \
> >                       -numa node,cpus=2,nodeid=2, -numa
> > node,cpus=3,nodeid=3, \
> >                       -numa dist,src=0,dst=1,val=20, -numa
> > dist,src=0,dst=2,val=30, \
> >                       -numa dist,src=0,dst=3,val=40, -numa
> > dist,src=1,dst=2,val=20, \
> >                       -numa dist,src=1,dst=3,val=30, -numa
> > dist,src=2,dst=3,val=20
> >
> > If you boot the above with CONFIG_SCHED_DEBUG, you'll get:
> >
> > [    0.245896] CPU0 attaching sched-domain(s):
> > [    0.246133]  domain-0: span=0-1 level=NUMA
> > [    0.246592]   groups: 0:{ span=0 cap=1011 }, 1:{ span=1 cap=1008 }
> > [    0.246998]   domain-1: span=0-2 level=NUMA
> > [    0.247145]    groups: 0:{ span=0-1 mask=0 cap=2019 }, 2:{ span=1-3
> > mask=2 cap=3025 }
> > [    0.247454] ERROR: groups don't span domain->span
> 
> Hi Valtentin,
> Thanks for your email. It seems it is quite clear. May I ask what is the real harm
> when group 2 is actually out of the span of diameter 2 here? What will happen
> when group2
> doesn't span cpu0_domain1->span?
> In domain-1, will scheduler fail to do load balance between group0 and
> group2?
> 
After second thought, would the harm be that scheduler should do load balance
among cpu0, cpu1 and cpu2 in domain1, but it is actually doing load balance
among all of cpu0, cpu1, cpu2 and cpu3 since cpu3 is incorrectly put in group2?
So it is possible that scheduler will make wrong decision to put tasks in cpu3 while
it actually should only begin to do that in domain2?

Thanks
Barry