NUMA topologies where the shortest path between some two nodes requires
three or more hops (i.e. diameter > 2) end up being misrepresented in the
scheduler topology structures.
This is currently detected when booting a kernel with CONFIG_SCHED_DEBUG=y
+ sched_debug on the cmdline, although this will only yield a warning about
sched_group spans not matching sched_domain spans:
ERROR: groups don't span domain->span
Add an explicit warning for that case, triggered regardless of
CONFIG_SCHED_DEBUG, and decorate it with an appropriate comment.
The topology described in the comment can be booted up on QEMU by appending
the following to your usual QEMU incantation:
-smp cores=4 \
-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
A somewhat more realistic topology (6-node mesh) with the same affliction
can be conjured with:
-smp cores=6 \
-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 node,cpus=4,nodeid=4, -numa node,cpus=5,nodeid=5, \
-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=0,dst=4,val=30, \
-numa dist,src=0,dst=5,val=20, \
-numa dist,src=1,dst=2,val=20, -numa dist,src=1,dst=3,val=30, \
-numa dist,src=1,dst=4,val=20, -numa dist,src=1,dst=5,val=30, \
-numa dist,src=2,dst=3,val=20, -numa dist,src=2,dst=4,val=30, \
-numa dist,src=2,dst=5,val=40, \
-numa dist,src=3,dst=4,val=20, -numa dist,src=3,dst=5,val=30, \
-numa dist,src=4,dst=5,val=20
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lore.kernel.org/lkml/jhjtux5edo2.mognet@arm.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.
b5b217346d