[PATCH 0/2] xen: credit2: limit the number of CPUs per runqueue

[Dario Faggioli <dfaggioli@suse.com>]

In Credit2, the CPUs are assigned to runqueues according to the host
topology. For instance, if we want per-socket runqueues (which is the
default), the CPUs that are in the same socket will end up in the same
runqueue.

This is generally good for scalability, at least until the number of
CPUs that end up in the same runqueue is not too high. In fact, all this
CPUs will compete for the same spinlock, for making scheduling decisions
and manipulating the scheduler data structures. Therefore, if they are
too many, that can become a bottleneck.

This has not been an issue so far, but architectures with 128 CPUs per
socket are now available, and it is certainly unideal to have so many
CPUs in the same runqueue, competing for the same locks, etc.

Let's therefore set a cap to the total number of CPUs that can share a
runqueue. The value is set to 16, by default, but can be changed with
a boot command line parameter.

Note that this is orthogonal and independent from the activity of
introducing runqueue arrangement mechanisms and chriteria. In fact,
we very well may want to implement the capability of having, say, per-LLC
runqueues, or per-die (where that is defined) runqueues. In fact, this
would work well on current system, but nothing guarantees that, in
future, there won't be an architecture with hundreds of CPUs per DIE or
LLC... And we'll be back to where we are now.

Therefore, even if we are actually planning to add some new runqueue
organization criteria, fixing a limit for the number of CPUs that
will ever share a runqueue, whatever the underlying topology is, is
still useful.

Note also that, if the host has hyperthreading (and we are not using
core-scheduling), additional care is taken to avoid splitting CPUs that
are hyperthread siblings among different runqueues.

---
Dario Faggioli (2):
      xen: credit2: factor cpu to runqueue matching in a function
      xen: credit2: limit the max number of CPUs in a runqueue

 xen/common/sched/cpupool.c |    2 -
 xen/common/sched/credit2.c |  130 +++++++++++++++++++++++++++++++++++++++-----
 xen/common/sched/private.h |    2 +
 3 files changed, 119 insertions(+), 15 deletions(-)

--
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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