[patch 0/2] RFC sched: Change nohz ilb logic from poll to push model

venkatesh.pallipadi
2009-06-17T18:36:46Z

Existing nohz idle load balance (ilb) logic uses the pull model, with one
idle load balancer CPU nominated on any partially idle system and that
balancer CPU not going into nohz mode. With the periodic tick, the
balancer does the idle balancing on behalf of all the CPUs in nohz mode.

This is not very optimal and has few issues:
* the balancer will continue to have periodic ticks and wakeup
frequently (HZ rate), even though it may not have any rebalancing to do on
behalf of any of the idle CPUs.
* On x86 and CPUs that have APIC timer stoppage on idle CPUs, this periodic
wakeup can result in an additional interrupt on a CPU doing the timer
broadcast.
* The balancer may end up spending a lot of time doing the balancing on
behalf of nohz CPUs, especially with increasing number of sockets and
cores in the platform.

The alternative is to have a push model, where all idle CPUs can enter nohz
mode and busy CPU kicks one of the idle CPUs to take care of idle balancing
on behalf of a group of idle CPUs.

Following patches tries that approach. There are still some rough edges
in the patches related to use of #defines around the code. But, wanted
to get opinion on this approach as an RFC (not for inclusion into the
tree yet).

Thanks,
Venki