[RFC PATCH 0/3] nohz: Move nohz kick out of scheduler IPI

From: Frederic Weisbecker
Date: Wed Mar 19 2014 - 14:28:38 EST

When a full dynticks CPU runs in single task mode then a new task gets
enqueued, we notify it through an IPI such that it restarts its tick.

The IPI used here is the scheduler IPI. There are a few reasons for that:
it can be called when interrupts are disabled, it can be called
concurrently... These properties altogether aren't offered by the IPI

Meanwhile, bloating that way the scheduler IPI with scheduler unrelated
code is an abuse of this fast path. We certainly don't want to start a
big kernel IPI (BKI, the new shiny piece of my collection of big kernel
things aside the big kernel tick, the big reiserfs lock, ..).

So this patchset adds a small helper to the IPI subsystem that allows
to queue an IPI from interrupt disabled code and handles concurrent
callers as well. Eventually the nohz kick gets converted to this new facility.

Partly inspired by a suggestion from Peter Zijlstra.




Frederic Weisbecker (3):
smp: Non busy-waiting IPI queue
nohz: Move full nohz kick to its own IPI
nohz: Use IPI implicit full barrier against rq->nr_running r/w

include/linux/smp.h | 12 ++++++++++++
include/linux/tick.h | 2 ++
kernel/sched/core.c | 14 ++++++--------
kernel/sched/sched.h | 12 +++++++++---
kernel/smp.c | 24 ++++++++++++++++++++++++
kernel/time/tick-sched.c | 20 ++++++++++++++++++++
6 files changed, 73 insertions(+), 11 deletions(-)
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