From: Nicholas Piggin npiggin@gmail.com
[ Upstream commit 9b019acb72e4b5741d88e8936d6f200ed44b66b2 ]
The NOHZ idle balancer runs on the lowest idle CPU. This can interfere with isolated CPUs, so confine it to HK_FLAG_MISC housekeeping CPUs.
HK_FLAG_SCHED is not used for this because it is not set anywhere at the moment. This could be folded into HK_FLAG_SCHED once that option is fixed.
The problem was observed with increased jitter on an application running on CPU0, caused by NOHZ idle load balancing being run on CPU1 (an SMT sibling).
Signed-off-by: Nicholas Piggin npiggin@gmail.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Frederic Weisbecker fweisbec@gmail.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Link: https://lkml.kernel.org/r/20190412042613.28930-1-npiggin@gmail.com Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 16 ++++++++++------ 1 file changed, 10 insertions(+), 6 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index be55a64748ba3..d905c443e10e5 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -9456,22 +9456,26 @@ static inline int on_null_domain(struct rq *rq) * - When one of the busy CPUs notice that there may be an idle rebalancing * needed, they will kick the idle load balancer, which then does idle * load balancing for all the idle CPUs. + * - HK_FLAG_MISC CPUs are used for this task, because HK_FLAG_SCHED not set + * anywhere yet. */
static inline int find_new_ilb(void) { - int ilb = cpumask_first(nohz.idle_cpus_mask); + int ilb;
- if (ilb < nr_cpu_ids && idle_cpu(ilb)) - return ilb; + for_each_cpu_and(ilb, nohz.idle_cpus_mask, + housekeeping_cpumask(HK_FLAG_MISC)) { + if (idle_cpu(ilb)) + return ilb; + }
return nr_cpu_ids; }
/* - * Kick a CPU to do the nohz balancing, if it is time for it. We pick the - * nohz_load_balancer CPU (if there is one) otherwise fallback to any idle - * CPU (if there is one). + * Kick a CPU to do the nohz balancing, if it is time for it. We pick any + * idle CPU in the HK_FLAG_MISC housekeeping set (if there is one). */ static void kick_ilb(unsigned int flags) {