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Date: Tue, 6 Jun 2017 14:30:11 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Jeffrey Hugo <jhugo@...eaurora.org> Cc: Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org, Dietmar Eggemann <dietmar.eggemann@....com>, Austin Christ <austinwc@...eaurora.org>, Tyler Baicar <tbaicar@...eaurora.org>, Timur Tabi <timur@...eaurora.org> Subject: Re: [PATCH V4 1/2] sched/fair: Fix load_balance() affinity redo path On Fri, Jun 02, 2017 at 04:27:11PM -0600, Jeffrey Hugo wrote: > If load_balance() fails to migrate any tasks because all tasks were > affined, load_balance() removes the source cpu from consideration and > attempts to redo and balance among the new subset of cpus. > > There is a bug in this code path where the algorithm considers all active > cpus in the system (minus the source that was just masked out). This is > not valid for two reasons: some active cpus may not be in the current > scheduling domain and one of the active cpus is dst_cpu. These cpus should > not be considered, as we cannot pull load from them. > > Instead of failing out of load_balance(), we may end up redoing the search > with no valid cpus and incorrectly concluding the domain is balanced. > Additionally, if the group_imbalance flag was just set, it may also be > incorrectly unset, thus the flag will not be seen by other cpus in future > load_balance() runs as that algorithm intends. > > Fix the check by removing cpus not in the current domain and the dst_cpu > from considertation, thus limiting the evaluation to valid remaining cpus > from which load might be migrated. > > Co-authored-by: Austin Christ <austinwc@...eaurora.org> > Co-authored-by: Dietmar Eggemann <dietmar.eggemann@....com> > Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.org> > Tested-by: Tyler Baicar <tbaicar@...eaurora.org> > --- > kernel/sched/fair.c | 22 ++++++++++------------ > 1 file changed, 10 insertions(+), 12 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d711093..84255ab 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6737,10 +6737,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) > * our sched_group. We may want to revisit it if we couldn't > * meet load balance goals by pulling other tasks on src_cpu. > * > - * Also avoid computing new_dst_cpu if we have already computed > - * one in current iteration. > + * Avoid computing new_dst_cpu for NEWLY_IDLE or if we have > + * already computed one in current iteration. > */ > - if (!env->dst_grpmask || (env->flags & LBF_DST_PINNED)) > + if (env->idle == CPU_NEWLY_IDLE || (env->flags & LBF_DST_PINNED)) > return 0; > > /* Prevent to re-select dst_cpu via env's cpus */ > @@ -8091,14 +8091,7 @@ static int load_balance(int this_cpu, struct rq *this_rq, > .tasks = LIST_HEAD_INIT(env.tasks), > }; > > - /* > - * For NEWLY_IDLE load_balancing, we don't need to consider > - * other cpus in our group > - */ > - if (idle == CPU_NEWLY_IDLE) > - env.dst_grpmask = NULL; > - > - cpumask_copy(cpus, cpu_active_mask); > + cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask); > > schedstat_inc(sd->lb_count[idle]); > > @@ -8220,7 +8213,12 @@ static int load_balance(int this_cpu, struct rq *this_rq, > /* All tasks on this runqueue were pinned by CPU affinity */ > if (unlikely(env.flags & LBF_ALL_PINNED)) { > cpumask_clear_cpu(cpu_of(busiest), cpus); > - if (!cpumask_empty(cpus)) { > + /* > + * Go back to "redo" iff the load-balance cpumask > + * contains other potential busiest cpus for the > + * current sched domain. > + */ > + if (!cpumask_subset(cpus, env.dst_grpmask)) { > env.loop = 0; > env.loop_break = sched_nr_migrate_break; > goto redo; So I was struggling with that subset condition. You want to ensure there are CPUs outside of dst_grpmask left, otherwise balancing at this SD level doesn't make sense anymore, right? I think you might want to spell that out a little in that comment. Currently the comment only explains what it does, which is something we can read from the code. Comments should explain _why_ we do things and its failing there. So with that the problem is that active_load_balance_cpu_stop() calls into can_migrate_task() with ->idle = CPU_IDLE and !dst_grpmask, which then goes *bang*. Now active_load_balance_cpu_stop() doesn't need to re-evaluate anything, so ideally it would just skip this entirely, right? So why not do #3: --- kernel/sched/fair.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 47a0c552c77b..fd639d32fa4c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8523,6 +8523,13 @@ static int active_load_balance_cpu_stop(void *data) .src_cpu = busiest_rq->cpu, .src_rq = busiest_rq, .idle = CPU_IDLE, + /* + * can_migrate_task() doesn't need to compute new_dst_cpu + * for active balancing. Since we have CPU_IDLE, but no + * @dst_grpmask we need to make that test go away with lying + * about DST_PINNED. + */ + .flags = LBF_DST_PINNED, }; schedstat_inc(sd->alb_count);
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