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    SubjectRe: [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@codeaurora.org>
    > Co-authored-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    > Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
    > Tested-by: Tyler Baicar <tbaicar@codeaurora.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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