Messages in this thread | | | Date | Tue, 25 Aug 2015 12:19:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH,RFC] sched: fix "impossible" load balancing oops |
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On Fri, Aug 21, 2015 at 02:21:22PM -0400, Rik van Riel wrote: > The load balancing code can run into the situation where > the source and destination runqueues are the same, in > the active balancing code. > > /* > * This condition is "impossible", if it occurs > * we need to fix it. Originally reported by > * Bjorn Helgaas on a 128-cpu setup. > */ > BUG_ON(busiest_rq == target_rq); > > This happens despite not triggering the BUG_ON(busiest == env.dst_rq) > line after find_busiest_queue. > > From code inspection, it appears there is a condition where this can happen. > > Specifically, if we encounter only pinned tasks on a CPU, can_migrate_task > will set env->new_dst_cpu to a CPU in the env->dst_grpmask. If the group > includes the source cpu, we may end up setting env.dst_cpu to the same > as dst.src_cpu. > > The fix would be to clear the source cpu from env.dst_grpmask, to ensure > we never select the source cpu as the destination. > > Signed-off-by: Rik van Riel <riel@redhat.com> > --- > kernel/sched/fair.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index d113c3b..514a369 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7172,6 +7172,8 @@ redo: > env.src_cpu = busiest->cpu; > env.src_rq = busiest; > > + cpumask_clear_cpu(busiest->cpu, env.dst_grpmask); > + > ld_moved = 0; > if (busiest->nr_running > 1) { > /*
Right, so real problem 'recently' introduced by:
88b8dac0a14c ("sched: Improve balance_cpu() to consider other cpus in its group as target of (pinned) task")
But I think there's a wee problem with the solution, if I'm not still entirely asleep, it appears dst_grpmask might be NULL in case of CPU_NEWLY_IDLE, which would make that cpumask_clear_cpu() do something naughty.
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