On 08/25/2015 06:19 AM, Peter Zijlstra wrote: > 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 <r...@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.
Ahhh, so that's what I've been seeing! Also, I am not sure that find_busiest_group will actually return the group the CPU is in, so the patch may be wrong in another way too. I have yet to wrap my head around the sd & sg stuff (again), to figure out that bit... -- All rights reversed -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/