Re: [PATCH] sched/rt: Do not do push/pull when there is only one CPU
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: 2017-12-02 12:53:38
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lkml
On 2017-12-01 13:32:22 [-0500], Steven Rostedt wrote:
quoted hunk ↗ jump to hunk
Daniel Wagner reported a crash on the beaglebone black. This is a single CPU architecture, and does not have a functional: arch_send_call_function_single_ipi() and can crash if that is called. As it only has one CPU, it shouldn't be called, but if the kernel is compiled for SMP, the push/pull RT scheduling logic now calls it for irq_work if the one CPU is overloaded, it can use that function to call itself and crash the kernel. There's no reason for the push/pull logic to even be called if there's only one CPU online. Have it bail if it sees that's the case. Link: http://lkml.kernel.org/r/8c913cc2-b2e3-8c2e-e503-aff1428f8ff5@monom.org Fixes: 4bdced5c9 ("sched/rt: Simplify the IPI based RT balancing logic") Cc: stable@vger.kernel.org Reported-by: Daniel Wagner <redacted> ---diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4056c19ca3f0..50d2f8179f70 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c@@ -1784,6 +1784,10 @@ static int push_rt_task(struct rq *rq) if (!rq->rt.overloaded) return 0; + /* If we are the only CPU, don't bother */ + if (num_online_cpus() == 1) + return 0; +
what about a check next to sched_feat(RT_PUSH_IPI)? I don't know if this is a hot path or not (due to bitmap_weight). If it is, then I would suggest something like a jump-label which is enabled if more than one CPU has been enabled on boot.
quoted hunk ↗ jump to hunk
next_task = pick_next_pushable_task(rq); if (!next_task) return 0;@@ -2038,6 +2042,10 @@ static void pull_rt_task(struct rq *this_rq) if (likely(!rt_overloaded(this_rq))) return; + /* If we are the only CPU, don't bother */ + if (num_online_cpus() == 1) + return; + /* * Match the barrier from rt_set_overloaded; this guarantees that if we * see overloaded we must also see the rto_mask bit.
Sebastian