Messages in this thread Patch in this message | | | Date | Wed, 26 Jul 2017 05:57:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH RFC] sched: Allow migrating kthreads into online but inactive CPUs |
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On Tue, Jul 25, 2017 at 06:58:21PM +0200, Peter Zijlstra wrote: > Hi, > > On Sat, Jun 17, 2017 at 08:10:08AM -0400, Tejun Heo wrote: > > Per-cpu workqueues have been tripping CPU affinity sanity checks while > > a CPU is being offlined. A per-cpu kworker ends up running on a CPU > > which isn't its target CPU while the CPU is online but inactive. > > > > While the scheduler allows kthreads to wake up on an online but > > inactive CPU, it doesn't allow a running kthread to be migrated to > > such a CPU, which leads to an odd situation where setting affinity on > > a sleeping and running kthread leads to different results. > > > > Each mem-reclaim workqueue has one rescuer which guarantees forward > > progress and the rescuer needs to bind itself to the CPU which needs > > help in making forward progress; however, due to the above issue, > > while set_cpus_allowed_ptr() succeeds, the rescuer doesn't end up on > > the correct CPU if the CPU is in the process of going offline, > > tripping the sanity check and executing the work item on the wrong > > CPU. > > > > This patch updates __migrate_task() so that kthreads can be migrated > > into an inactive but online CPU. > > > > Signed-off-by: Tejun Heo <tj@kernel.org> > > Reported-by: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> > > Reported-by: Steven Rostedt <rostedt@goodmis.org> > > Hmm.. so the rules for running on !active && online are slightly > stricter than just being a kthread, how about the below, does that work > too?
Of 24 one-hour runs of the TREE07 rcutorture scenario, two had stalled tasks with this patch. One of them had more than 200 instances, the other two instances. In contrast, a 30-hour run a week ago with Tejun's patch completed cleanly. Here "stalled task" means that one of rcutorture's update-side kthreads fails to make any progress for more than 15 seconds. Grace periods are progressing, but a kthread waiting for a grace period isn't making progress, and is stuck with its ->state field at 0x402, that is TASK_NOLOAD|TASK_UNINTERRUPTIBLE. Which is as if it never got the wakeup, given that it is sleeping on schedule_timeout_idle().
Now, two of 24 might just be bad luck, but I haven't seen anything like this out of TREE07 since I queued Tejun's patch, so I am inclined to view your patch below with considerable suspicion.
I -am- seeing this out of TREE01, even with Tejun's patch, but that scenario sets maxcpu=8 and nr_cpus=43, which seems to be tickling an issue that several other people are seeing. Others' testing seems to indicate that setting CONFIG_SOFTLOCKUP_DETECTOR=y suppresses this issue, but I need to do an overnight run to check my test cases, and that is tonight.
So there might be something else going on as well.
Thanx, Paul
> kernel/sched/core.c | 36 ++++++++++++++++++++++++++++++------ > 1 file changed, 30 insertions(+), 6 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index d3d39a283beb..59b667c16826 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -894,6 +894,22 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags) > } > > #ifdef CONFIG_SMP > + > +/* > + * Per-CPU kthreads are allowed to run on !actie && online CPUs, see > + * __set_cpus_allowed_ptr() and select_fallback_rq(). > + */ > +static inline bool is_per_cpu_kthread(struct task_struct *p) > +{ > + if (!(p->flags & PF_KTHREAD)) > + return false; > + > + if (p->nr_cpus_allowed != 1) > + return false; > + > + return true; > +} > + > /* > * This is how migration works: > * > @@ -951,8 +967,13 @@ struct migration_arg { > static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf, > struct task_struct *p, int dest_cpu) > { > - if (unlikely(!cpu_active(dest_cpu))) > - return rq; > + if (is_per_cpu_kthread(p)) { > + if (unlikely(!cpu_online(dest_cpu))) > + return rq; > + } else { > + if (unlikely(!cpu_active(dest_cpu))) > + return rq; > + } > > /* Affinity changed (again). */ > if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed)) > @@ -1482,10 +1503,13 @@ static int select_fallback_rq(int cpu, struct task_struct *p) > for (;;) { > /* Any allowed, online CPU? */ > for_each_cpu(dest_cpu, &p->cpus_allowed) { > - if (!(p->flags & PF_KTHREAD) && !cpu_active(dest_cpu)) > - continue; > - if (!cpu_online(dest_cpu)) > - continue; > + if (is_per_cpu_kthread(p)) { > + if (!cpu_online(dest_cpu)) > + continue; > + } else { > + if (!cpu_active(dest_cpu)) > + continue; > + } > goto out; > } > >
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