Messages in this thread Patch in this message | | | Subject | Re: [Question] report a race condition between CPU hotplug state machine and hrtimer 'sched_cfs_period_timer' for cfs bandwidth throttling | From | Xiongfeng Wang <> | Date | Thu, 29 Jun 2023 09:41:59 +0800 |
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On 2023/6/29 6:01, Thomas Gleixner wrote: > On Wed, Jun 28 2023 at 14:35, Vincent Guittot wrote: >> On Wed, 28 Jun 2023 at 14:03, Thomas Gleixner <tglx@linutronix.de> wrote: >>> No, because this is fundamentally wrong. >>> >>> If the CPU is on the way out, then the scheduler hotplug machinery >>> has to handle the period timer so that the problem Xiongfeng analyzed >>> does not happen in the first place. >> >> But the hrtimer was enqueued before it starts to offline the cpu > > It does not really matter when it was enqueued. The important point is > that it was enqueued on that outgoing CPU for whatever reason. > >> Then, hrtimers_dead_cpu should take care of migrating the hrtimer out >> of the outgoing cpu but : >> - it must run on another target cpu to migrate the hrtimer. >> - it runs in the context of the caller which can be throttled. > > Sure. I completely understand the problem. The hrtimer hotplug callback > does not run because the task is stuck and waits for the timer to > expire. Circular dependency. > >>> sched_cpu_wait_empty() would be the obvious place to cleanup armed CFS >>> timers, but let me look into whether we can migrate hrtimers early in >>> general. >> >> but for that we must check if the timer is enqueued on the outgoing >> cpu and we then need to choose a target cpu. > > You're right. I somehow assumed that cfs knows where it queued stuff, > but obviously it does not. > > I think we can avoid all that by simply taking that user space task out > of the picture completely, which avoids debating whether there are other > possible weird conditions to consider alltogether. > > Something like the untested below should just work. > > Thanks, > > tglx > --- > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -1490,6 +1490,13 @@ static int cpu_down(unsigned int cpu, en > return err; > } > > +static long __cpu_device_down(void *arg) > +{ > + struct device *dev = arg; > + > + return cpu_down(dev->id, CPUHP_OFFLINE); > +} > + > /** > * cpu_device_down - Bring down a cpu device > * @dev: Pointer to the cpu device to offline > @@ -1502,7 +1509,12 @@ static int cpu_down(unsigned int cpu, en > */ > int cpu_device_down(struct device *dev) > { > - return cpu_down(dev->id, CPUHP_OFFLINE); > + unsigned int cpu = cpumask_any_but(cpu_online_mask, dev->id); > + > + if (cpu >= nr_cpu_ids) > + return -EBUSY; > + > + return work_on_cpu(cpu, __cpu_device_down, dev); > } > > int remove_cpu(unsigned int cpu) > . >
Test with the following kernel modification which helps reproduce the issue. The hang task does not happen any more. Thanks a lot.
Thanks, Xiongfeng
--- a/kernel/fork.c +++ b/kernel/fork.c @@ -110,6 +110,8 @@ #define CREATE_TRACE_POINTS #include <trace/events/task.h>
+#include <linux/delay.h> + /* * Minimum number of threads to boot the kernel */ @@ -199,6 +201,9 @@ static int free_vm_stack_cache(unsigned int cpu) struct vm_struct **cached_vm_stacks = per_cpu_ptr(cached_stacks, cpu); int i;
+ mdelay(2000); + cond_resched(); + for (i = 0; i < NR_CACHED_STACKS; i++) { struct vm_struct *vm_stack = cached_vm_stacks[i];
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