Messages in this thread | | | Date | Tue, 19 Apr 2022 14:34:37 +0200 | From | David Hildenbrand <> | Subject | Re: [RFC PATCH] kernel/cpu: restart cpu_up when hotplug is disabled |
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On 18.04.22 21:54, Joel Savitz wrote: > The cpu hotplug path may be utilized while hotplug is disabled for a > brief moment leading to failures. As an example, attempts to perform > cpu hotplug by userspace soon after boot may race with pci_device_probe > leading to inconsistent results.
You might want to extend a bit in which situation we observed that issue fairly reliably.
When restricting the number of boot cpus on the kernel cmdline, e.g., via "maxcpus=2", udev will find the offline cpus when enumerating all cpus and try onlining them. Due to the race, onlining of some cpus fails e.g., when racing with pci_device_probe().
While teaching udev to not online coldplugged CPUs when "maxcpus" was specified ("policy"), it revealed the underlying issue that onlining a CPU can fail with -EBUSY in corner cases when cpu hotplug is temporarily disabled.
> > Proposed idea: > Call restart_syscall instead of returning -EBUSY since > cpu_hotplug_disabled seems to only have a positive value > for short, temporary amounts of time. > > Does anyone see any serious problems with this? > > Signed-off-by: Joel Savitz <jsavitz@redhat.com> > --- > kernel/cpu.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 5797c2a7a93f..2992c7d1d24e 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -35,6 +35,7 @@ > #include <linux/percpu-rwsem.h> > #include <linux/cpuset.h> > #include <linux/random.h> > +#include <linux/delay.h> > > #include <trace/events/power.h> > #define CREATE_TRACE_POINTS > @@ -1401,7 +1402,9 @@ static int cpu_up(unsigned int cpu, enum cpuhp_state target) > cpu_maps_update_begin(); > > if (cpu_hotplug_disabled) { > - err = -EBUSY; > + /* avoid busy looping (5ms of sleep should be enough) */ > + msleep(5); > + err = restart_syscall();
It's worth noting that we use the same approach in lock_device_hotplug_sysfs(). It's far from perfect I would say, but we really wanted to avoid letting user space having to deal with retry logic.
For example, while memory onlining can fail with -EBUSY, it's not expected to fail during memory onlining (we only fail in very rare cases, when a memory notifier fails -- for example when kasan fails to allocate memory).
-- Thanks,
David / dhildenb
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