Messages in this thread | | | Subject | Re: [PATCH] cpu/hotplug: Fix rollback during error-out in takedown_cpu() | From | Neeraj Upadhyay <> | Date | Thu, 6 Sep 2018 14:30:41 +0530 |
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On 09/06/2018 01:48 PM, Thomas Gleixner wrote: > On Thu, 6 Sep 2018, Neeraj Upadhyay wrote: >> On 09/05/2018 06:47 PM, Thomas Gleixner wrote: >>> On Wed, 5 Sep 2018, Neeraj Upadhyay wrote: >>>> On 09/05/2018 05:53 PM, Thomas Gleixner wrote: >>>>> And looking closer this is a general issue. Just that the TEARDOWN state >>>>> makes it simple to observe. It's universaly broken, when the first >>>>> teardown >>>>> callback fails because, st->state is only decremented _AFTER_ the >>>>> callback >>>>> returns success, but undo_cpu_down() increments unconditionally. >>>>> >>>> As per my understanding, there are 2 problems here; one is fixed with your >>>> patch, and other is cpuhp_reset_state() is used during rollback from >>>> non-AP to >>>> AP state, which seem to result in 2 increments of st->state (one increment >>>> done by cpuhp_reset_state() and another by cpu_thread_fun()) . >>> And how did your hack fix that up magically? I'll have a look later today. >>> >>> Thanks, >>> >>> tglx >> >> The hack fixes it by not calling cpuhp_reset_state() and doing rollback state >> reset inline in _cpu_down(). > > And how is that any different from the proper patch I sent? It ends up in > the same state. So I have a hard time to understand your blurb above where > you claim that my patch just solves one of two problems? > > Thanks, > > tglx >
Yes, your patch solves the problem related to smpboot unparking being skipped during rollback and with the hack we end up in same state. The second thing, which I am referring to, is that there is one additional state increment. I missed the part that, it could be required, so that we reach CPUHP_AP_ONLINE_IDLE before calling __cpuhp_kick_ap(). So, it's not a problem.
* cpuhp_reset_state() does one state increment and we reach CPUHP_AP_ONLINE_IDLE.
if (ret && st->state > CPUHP_TEARDOWN_CPU && st->state < prev_state) { cpuhp_reset_state(st, prev_state); __cpuhp_kick_ap(st); }
CPUHP_TEARDOWN_CPU, CPUHP_AP_ONLINE_IDLE, CPUHP_AP_SMPBOOT_THREADS,
* cpuhp_thread_fun() does one more increment before invoking state callback (so, we skip CPUHP_AP_ONLINE_IDLE) and we reach CPUHP_AP_SMPBOOT_THREADS:
static void cpuhp_thread_fun(unsigned int cpu) <snip> if (bringup) { st->state++; state = st->state; <snip>
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