Messages in this thread Patch in this message | | | Date | Wed, 30 Mar 2011 15:09:51 +0200 | From | Jiri Olsa <> | Subject | Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx value |
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On Mon, Mar 28, 2011 at 06:56:48PM +0200, Oleg Nesterov wrote: > On 03/28, Peter Zijlstra wrote: > > > > Another fun race, suppose we do properly remove task_ctx and is_active, > > but then the task gets scheduled back in before free_event() gets around > > to disabling the jump_label.. > > Yes, this too... > > Well, ignoring the HAVE_JUMP_LABEL case... perhaps we can split > perf_sched_events into 2 counters? I mean, > > atomic_t perf_sched_events_in, perf_sched_events_out; > > static inline void perf_event_task_sched_in(struct task_struct *task) > { > COND_STMT(&perf_sched_events_in, __perf_event_task_sched_in(task)); > } > > static inline > void perf_event_task_sched_out(struct task_struct *task, struct task_struct *next) > { > perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 1, NULL, 0); > > COND_STMT(&perf_sched_events_out, __perf_event_task_sched_out(task, next)); > } > > void perf_sched_events_inc(void) > { > atomic_inc(&perf_sched_events_out); > smp_mb__after_atomic_inc(); > atomic_inc(&perf_sched_events_in); > } > > void perf_sched_events_dec(void) > { > if (atomic_dec_and_test(&perf_sched_events_in)) > synchronize_sched(); > atomic_dec(&perf_sched_events_out); > } > > The last 2 helpers should be used instead of jump_label_inc/dec.
this approach seems to work, plz check attached patch
> > > > Or we can remove COND_STMT() from perf_event_task_sched_out(). Say, > __perf_event_task_sched_in() can set PF_PERF_XXX or something, then > perf_event_task_sched_out() can check this bit. > > > As for HAVE_JUMP_LABEL, I still can't understand why this test-case > triggers the problem. But jump_label_inc/dec logic looks obviously > racy.
I think the reason is, that there are actually 2 places that needs to be updated by jump label code - perf_event_task_sched_in - perf_event_task_sched_out
As for my image it first patches perf_event_task_sched_out and then perf_event_task_sched_in
Now, when the code is updated, migration thread is scheduled several times and perf schedule callbacks are called.
This seems to be happening in my setup:
perf release: - jump_label_dec(perf_sched_events) - migration thread is scheduled for disabling perf_event_task_sched_out jump label - perf_event_task_sched_out/perf_event_task_sched_in is called - task_ctx is NULL, since migration thread does not have any events - perf_event_task_sched_out jump label got disabled - release code is rescheduled -> now only perf_event_task_sched_in jump label is called - task_ctx is set - migration thread is scheduled for disabling perf_event_task_sched_in jump label - perf_event_task_sched_in is called for migration thread, task_ctx stays, since migration thread does not have any events - perf_event_task_sched_in jump label is disabled - release code is scheduled -> no sched callback is called - task_ctx stays != NULL
I'm almost 5% sure it could happen as I described ;)
Back to the patch below:
IIUIC that calling synchronize_sched in perf_sched_events_dec ensures that final schedule back to the release code will not call perf_event_task_sched_in, so task_ctx stays sane (NULL).
The cool think is, that it also fixies the original issue, which was: wrong counters for perf builtin test #3.. which I started with from the very beggining :)
please let me know what you think, thanks jirka
--- include/linux/perf_event.h | 44 +++++++++++++++++++++++++++++++++++++++++--- kernel/perf_event.c | 11 ++++++----- 2 files changed, 47 insertions(+), 8 deletions(-)
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index 311b4dc..4b4f114 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -1074,11 +1074,12 @@ have_event: __perf_sw_event(event_id, nr, nmi, regs, addr); } -extern atomic_t perf_sched_events; +extern atomic_t perf_sched_events_in; +extern atomic_t perf_sched_events_out; static inline void perf_event_task_sched_in(struct task_struct *task) { - COND_STMT(&perf_sched_events, __perf_event_task_sched_in(task)); + COND_STMT(&perf_sched_events_in, __perf_event_task_sched_in(task)); } static inline @@ -1086,9 +1087,46 @@ void perf_event_task_sched_out(struct task_struct *task, struct task_struct *nex { perf_sw_event(PERF_COUNT_SW_CONTEXT_SWITCHES, 1, 1, NULL, 0); - COND_STMT(&perf_sched_events, __perf_event_task_sched_out(task, next)); + COND_STMT(&perf_sched_events_out, __perf_event_task_sched_out(task, next)); } +#ifdef HAVE_JUMP_LABEL +static inline +void perf_sched_events_inc(void) +{ + jump_label_inc(&perf_sched_events_out); + smp_mb__after_atomic_inc(); + jump_label_inc(&perf_sched_events_in); +} + +static inline +void perf_sched_events_dec(void) +{ + if (atomic_dec_and_test(&perf_sched_events_in)) { + jump_label_disable(&perf_sched_events_in); + synchronize_sched(); + } + + jump_label_dec(&perf_sched_events_out); +} +#else +static inline +void perf_sched_events_inc(void) +{ + atomic_inc(&perf_sched_events_out); + smp_mb__after_atomic_inc(); + atomic_inc(&perf_sched_events_in); +} + +static inline +void perf_sched_events_dec(void) +{ + if (atomic_dec_and_test(&perf_sched_events_in)) + synchronize_sched(); + atomic_dec(&perf_sched_events_out); +} +#endif /* HAVE_JUMP_LABEL */ + extern void perf_event_mmap(struct vm_area_struct *vma); extern struct perf_guest_info_callbacks *perf_guest_cbs; extern int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *callbacks); diff --git a/kernel/perf_event.c b/kernel/perf_event.c index c75925c..db38d6e 100644 --- a/kernel/perf_event.c +++ b/kernel/perf_event.c @@ -125,7 +125,8 @@ enum event_type_t { * perf_sched_events : >0 events exist * perf_cgroup_events: >0 per-cpu cgroup events exist on this cpu */ -atomic_t perf_sched_events __read_mostly; +atomic_t perf_sched_events_in __read_mostly; +atomic_t perf_sched_events_out __read_mostly; static DEFINE_PER_CPU(atomic_t, perf_cgroup_events); static atomic_t nr_mmap_events __read_mostly; @@ -2894,7 +2895,7 @@ static void free_event(struct perf_event *event) if (!event->parent) { if (event->attach_state & PERF_ATTACH_TASK) - jump_label_dec(&perf_sched_events); + perf_sched_events_dec(); if (event->attr.mmap || event->attr.mmap_data) atomic_dec(&nr_mmap_events); if (event->attr.comm) @@ -2905,7 +2906,7 @@ static void free_event(struct perf_event *event) put_callchain_buffers(); if (is_cgroup_event(event)) { atomic_dec(&per_cpu(perf_cgroup_events, event->cpu)); - jump_label_dec(&perf_sched_events); + perf_sched_events_dec(); } } @@ -6248,7 +6249,7 @@ done: if (!event->parent) { if (event->attach_state & PERF_ATTACH_TASK) - jump_label_inc(&perf_sched_events); + perf_sched_events_inc(); if (event->attr.mmap || event->attr.mmap_data) atomic_inc(&nr_mmap_events); if (event->attr.comm) @@ -6490,7 +6491,7 @@ SYSCALL_DEFINE5(perf_event_open, * - that may need work on context switch */ atomic_inc(&per_cpu(perf_cgroup_events, event->cpu)); - jump_label_inc(&perf_sched_events); + perf_sched_events_inc(); } /* -- 1.7.1
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