Re: [PATCH 2/2] perf: Fix mixed hw/sw event group initialization

Namhyung Kim
2013-03-11T11:20:58Z

Hi Peter,

On Mon, 11 Mar 2013 11:01:14 +0100, Peter Zijlstra wrote:
> On Thu, 2013-03-07 at 13:19 +0900, Namhyung Kim wrote:
>> From: Namhyung Kim <namhyung.kim@lge.com>
>>
>> There's a problem with mixed hw/sw group when the leader is a software
>> event. For instance:
>
>> Jiri's patch 0231bb533675 ("perf: Fix event group context move") fixed
>> a part of problem but there's a devil still..
>>
>> The problem arose when a sw event is added to already moved (to hw
>> context) group whose leader also is a sw event. In the above example
>>
>> 1. task-clock (sw event) is a group leader (has PERF_GROUP_SOFTWARE)
>> 2. cycles (hw event) is added, so the leader moved to the hw context
>> 3. faults (sw event) is added but the leader also is a sw event
>> 4. after find_get_context(), ctx is not same as leader->ctx since the
>> leader had moved to the hw context (-EINVAL)
>>
>> Fix it by adding new PERF_GROUP_MIXED flag and use leader's ctx->pmu
>> if it's set.
>
>> Reported-by: Andreas Hollmann <hollmann@in.tum.de>
>> Cc: Jiri Olsa <jolsa@redhat.com>
>> Cc: Vince Weaver <vince@deater.net>
>> Cc: Frederic Weisbecker <fweisbec@gmail.com>
>> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
>> ---
>> include/linux/perf_event.h | 1 +
>> kernel/events/core.c | 37 ++++++++++++++++++++++---------------
>> 2 files changed, 23 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index e47ee462c2f2..001a3b64fe61 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -285,6 +285,7 @@ typedef void (*perf_overflow_handler_t)(struct perf_event *,
>>
>> enum perf_group_flag {
>> PERF_GROUP_SOFTWARE = 0x1,
>> + PERF_GROUP_MIXED = 0x2,
>> };
>>
>> #define SWEVENT_HLIST_BITS 8
>> diff --git a/kernel/events/core.c b/kernel/events/core.c
>> index 007dfe846d4d..06266d5ed500 100644
>> --- a/kernel/events/core.c
>> +++ b/kernel/events/core.c
>> @@ -6441,6 +6441,8 @@ out:
>> * @pid: target pid
>> * @cpu: target cpu
>> * @group_fd: group leader event fd
>> + * @flags: flags which controls the meaning of arguments.
>> + * see PERF_FLAG_*
>> */
>> SYSCALL_DEFINE5(perf_event_open,
>> struct perf_event_attr __user *, attr_uptr,
>> @@ -6536,26 +6538,30 @@ SYSCALL_DEFINE5(perf_event_open,
>> */
>> pmu = event->pmu;
>>
>> - if (group_leader &&
>> - (is_software_event(event) != is_software_event(group_leader))) {
>> - if (is_software_event(event)) {
>> - /*
>> - * If event and group_leader are not both a software
>> - * event, and event is, then group leader is not.
>> - *
>> - * Allow the addition of software events to !software
>> - * groups, this is safe because software events never
>> - * fail to schedule.
>> - */
>> - pmu = group_leader->pmu;
>> - } else if (is_software_event(group_leader) &&
>> - (group_leader->group_flags & PERF_GROUP_SOFTWARE)) {
>> + if (group_leader) {
>> + if (group_leader->group_flags & PERF_GROUP_SOFTWARE) {
>> /*
>> * In case the group is a pure software group, and we
>> * try to add a hardware event, move the whole group to
>> * the hardware context.
>> */
>> - move_group = 1;
>> + if (!is_software_event(event))
>> + move_group = 1;
>> + } else if (group_leader->group_flags & PERF_GROUP_MIXED) {
>> + /*
>> + * The group leader was moved on to a hardware context,
>> + * so move this event also.
>> + */
>> + if (is_software_event(event))
>> + pmu = group_leader->ctx->pmu;
>> + } else if (!is_software_event(group_leader)) {
>> + /*
>> + * Allow the addition of software events to !software
>> + * groups, this is safe because software events never
>> + * fail to schedule.
>> + */
>> + if (is_software_event(event))
>> + pmu = group_leader->pmu;
>> }
>> }
>>
>> @@ -6650,6 +6656,7 @@ SYSCALL_DEFINE5(perf_event_open,
>> perf_install_in_context(ctx, sibling, event->cpu);
>> get_ctx(ctx);
>> }
>> + group_leader->group_flags = PERF_GROUP_MIXED;
>> }
>>
>> perf_install_in_context(ctx, event, event->cpu);
>
>
> This seems reasonable, but I think the perf_group_detach thing needs to
> migrate events between pmu's as well to complete the whole mess.

Hmm.. it seems that it needs whole perf_{remove_from,install_in}_context
step to migrate them, right? Looks like not a simple job for me. :(

Any advice?

Thanks,
Namhyung