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

Peter Zijlstra
2013-03-11T10:01:29Z

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.