Re: [PATCH] perf: fix group with mixed hw and sw events

[Song Liu <songliubraving@fb.com>]

Dear Peter,

Could you please share your comments on this minor fix?

Best,
Song


> On May 3, 2018, at 12:47 PM, Song Liu <songliubraving@fb.com> wrote:
> 
> When hw and sw events are mixed in the same group, they are all attached
> to the hw perf_event_context. This sometimes requires moving group of
> perf_event to a different context. We found an issue in the moving. Here
> is an example of it.
> 
>   perf stat -e '{faults,ref-cycles,faults}'  -I 1000
> 
>     1.005591180              1,297      faults
>     1.005591180        457,476,576      ref-cycles
>     1.005591180    <not supported>      faults
> 
> First, sw event "faults" is attached to the sw context, and become the
> group leader. Then, hw event "ref-cycles" is attached, so both events
> are moved to hw context. Last, another sw "faults" tries to attach,
> but it fails because of mismatch between the new target ctx (from sw
> pmu) and the group_leader's ctx (hw context, same as ref-cycles).
> 
> The broken condition is:
>   group_leader is sw event;
>   group_leader is on hw context;
>   add a sw event to the group.
> 
> This patch fixes this scenario by checking group_leader's context
> (instead of just event type). If group_leader is on hw context, use
> pmu of this context to look up context for the new event.
> 
> Fixes: b04243ef7006 ("perf: Complete software pmu grouping")
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
> include/linux/perf_event.h |  8 ++++++++
> kernel/events/core.c       | 21 +++++++++++----------
> 2 files changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index e71e99e..def866f 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -1016,6 +1016,14 @@ static inline int is_software_event(struct perf_event *event)
> 	return event->event_caps & PERF_EV_CAP_SOFTWARE;
> }
> 
> +/*
> + * Return 1 for event in sw context, 0 for event in hw context
> + */
> +static inline int in_software_context(struct perf_event *event)
> +{
> +	return event->ctx->pmu->task_ctx_nr == perf_sw_context;
> +}
> +
> extern struct static_key perf_swevent_enabled[PERF_COUNT_SW_MAX];
> 
> extern void ___perf_sw_event(u32, u64, struct pt_regs *, u64);
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 67612ce..ce6aa5f 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10521,19 +10521,20 @@ SYSCALL_DEFINE5(perf_event_open,
> 	if (pmu->task_ctx_nr == perf_sw_context)
> 		event->event_caps |= PERF_EV_CAP_SOFTWARE;
> 
> -	if (group_leader &&
> -	    (is_software_event(event) != is_software_event(group_leader))) {
> -		if (is_software_event(event)) {
> +	if (group_leader) {
> +		if (is_software_event(event) &&
> +		    !in_software_context(group_leader)) {
> 			/*
> -			 * If event and group_leader are not both a software
> -			 * event, and event is, then group leader is not.
> +			 * If the event is a sw event, but the group_leader
> +			 * is on hw context.
> 			 *
> -			 * Allow the addition of software events to !software
> -			 * groups, this is safe because software events never
> -			 * fail to schedule.
> +			 * Allow the addition of software events to hw
> +			 * groups, this is safe because software events
> +			 * never fail to schedule.
> 			 */
> -			pmu = group_leader->pmu;
> -		} else if (is_software_event(group_leader) &&
> +			pmu = group_leader->ctx->pmu;
> +		} else if (!is_software_event(event) &&
> +			   is_software_event(group_leader) &&
> 			   (group_leader->group_caps & PERF_EV_CAP_SOFTWARE)) {
> 			/*
> 			 * In case the group is a pure software group, and we
> -- 
> 2.9.5
>