Re: [PATCH 1/4] perf topdown: Correct leader selection with sample_read enabled

Liang, Kan
2024-07-02T16:05:23Z

On 2024-07-02 6:40 p.m., Dapeng Mi wrote:
> Addresses an issue where, in the absence of a topdown metrics event
> within a sampling group, the slots event was incorrectly bypassed as
> the sampling leader when sample_read was enabled.
>
> perf record -e '{slots,branches}:S' -c 10000 -vv sleep 1
>
> In this case, the slots event should be sampled as leader but the
> branches event is sampled in fact like the verbose output shows.
>
> perf_event_attr:
> type 4 (cpu)
> size 168
> config 0x400 (slots)
> sample_type IP|TID|TIME|READ|CPU|IDENTIFIER
> read_format ID|GROUP|LOST
> disabled 1
> sample_id_all 1
> exclude_guest 1
> ------------------------------------------------------------
> sys_perf_event_open: pid -1 cpu 0 group_fd -1 flags 0x8 = 5
> ------------------------------------------------------------
> perf_event_attr:
> type 0 (PERF_TYPE_HARDWARE)
> size 168
> config 0x4 (PERF_COUNT_HW_BRANCH_INSTRUCTIONS)
> { sample_period, sample_freq } 10000
> sample_type IP|TID|TIME|READ|CPU|IDENTIFIER
> read_format ID|GROUP|LOST
> sample_id_all 1
> exclude_guest 1
>
> The sample period of slots event instead of branches event is reset to
> 0.
>
> This fix ensures the slots event remains the leader under these
> conditions.
>
> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> ---
> tools/perf/arch/x86/util/topdown.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/tools/perf/arch/x86/util/topdown.c b/tools/perf/arch/x86/util/topdown.c
> index 3f9a267d4501..5d7b78eb7516 100644
> --- a/tools/perf/arch/x86/util/topdown.c
> +++ b/tools/perf/arch/x86/util/topdown.c
> @@ -1,6 +1,7 @@
> // SPDX-License-Identifier: GPL-2.0
> #include "api/fs/fs.h"
> #include "util/evsel.h"
> +#include "util/evlist.h"
> #include "util/pmu.h"
> #include "util/pmus.h"
> #include "util/topdown.h"
> @@ -41,11 +42,22 @@ bool topdown_sys_has_perf_metrics(void)
> */
> bool arch_topdown_sample_read(struct evsel *leader)
> {
> + struct evsel *event;
> +
> if (!evsel__sys_has_perf_metrics(leader))
> return false;
>
> - if (leader->core.attr.config == TOPDOWN_SLOTS)
> - return true;
> + if (leader->core.attr.config != TOPDOWN_SLOTS)
> + return false;
> +
> + /*
> + * If slots event as leader event but no topdown metric events in group,
> + * slots event should still sample as leader.
> + */
> + evlist__for_each_entry(leader->evlist, event) {
> + if (event != leader && strcasestr(event->name, "topdown"))

User may uses the RAW format. It may not be good enough to just check
the event name.

I recall you have a complete support for this in the previous patch. Why
drop it?

Thanks,
Kan

> + return true;
> + }
>
> return false;
> }