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

[Liang, Kan]

Hi Dapeng,

On 2024-06-14 5:39 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.

This should be just one of the issues with the slots/topdown related
sampling read.

If adding one more topdown event, the sampling read may still be broken.
 perf record -e "{slots,instructions,topdown-retiring}:S"  -C0 sleep 1
 WARNING: events were regrouped to match PMUs
 Error:
 The sys_perf_event_open() syscall returned with 22 (Invalid argument)
for event (topdown-retiring).

That may require Yanfei's patch.
https://lore.kernel.org/lkml/20240411144852.2507143-1-yanfei.xu@xxxxxxxxx/

Please give it try and summarize all the required patches for the
topdown sampling read feature.

Besides, we need a test for the sampling read as well.
Ian has provided a very good base. Please add a topdown sampling read
case on top of it as well.
https://lore.kernel.org/lkml/CAP-5=fUkg-cAXTb+3wbFOQCfdXgpQeZw40XHjfrNFbnBD=NMXg@xxxxxxxxxxxxxx/


Thanks,
Kan

> 
> Reviewed-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> ---
>  tools/perf/arch/x86/util/topdown.c | 42 ++++++++++++++++++++++++++++--
>  1 file changed, 40 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/arch/x86/util/topdown.c b/tools/perf/arch/x86/util/topdown.c
> index 3f9a267d4501..aea6896bbb57 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"
> @@ -31,6 +32,32 @@ bool topdown_sys_has_perf_metrics(void)
>  	return has_perf_metrics;
>  }
>  
> +static int perf_pmus__topdown_event(void *vstate, struct pmu_event_info *info)
> +{
> +	if (!strcmp(info->name, (char *)vstate))
> +		return 1;
> +
> +	return 0;
> +}
> +
> +static bool is_topdown_metric_event(struct evsel *event)
> +{
> +	struct perf_pmu *pmu;
> +
> +	if (!topdown_sys_has_perf_metrics())
> +		return false;
> +
> +	if (event->core.attr.type != PERF_TYPE_RAW)
> +		return false;
> +
> +	pmu = perf_pmus__find_by_type(PERF_TYPE_RAW);
> +	if (pmu && perf_pmu__for_each_event(pmu, false, event->name,
> +					    perf_pmus__topdown_event))
> +		return true;
> +
> +	return false;
> +}
> +
>  #define TOPDOWN_SLOTS		0x0400
>  
>  /*
> @@ -41,11 +68,22 @@ 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 && is_topdown_metric_event(event))
> +			return true;
> +	}
>  
>  	return false;
>  }
> 
> base-commit: 92e5605a199efbaee59fb19e15d6cc2103a04ec2