linux-kernel - Re: [PATCH -next 1/2] perf stat: Increase perf_attr

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Message-ID: <1222cf96-0382-4396-bb2d-475615070ccf@huaweicloud.com>
Date: Sun, 29 Sep 2024 08:54:38 +0800
From: Tengda Wu <wutengda@...weicloud.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, song@...nel.org,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, kan.liang@...ux.intel.com,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 bpf@...r.kernel.org
Subject: Re: [PATCH -next 1/2] perf stat: Increase perf_attr_map entries



On 2024/9/28 1:12, Namhyung Kim wrote:
> On Fri, Sep 27, 2024 at 10:35:54AM +0800, Tengda Wu wrote:
>>
>>
>> On 2024/9/26 12:16, Namhyung Kim wrote:
>>> On Wed, Sep 25, 2024 at 01:55:22PM +0000, Tengda Wu wrote:
>>>> bperf restricts the size of perf_attr_map's entries to 16, which
>>>> cannot hold all events in many scenarios. A typical example is
>>>> when the user specifies `-a -ddd` ([0]). And in other cases such as
>>>> top-down analysis, which often requires a set of more than 16 PMUs
>>>> to be collected simultaneously.
>>>>
>>>> Fix this by increase perf_attr_map entries to 100, and an event
>>>> number check has been introduced when bperf__load() to ensure that
>>>> users receive a more friendly prompt when the event limit is reached.
>>>>
>>>>   [0] https://lore.kernel.org/all/20230104064402.1551516-3-namhyung@kernel.org/
>>>
>>> Apparently this patch was never applied.  I don't know how much you need
>>> but having too many events at the same time won't be very useful because
>>> multiplexing could reduce the accuracy.
>>>
>>
>> Could you please explain why patch [0] was not merged at that time? I couldn't
>> find this information from the previous emails.
> 
> I guess it's just fell through the crack. :)

Hope it won't happen again. 😆

> 
>>
>> In my scenario, we collect more than 40+ events to support necessary metric
>> calculations, which multiplexing is inevitable. Although multiplexing may
>> reduce accuracy, for the purpose of supporting metric calculations, these
>> accuracy losses can be acceptable. Perf also has the same issue with multiplexing.
>> Removing the event limit for bperf can provide users with additional options.
>>
>> In addition to accuracy, we also care about overhead. I compared the overhead
>> of bperf and perf by testing ./lat_ctx in lmbench [1], and found that the
>> overhead of bperf stat is about 4% less than perf. This is why we choose to
>> use bperf in some extreme scenarios.
> 
> Ok, thanks for explanation.  I think it's ok to increase the limit.
> 
> Thanks,
> Namhyung
> 
>>
>>   [1] https://github.com/intel/lmbench
>>
>> Thanks,
>> Tengda
>>
>>>
>>>>
>>>> Fixes: 7fac83aaf2ee ("perf stat: Introduce 'bperf' to share hardware PMCs with BPF")
>>>> Signed-off-by: Tengda Wu <wutengda@...weicloud.com>
>>>> ---
>>>>  tools/perf/util/bpf_counter.c | 8 +++++++-
>>>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/tools/perf/util/bpf_counter.c b/tools/perf/util/bpf_counter.c
>>>> index 7a8af60e0f51..3346129c20cf 100644
>>>> --- a/tools/perf/util/bpf_counter.c
>>>> +++ b/tools/perf/util/bpf_counter.c
>>>> @@ -28,7 +28,7 @@
>>>>  #include "bpf_skel/bperf_leader.skel.h"
>>>>  #include "bpf_skel/bperf_follower.skel.h"
>>>>  
>>>> -#define ATTR_MAP_SIZE 16
>>>> +#define ATTR_MAP_SIZE 100
>>>>  
>>>>  static inline void *u64_to_ptr(__u64 ptr)
>>>>  {
>>>> @@ -451,6 +451,12 @@ static int bperf__load(struct evsel *evsel, struct target *target)
>>>>  	enum bperf_filter_type filter_type;
>>>>  	__u32 filter_entry_cnt, i;
>>>>  
>>>> +	if (evsel->evlist->core.nr_entries > ATTR_MAP_SIZE) {
>>>> +		pr_err("Too many events, please limit to %d or less\n",
>>>> +			ATTR_MAP_SIZE);
>>>> +		return -1;
>>>> +	}
>>>> +
>>>>  	if (bperf_check_target(evsel, target, &filter_type, &filter_entry_cnt))
>>>>  		return -1;
>>>>  
>>>> -- 
>>>> 2.34.1
>>>>
>>