Re: [PATCH V2 3/5] perf mem: Clean up perf_mem_events__name()

[Liang, Kan]

On 2023-12-09 12:48 a.m., Leo Yan wrote:
> On Thu, Dec 07, 2023 at 11:23:36AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
>> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
>>
>> Introduce a generic perf_mem_events__name(). Remove the ARCH-specific
>> one.
> 
> Now memory event naming is arch dependent, this is because every arch
> has different memory PMU names, and it appends specific configurations
> (e.g. aarch64 appends 'ts_enable=1,...,min_latency=%u', and x86_64
> appends 'ldlat=%u', etc).  This is not friendly for extension, e.g. it's
> impossible for users to specify any extra attributes for memory events.
> 
> This patch tries to consolidate in a central place for generating memory
> event names, its approach is to add more flags to meet special usage
> cases, which means the code gets more complex (and more difficult for
> later's maintenance).
> 
> I think we need to distinguish the event naming into two levels: in
> util/mem-events.c, we can consider it as a common layer, and we maintain
> common options in it, e.g. 'latency', 'all-user', 'all-kernel',
> 'timestamp', 'physical_address', etc.  In every arch's mem-events.c
> file, it converts the common options to arch specific configurations.
> 
> In the end, this can avoid to add more and more flags into the
> structure perf_mem_event.

The purpose of this cleanup patch set is to remove the unnecessary
__weak functions, and try to make the code more generic.
The two flags has already covered all the current usage.
For now, it's good enough.

I agree that if there are more flags, it should not be a perfect
solution. But we don't have the third flag right now. Could we clean up
it later e.g., when introducing the third flag?

I just don't want to make the patch bigger and bigger. Also, I don't
think we usually implement something just for future possibilities.


> 
> Anyway, I also have a question for this patch itself, please see below
> inline comment.
> 
> [...]
> 
>> diff --git a/tools/perf/arch/arm64/util/mem-events.c b/tools/perf/arch/arm64/util/mem-events.c
>> index 2602e8688727..eb2ef84f0fc8 100644
>> --- a/tools/perf/arch/arm64/util/mem-events.c
>> +++ b/tools/perf/arch/arm64/util/mem-events.c
>> @@ -2,28 +2,10 @@
>>  #include "map_symbol.h"
>>  #include "mem-events.h"
>>  
>> -#define E(t, n, s) { .tag = t, .name = n, .sysfs_name = s }
>> +#define E(t, n, s, l, a) { .tag = t, .name = n, .sysfs_name = s, .ldlat = l, .aux_event = a }
>>  
>>  struct perf_mem_event perf_mem_events_arm[PERF_MEM_EVENTS__MAX] = {
>> -	E("spe-load",	"arm_spe_0/ts_enable=1,pa_enable=1,load_filter=1,store_filter=0,min_latency=%u/",	"arm_spe_0"),
>> -	E("spe-store",	"arm_spe_0/ts_enable=1,pa_enable=1,load_filter=0,store_filter=1/",			"arm_spe_0"),
>> -	E("spe-ldst",	"arm_spe_0/ts_enable=1,pa_enable=1,load_filter=1,store_filter=1,min_latency=%u/",	"arm_spe_0"),
>> +	E("spe-load",	"%s/ts_enable=1,pa_enable=1,load_filter=1,store_filter=0,min_latency=%u/",	"arm_spe_0",	true,	0),
>> +	E("spe-store",	"%s/ts_enable=1,pa_enable=1,load_filter=0,store_filter=1/",			"arm_spe_0",	false,	0),
>> +	E("spe-ldst",	"%s/ts_enable=1,pa_enable=1,load_filter=1,store_filter=1,min_latency=%u/",	"arm_spe_0",	true,	0),
> 
> Arm SPE is AUX event, should set '1' to the field '.aux_event'.

It actually means whether an extra event is required with a mem_loads
event.  I guess for ARM the answer is no.

> 
> I am a bit suspect if we really need the field '.aux_event', the
> '.aux_event' field is only used for generating event string.

No, it stores the event encoding for the extra event.
ARM doesn't need it, so it's 0.

> 
> So in the below function perf_pmu__mem_events_name(), I prefer to call
> an arch specific function, e.g. arch_memory_event_name(event_id, cfg),
> the parameter 'event_id' passes memory event ID for load, store, and
> load-store, and the parameter 'cfg' which is a pointer to the common
> memory options (as mentioned above for latency, all-user or all-kernel
> mode, timestamp tracing, etc).

If I understand correctly, I think we try to avoid the __weak function
for the perf tool. If there will be more parameters later, a mask may be
used for the parameters. But, again, I think it should be an improvement
for later.

Thanks,
Kan
> 
> In the end, the common layer just passes the common options to low
> level arch's layer and get a event string for recording.
> 
> Thanks,
> Leo