Re: [PATCH 2/7] perf: Create a symlink for a PMU

Liang, Kan
2021-06-25T15:04:46Z

On 6/25/2021 10:49 AM, Andi Kleen wrote:
>
>> Device names will change, that's always a given, as the kernel can never
>> always make them the same.  That's why userspace needs to scan the bus
>> for all devices and then pick out the one that it wants to look at.
>
> In perf the tool doesn't normally know what devices (= pmu) the users
> want to look at. It's all specified on the command line depending on
> what events you want to measure. There's no way for the tool to figure
> that out on its own.
>
>
>> Don't hard-encode device names into userspace tools, that way lies
>> madness.
>
> There's no hard coding in the tools (or at least not for the non json
> event list case). It all comes from the command line. But that is where
> the problem comes from.
>
>>
>>> Anyways thinking about it if Greg doesn't want symlinks (even though
>>> sysfs
>>> already has symlinks elsewhere), maybe we could just create two devices
>>> without symlinks. Kan, do you think that would work?
>> Do not have 2 different structures represent the same hardware device,
>> that too is a shortcut to madness.
>>
>> What prevents userspace from handling device names changing today?  Why
>> are you forcing userspace to pick a specific device name at all?
>
> The way the perf tool works is that you have to specify the names on the
> command line:
>
> perf stat -a -e uncore_cha/event=1/ ...
>
> With the numeric identifiers it would be
>
> perf stat -a -e uncore_type_X_Y/event=1/
>
> The tool handles it all abstractly.
>
> So yes the user tools itself can handle it. But the problem is that it
> is directly exposed to the users, so the users would need to change all
> their scripts when switching between the two cases. That is what we're
> trying to avoid -- provide them a way that works on both.
>

We have an attribute "caps/pmu_name" for the core PMU. Maybe we should
add it for uncore PMU as well. For example,

$ cat /sys/devices/uncore_type_0_0/caps/pmu_name
cha_0

Userspace tool can get clues about what type_0_0 is.

Thanks,
Kan