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

From: "Liang, Kan" <kan.liang@linux.intel.com>
To: Andi Kleen <ak@linux.intel.com>, Greg KH <greg@kroah.com>
Cc: peterz@infradead.org, mingo@redhat.com,
	linux-kernel@vger.kernel.org, eranian@google.com,
	namhyung@kernel.org, acme@kernel.org, jolsa@redhat.com
Subject: Re: [PATCH 2/7] perf: Create a symlink for a PMU
Date: Fri, 25 Jun 2021 11:03:18 -0400	[thread overview]
Message-ID: <c3ffcdd4-fabb-38fd-6ccd-3497389f94ec@linux.intel.com> (raw)
In-Reply-To: <e670abe2-67b9-a602-410a-0c4170796ec7@linux.intel.com>

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