Re: [PATCH V2 4/4] perf/x86: fix: disable userspace RDPMC usage for large PEBS

["Liang, Kan" <kan.liang@linux.intel.com>]

On 12/20/2017 4:41 PM, Andi Kleen wrote:
> On Wed, Dec 20, 2017 at 11:42:51AM -0800, kan.liang@linux.intel.com wrote:
>> From: Kan Liang <kan.liang@linux.intel.com>
>>
>> The userspace RDPMC usage never works for large PEBS since the large
>> PEBS is introduced by
>> commit b8241d20699e ("perf/x86/intel: Implement batched PEBS interrupt
>> handling (large PEBS interrupt threshold)")
>>
>> When the PEBS interrupt threshold is larger than one, there is no way to
>> get exact auto-reload times and value for userspace RDPMC.
>>
>> Disable the userspace RDPMC usage when large PEBS is enabled.
> 
> The only drawback is that the event wraps quickly. 

It's broken. The value read from RDPMC command will always between 
-reload_val and 0. Without reload times, I think it's impossible to 
calculate the total count number.

> I suspect in many
> cases it's still usable for measuring short regions.
>

Unless the short region is less than reload_val.
Otherwise, reload times is needed.

Thanks,
Kan

> I wouldn't force disable it, just let the users deal with it.
> 
> -Andi
>