linux-kernel - Re: [PATCH] perf/x86/intel: Drop kernel samples even though :u is specified

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Message-ID: <f67248f7-1ebd-98de-ae13-5ca3920d50e5@linux.intel.com>
Date:   Mon, 22 May 2017 10:12:22 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, jolsa@...nel.org, mingo@...hat.com,
        alexander.shishkin@...ux.intel.com, Linux-kernel@...r.kernel.org,
        ak@...ux.intel.com, kan.liang@...el.com, yao.jin@...el.com,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] perf/x86/intel: Drop kernel samples even though :u is
 specified



On 5/19/2017 9:33 PM, Jin, Yao wrote:
>
>
> On 5/19/2017 8:36 PM, Peter Zijlstra wrote:
>> On Fri, May 19, 2017 at 08:24:19PM +0800, Jin, Yao wrote:
>>>> Ah, I was more thinking of something like PERF_PMU_CAP_NO_SKID or
>>>> something that would skip the test and preserve current behaviour.
>>> OK, I understand now. For example, for PEBS event, its capabilities 
>>> should
>>> be set with PERF_PMU_CAP_NO_SKID.
>> Except you cannot in fact do that, since PEBS is the same struct pmu as
>> the normal counters (they share counter space after all).
>>
>> Also, weren't there PEBS errata that would allow this to happen?
>>
>> But no, more for other architectures to opt out for some reason. But I'm
>> thinking we want to start out by unconditionally doing this. It would be
>> good to try and Cc most arch pmu maintainers on this though, so they can
>> object.
>>
> I'm thinking v2 of patch will only do simple tasks:
>
> 1. Define PERF_PMU_CAP_NO_SKID but don't bind it to any event.
>
> 2. Move the skid checking from x86 specific code to generic code. 
> Before performing skid checking, test the PERF_PMU_CAP_NO_SKID bit first.
>
> For binding PERF_PMU_CAP_NO_SKID to event, that may be other arch 
> related patches.
>
> Thanks
> Jin Yao
>
>
Hi Peter,

Maybe it's not very easy to move the skid checking to generic code 
because we don't have a common kernel_ip() available to determine if ip 
is a kernel address.

I was trying to move kernel_ip() from arch/x86/events/perf_event.h to 
generic code, but some difficulties I have:

For example, in new kernel_ip(), we may use many conditional-compilation 
for all arch, for example:

#ifdef CONFIG_X86_32
     return ip > PAGE_OFFSET;
#endif

#ifdef CONFIG_X86_64
     return (long)ip < 0;
#endif

#ifdef CONFIG_ARM....
......
#ifdef CONFIG_MIPS....
......

But the code is being ugly and hard to maintain. And frankly I don't 
know kernel address space for all arch.

Any idea? Could we just do at x86 side this time?

Thanks
Jin Yao