linux-kernel - Re: [PATCH] perf/x86/intel/lbr: Optimize context switches for LBR

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Message-ID: <c9f2436a-f996-026d-5fee-557e6d91c613@linux.intel.com>
Date:   Fri, 14 Sep 2018 10:57:31 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     Alexey Budankov <alexey.budankov@...ux.intel.com>,
        linux-kernel-owner@...r.kernel.org, peterz@...radead.org,
        tglx@...utronix.de, acme@...nel.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, jolsa@...hat.com, namhyung@...nel.org
Subject: Re: [PATCH] perf/x86/intel/lbr: Optimize context switches for LBR



On 9/14/2018 10:27 AM, Andi Kleen wrote:
> On Fri, Sep 14, 2018 at 08:39:36AM -0400, Liang, Kan wrote:
>>
>>
>> On 9/14/2018 5:22 AM, Alexey Budankov wrote:
>>>
>>> Hi Andi,
>>>
>>> On 14.09.2018 11:54, Andi Kleen wrote:
>>>>>> In principle the LBRs need to be flushed between threads. So does
>>>>>> current code.
>>>>>
>>>>> IMHO, ideally, LBRs stack would be preserved and restored when
>>>>> switching between execution stacks. That would allow implementing
>>>>> per-thread statistical call graph view in Perf tools, fully based
>>>>> on HW capabilities. It could be advantageous for some cases, in
>>>>> comparison with traditional dwarf based call graph.
>>>>
>>>> This is already supported when you use LBR call stack mode
>>>> (perf record --call-graph lbr)
>>>
>>> Which kernel versions does it make sense to try?
>>>
>>
>> The optimization for LBR call stack has been merged into 4.19.
>> commit id: 8b077e4a69bef5c4121426e99497975860191e53
>> perf/x86/intel/lbr: Optimize context switches for the LBR call stack
> 
> I think he mean support for LBR call stack in general. This has been there
> for a long time (since Haswell) Any reasonable kernel version should
> support it.
>

Oh I see. Yes, the feature of LBR call stack was added long time ago.
But I still recommend 4.19. Because it includes a recent bug fix for LBR 
call stack.

commit id: 0592e57b24e7e05ec1f4c50b9666c013abff7017
perf/x86/intel/lbr: Fix incomplete LBR call stack

Thanks,
Kan