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

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Date:   Fri, 14 Sep 2018 12:22:36 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     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, Kan Liang <kan.liang@...ux.intel.com>
Subject: Re: [PATCH] perf/x86/intel/lbr: Optimize context switches for LBR


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?

Thanks,
Alexey

> 
> This change is only optimizing the case when call stack mode is not used.
> 
> Of course in call stack mode the context switch overhead is even higher,
> because it not only writes, but also reads.
> 
> -Andi
>