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

[Liang, Kan]

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

Thanks,
Kan

> 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