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

[Liang, Kan]

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