Re: [PATCH v1 00/13] perf/x86/amd: Add AMD Fam19h Branch Sampling support

[Song Liu]

Hi Stephane,

On Tue, Sep 14, 2021 at 10:57 PM Stephane Eranian <eranian@xxxxxxxxxx> wrote:
>
> On Thu, Sep 9, 2021 at 1:55 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Sep 09, 2021 at 12:56:47AM -0700, Stephane Eranian wrote:
> > > This patch series adds support for the AMD Fam19h 16-deep branch sampling
> > > feature as described in the AMD PPR Fam19h Model 01h Revision B1 section 2.1.13.
> >
> > Yay..
> >
> > > BRS interacts with the NMI interrupt as well. Because enabling BRS is expensive,
> > > it is only activated after P event occurrences, where P is the desired sampling period.
> > > At P occurrences of the event, the counter overflows, the CPU catches the NMI interrupt,
> > > activates BRS for 16 branches until it saturates, and then delivers the NMI to the kernel.
> >
> > WTF... ?!? Srsly? You're joking right?
> >
>
> As I said, this is because of the cost of running BRS usually for
> millions of branches to keep only the last 16.
> Running branch sampling in general on any arch is  never totally free.

Could you please share some data on how expensive the BRS is? We are
hoping to use
BRS/LBR without PMI (bpf_get_branch_snapshot). If it is too expensive,
we may need
some heuristic to turn it on/off.

Thanks,
Song