Messages in this thread | | | From | Song Liu <> | Date | Mon, 27 Sep 2021 13:17:53 -0700 | Subject | Re: [PATCH v1 00/13] perf/x86/amd: Add AMD Fam19h Branch Sampling support |
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Hi Stephane,
On Tue, Sep 14, 2021 at 10:57 PM Stephane Eranian <eranian@google.com> wrote: > > On Thu, Sep 9, 2021 at 1:55 AM Peter Zijlstra <peterz@infradead.org> 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
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