Messages in this thread | | | From | Rob Herring <> | Date | Thu, 27 Jan 2022 08:31:45 -0600 | Subject | Re: [RFC V1 06/11] arm64/perf: Drive BRBE from perf event states |
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On Thu, Jan 27, 2022 at 6:20 AM Anshuman Khandual <anshuman.khandual@arm.com> wrote: > > > On 1/26/22 10:37 PM, Rob Herring wrote: > > On Mon, Jan 24, 2022 at 10:00:48AM +0530, Anshuman Khandual wrote: > >> Branch stack sampling rides along the normal perf event and all the branch > >> records get captured during the PMU interrupt. This just changes perf event > >> handling on the arm64 platform to accommodate required BRBE operations that > >> will enable branch stack sampling support. > >> > >> Cc: Peter Zijlstra <peterz@infradead.org> > >> Cc: Ingo Molnar <mingo@redhat.com> > >> Cc: Arnaldo Carvalho de Melo <acme@kernel.org> > >> Cc: Mark Rutland <mark.rutland@arm.com> > >> Cc: Will Deacon <will@kernel.org> > >> Cc: Catalin Marinas <catalin.marinas@arm.com> > >> Cc: linux-perf-users@vger.kernel.org > >> Cc: linux-kernel@vger.kernel.org > >> Cc: linux-arm-kernel@lists.infradead.org > >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com> > >> --- > >> arch/arm64/kernel/perf_event.c | 6 +++++ > >> drivers/perf/arm_pmu.c | 40 ++++++++++++++++++++++++++++++++++ > >> 2 files changed, 46 insertions(+) > >> > >> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c > >> index f6a47036b0b4..11c82c8f2eec 100644 > >> --- a/arch/arm64/kernel/perf_event.c > >> +++ b/arch/arm64/kernel/perf_event.c > >> @@ -864,6 +864,12 @@ static irqreturn_t armv8pmu_handle_irq(struct arm_pmu *cpu_pmu) > >> if (!armpmu_event_set_period(event)) > >> continue; > >> > >> + if (has_branch_stack(event)) { > >> + cpu_pmu->brbe_read(cpuc, event); > > > > Is has_branch_stack() guaranteed to be false on arm32? If not, this will > > be a NULL function ptr. > > armpmu_event_init() blocks a perf event from being created with branch > stack sampling request without CONFIG_ARM_BRBE_PMU option being enabled > first, which has dependency on CONFIG_ARM64. So has_branch_stack() is > guaranteed to be false on arm32.
Then the stub functions in patch 3 are also not needed. The fact that you create dummy functions makes it look like you can't have NULL function ptrs, but you don't. This is what I mean about the structure of the series being hard to review.
> static int armpmu_event_init(struct perf_event *event) > { > .... > if (has_branch_stack(event)) { > /* > * BRBE support is absent. Select CONFIG_ARM_BRBE_PMU > * in the config, before branch stack sampling events > * can be requested. > */ > if (!IS_ENABLED(CONFIG_ARM_BRBE_PMU)) { > pr_warn_once("BRBE is disabled, select CONFIG_ARM_BRBE_PMU\n"); > return -EOPNOTSUPP; > } > > > config ARM_BRBE_PMU > tristate "Enable support for Branch Record Buffer Extension (BRBE)" > depends on ARM64 && ARM_PMU > default y > help > Enable perf support for Branch Record Buffer Extension (BRBE) which > records all branches taken in an execution path. This supports some > branch types and privilege based filtering. It captured additional > relevant information such as cycle count, misprediction and branch > type, branch privilege level etc. > > > > > To add to my other comments, this patch is where I would add > > brbe_read(), etc. to arm_pmu. > > Because all new arm_pmu helpers get added and get used in the perf driver > in the same patch, although the actual helper implementation would still > come by bit later via the driver. This also uses updates to pmu_hw_events > struct as well, then that patch needs to be folded here as well. > > There is no problem as such, kind of bit subjective. I just feel inclined > to keep the independent infrastructure changes separate making it easy to > review while also creating a flow.
Everything about kernel development is subjective until it's a requirement by the maintainer. I'm not here, so it's just advice.
Rob
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