> On 20-Jul-2021, at 7:45 PM, Nicholas Piggin <npig...@gmail.com> wrote:
>
> If an interrupt is taken in kernel mode, always use SIAR for it rather than
> looking at regs_sipr. This prevents samples piling up around interrupt
> enable (hard enable or interrupt replay via soft enable) in PMUs / modes
> where the PR sample indication is not in synch with SIAR.
>
> This results in better sampling of interrupt entry and exit in particular.
>
> Signed-off-by: Nicholas Piggin <npig...@gmail.com>
> ---
> arch/powerpc/perf/core-book3s.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index bb0ee716de91..91203ed9d0ff 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -340,6 +340,13 @@ static inline void perf_read_regs(struct pt_regs *regs)
> * If the PMU doesn't update the SIAR for non marked events use
> * pt_regs.
> *
> + * If regs is a kernel interrupt, always use SIAR. Some PMUs have an
> + * issue with regs_sipr not being in synch with SIAR in interrupt entry
> + * and return sequences, which can result in regs_sipr being true for
> + * kernel interrupts and SIAR, which has the effect of causing samples
> + * to pile up at mtmsrd MSR[EE] 0->1 or pending irq replay around
> + * interrupt entry/exit.
> + *
> * If the PMU has HV/PR flags then check to see if they
> * place the exception in userspace. If so, use pt_regs. In
> * continuous sampling mode the SIAR and the PMU exception are
> @@ -356,6 +363,8 @@ static inline void perf_read_regs(struct pt_regs *regs)
> use_siar = 1;
> else if ((ppmu->flags & PPMU_NO_CONT_SAMPLING))
> use_siar = 0;
> + else if (!user_mode(regs))
> + use_siar = 1;
Tested the change.
Workload used: Testcase that runs a loop of “scv” syscalls
Before the patch:
# perf record <test>
# perf report|grep replay
3.90% scvonly [kernel.vmlinux] [k] replay_soft_interrupts
Samples were present around interrupt replay code.
After the fix, perf report didn’t had samples pointing to replay code.
Tested-by: Athira Rajeev <atraj...@linux.vnet.ibm.com>
Thanks
Athira
> else if (!(ppmu->flags & PPMU_NO_SIPR) && regs_sipr(regs))
> use_siar = 0;
> else
> --
> 2.23.0
>
