On Tue, 2015-05-26 at 15:10 +1000, Anton Blanchard wrote:
> When we take a PMU exception or a software event we call
> perf_read_regs(). This overloads regs->result with a boolean that
> describes if we should use the sampled instruction address register
> (SIAR) or the regs.
>
> If the exception is in kernel, we start with the kernel regs and
> backtrace through the kernel stack. At this point we switch to the
> userspace regs and backtrace the user stack with perf_callchain_user().
>
> Unfortunately these regs have not got the perf_read_regs() treatment,
> so regs->result could be anything. If it is non zero,
> perf_instruction_pointer() decides to use the SIAR, and we get issues
> like this:
CC stable ?
> 0.11% qemu-system-ppc [kernel.kallsyms] [k] _raw_spin_lock_irqsave
> |
> ---_raw_spin_lock_irqsave
> |
> |--52.35%-- 0
> | |
> | |--46.39%-- __hrtimer_start_range_ns
> | | kvmppc_run_core
> | | kvmppc_vcpu_run_hv
> | | kvmppc_vcpu_run
> | | kvm_arch_vcpu_ioctl_run
> | | kvm_vcpu_ioctl
> | | do_vfs_ioctl
> | | sys_ioctl
> | | system_call
> | | |
> | | |--67.08%-- _raw_spin_lock_irqsave <--- hi mum
> | | | |
> | | | --100.00%-- 0x7e714
> | | | 0x7e714
>
> Notice the bogus _raw_spin_irqsave when we transition from kernel
> (system_call) to userspace (0x7e714). We inserted what was in the SIAR.
>
> Add a check in regs_use_siar() to check that the regs in question
> are from a PMU exception. With this fix the backtrace makes sense:
>
> 0.47% qemu-system-ppc [kernel.vmlinux] [k]
> _raw_spin_lock_irqsave
> |
> ---_raw_spin_lock_irqsave
> |
> |--53.83%-- 0
> | |
> | |--44.73%-- hrtimer_try_to_cancel
> | | kvmppc_start_thread
> | | kvmppc_run_core
> | | kvmppc_vcpu_run_hv
> | | kvmppc_vcpu_run
> | | kvm_arch_vcpu_ioctl_run
> | | kvm_vcpu_ioctl
> | | do_vfs_ioctl
> | | sys_ioctl
> | | system_call
> | | __ioctl
> | | 0x7e714
> | | 0x7e714
>
> Signed-off-by: Anton Blanchard <an...@samba.org>
> ---
> arch/powerpc/perf/core-book3s.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/perf/core-book3s.c b/arch/powerpc/perf/core-book3s.c
> index 12b6384..42a3725 100644
> --- a/arch/powerpc/perf/core-book3s.c
> +++ b/arch/powerpc/perf/core-book3s.c
> @@ -131,7 +131,10 @@ static void pmao_restore_workaround(bool ebb) { }
>
> static bool regs_use_siar(struct pt_regs *regs)
> {
> - return !!regs->result;
> + if ((TRAP(regs) == 0xf00) && regs->result)
> + return true;
> +
> + return false;
> }
>
> /*
_______________________________________________
Linuxppc-dev mailing list
Linuxppc-dev@lists.ozlabs.org
https://lists.ozlabs.org/listinfo/linuxppc-dev
