On Thu, Oct 15, 2015 at 01:21:54PM +0000, Jungseok Lee wrote:
> dump_backtrace() has its own backtrace logic unlike perf callchain which
> relies on walk_stackframe(). They behave differently when a symbol is
> recorded. Perf writes it down *before* calling unwind_frame(), but
> dump_backtrace() prints it out *after* unwind_frame(). As a result, the
> last valid symbol is not added to a list in case of dump_backtrace().
>
> This patch catches up the last symbol as synchronising dump_backtrace()
> with perf callchain. However, the patch does not cover a case where MMU
> is disabled. That is, a physical address can be stored in stack frame,
> but it's not handled. For example, a swapper process falls into this case.
> Unlike a swapper from a secondary core, a swapper on a boot cpu, which
> starting from __mmap_switched(), can't be tracked down with a simple
> conversion, phys_to_virt(), because PC is retrieved from LR - 4, not LR.
It would be good to have an example backtrace before and after this patch
is applied, to show what it fixes.
> It is a big tradeoff to change both head.S and unwind_frame() structure
> for a few of symbols in *.S, so this hunk does not take care of the case.
>
> Cc: AKASHI Takahiro <takahiro.aka...@linaro.org>
> Cc: James Morse <james.mo...@arm.com>
> Cc: Mark Rutland <mark.rutl...@arm.com>
> Signed-off-by: Jungseok Lee <jungseokle...@gmail.com>
> ---
> arch/arm64/kernel/traps.c | 16 +++++++++++-----
> 1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> index f93aae5..4ddb928 100644
> --- a/arch/arm64/kernel/traps.c
> +++ b/arch/arm64/kernel/traps.c
> @@ -103,12 +103,13 @@ static void dump_mem(const char *lvl, const char *str,
> unsigned long bottom,
> set_fs(fs);
> }
>
> -static void dump_backtrace_entry(unsigned long where, unsigned long stack)
> +static void dump_backtrace_entry(unsigned long where)
> {
> + /*
> + * The highest stack frame of a swapper process stores PC in a form
> + * of physical address, but this case is not handled.
> + */
> print_ip_sym(where);
> - if (in_exception_text(where))
> - dump_mem("", "Exception stack", stack,
> - stack + sizeof(struct pt_regs), false);
> }
>
> static void dump_instr(const char *lvl, struct pt_regs *regs)
> @@ -172,12 +173,17 @@ static void dump_backtrace(struct pt_regs *regs, struct
> task_struct *tsk)
> pr_emerg("Call trace:\n");
> while (1) {
> unsigned long where = frame.pc;
> + unsigned long stack;
> int ret;
>
> + dump_backtrace_entry(where);
> ret = unwind_frame(&frame);
> if (ret < 0)
> break;
> - dump_backtrace_entry(where, frame.sp);
> + stack = frame.sp;
> + if (in_exception_text(where))
> + dump_mem("", "Exception stack", stack,
> + stack + sizeof(struct pt_regs), false);
AFAICT, the original code is all based on unwind_backtrace in
arch/arm/kernel/unwind.c. Does that need updating too (as a separate patch)?
Will
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