Re: [Libunwind-devel] unw_backtrace(): Performance slowdown for ARM?

[Chris January]

Hello Luke,

On 18/01/18 22:22, Luke Diamand wrote:
> > On aarch32 ARM libunwind also defaults to --enable-debug-frame=yes. This
> > can cause libunwind to open and read the executable and any shared
> > libraries every unwind if the .eh_frame section is missing or does not
> > cover all addresses, which matches your comment that it is opening and
> > closing files a lot. If you know you have .ARM.exidx sections then try
> > configuring libunwind with --enable-debug-frame=no.
> I've been building with --disable-debug-frame, which I assume does the
> same thing (at least configure doesn't complain about it).
>
> That results in defining CONFIG_DEBUG_FRAME, which cuts out a bit of
> code from dwarf_find_unwind_table() and unw_step().
>
> But arm_find_proc_info() will still call dwarf_find_proc_info(), which
> will go and call dl_iterate_phdr() to try to find the DWARF unwind
> tables on each and every call.
>
> Do we just need something like this:
>
> @@ -528,8 +527,10 @@ arm_find_proc_info (unw_addr_space_t as, unw_word_t ip,
>
>     Debug (14, "looking for IP=0x%lx\n", (long) ip);
>
> +#ifdef CONFIG_DEBUG_FRAME
>     if (UNW_TRY_METHOD(UNW_ARM_METHOD_DWARF))
>       ret = dwarf_find_proc_info (as, ip, pi, need_unwind_info, arg);
> +#endif
>
>     if (ret < 0 && UNW_TRY_METHOD (UNW_ARM_METHOD_EXIDX))
>       {
The dwarf_find_proc_info looks for .eh_frame as well as .debug_frame, so
it wouldn't be appropriate to wrap it in a CONFIG_DEBUG_FRAME guard.

Although libunwind, as a generic unwinder, needs to support both
.eh_frame and .ARM.exidx, a particular distribution that is built with a
particular toolchain is most likely only going to use one or the other.
The unwinders that libunwind tries on ARM can be controlled at runtime
using the UNW_ARM_UNWIND_METHOD environment variable, as you know, but
if a distribution only uses one and sticks with it, then it would make
sense to add an option to libunwind's configure to control the choice at
configure time -- i.e. to set the default value of unwi_unwind_method in
src/arm/Gglobal.c.

Yours,
Chris

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