Re: [PATCH] perf probe: Ignore tail calls to probed functions

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

On 2015/04/30 20:42, Naveen N. Rao wrote:
> perf probe currently errors out if there are any tail calls to probed
> functions:
> 
> [root@rhel71be]# perf probe do_fork
> Failed to find probe point in any functions.
>   Error: Failed to add events.
> 
> Fix this by teaching perf to ignore tail calls.

Hmm, why just ignore it, can't we use it?
Also, could you show us how the result will be changed with this?

Thank you,

> 
> Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
> I'm seeing this with RHEL7 kernels on ppc64. The specific example in this case is do_fork:
> 
>    <1><82e7e1>: Abbrev Number: 118 (DW_TAG_subprogram)
>       <82e7e2>   DW_AT_external    : 1
>       <82e7e2>   DW_AT_name        : (indirect string, offset: 0x3dfe0): do_fork
>       <82e7e6>   DW_AT_decl_file   : 7
>       <82e7e7>   DW_AT_decl_line   : 1583
>       <82e7e9>   DW_AT_prototyped  : 1
>       <82e7e9>   DW_AT_type        : <0x8110eb>
>       <82e7ed>   DW_AT_inline      : 1    (inlined)
>       <82e7ee>   DW_AT_sibling     : <0x82e878>
> 
> <snip>
> 
>    <1><830081>: Abbrev Number: 133 (DW_TAG_subprogram)
>       <830083>   DW_AT_external    : 1
>       <830083>   DW_AT_name        : (indirect string, offset: 0x3cea3): SyS_clone
>       <830087>   DW_AT_decl_file   : 7
>       <830088>   DW_AT_decl_line   : 1689
>       <83008a>   DW_AT_prototyped  : 1
>       <83008a>   DW_AT_type        : <0x8110eb>
>       <83008e>   DW_AT_low_pc      : 0xc0000000000bc270
>       <830096>   DW_AT_high_pc     : 0xc
>       <83009e>   DW_AT_frame_base  : 1 byte block: 9c (DW_OP_call_frame_cfa)
>       <8300a0>   DW_AT_GNU_all_call_sites: 1
>       <8300a0>   DW_AT_sibling     : <0x830178>
> 
> <snip>
> 
>    <3><830147>: Abbrev Number: 125 (DW_TAG_GNU_call_site)
>       <830148>   DW_AT_low_pc      : 0xc0000000000bc27c
>       <830150>   DW_AT_GNU_tail_call: 1
>       <830150>   DW_AT_abstract_origin: <0x82e7e1>
> 
> 
> - Naveen
> 
>  tools/perf/util/dwarf-aux.c    | 37 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dwarf-aux.h    |  4 ++++
>  tools/perf/util/probe-finder.c | 12 +++++++++---
>  3 files changed, 50 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index c34e024..851a76f 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -417,6 +417,43 @@ struct __addr_die_search_param {
>  	Dwarf_Die	*die_mem;
>  };
>  
> +static int __die_search_func_tail_cb(Dwarf_Die *fn_die, void *data)
> +{
> +	struct __addr_die_search_param *ad = data;
> +	Dwarf_Addr addr = 0;
> +
> +	if (dwarf_tag(fn_die) == DW_TAG_subprogram &&
> +	    !dwarf_highpc(fn_die, &addr) &&
> +	    addr == ad->addr) {
> +		memcpy(ad->die_mem, fn_die, sizeof(Dwarf_Die));
> +		return DWARF_CB_ABORT;
> +	}
> +	return DWARF_CB_OK;
> +}
> +
> +/**
> + * die_find_tailfunc - Search for a non-inlined function with tail call at
> + * given address
> + * @cu_die: a CU DIE which including @addr
> + * @addr: target address
> + * @die_mem: a buffer for result DIE
> + *
> + * Search for a non-inlined function DIE with tail call at @addr. Stores the
> + * DIE to @die_mem and returns it if found. Returns NULL if failed.
> + */
> +Dwarf_Die *die_find_tailfunc(Dwarf_Die *cu_die, Dwarf_Addr addr,
> +				    Dwarf_Die *die_mem)
> +{
> +	struct __addr_die_search_param ad;
> +	ad.addr = addr;
> +	ad.die_mem = die_mem;
> +	/* dwarf_getscopes can't find subprogram. */
> +	if (!dwarf_getfuncs(cu_die, __die_search_func_tail_cb, &ad, 0))
> +		return NULL;
> +	else
> +		return die_mem;
> +}
> +
>  /* die_find callback for non-inlined function search */
>  static int __die_search_func_cb(Dwarf_Die *fn_die, void *data)
>  {
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index af7dbcd..c9278ed 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -82,6 +82,10 @@ extern Dwarf_Die *die_find_child(Dwarf_Die *rt_die,
>  extern Dwarf_Die *die_find_realfunc(Dwarf_Die *cu_die, Dwarf_Addr addr,
>  				    Dwarf_Die *die_mem);
>  
> +/* Search a non-inlined function with tail call at given address */
> +Dwarf_Die *die_find_tailfunc(Dwarf_Die *cu_die, Dwarf_Addr addr,
> +				    Dwarf_Die *die_mem);
> +
>  /* Search the top inlined function including given address */
>  extern Dwarf_Die *die_find_top_inlinefunc(Dwarf_Die *sp_die, Dwarf_Addr addr,
>  					  Dwarf_Die *die_mem);
> diff --git a/tools/perf/util/probe-finder.c b/tools/perf/util/probe-finder.c
> index b5bf9d5..4cb461e 100644
> --- a/tools/perf/util/probe-finder.c
> +++ b/tools/perf/util/probe-finder.c
> @@ -660,9 +660,15 @@ static int call_probe_finder(Dwarf_Die *sc_die, struct probe_finder *pf)
>  	/* If not a real subprogram, find a real one */
>  	if (!die_is_func_def(sc_die)) {
>  		if (!die_find_realfunc(&pf->cu_die, pf->addr, &pf->sp_die)) {
> -			pr_warning("Failed to find probe point in any "
> -				   "functions.\n");
> -			return -ENOENT;
> +			if (die_find_tailfunc(&pf->cu_die, pf->addr, &pf->sp_die)) {
> +				pr_warning("Ignoring tail call from %s\n",
> +						dwarf_diename(&pf->sp_die));
> +				return 0;
> +			} else {
> +				pr_warning("Failed to find probe point in any "
> +					   "functions.\n");
> +				return -ENOENT;
> +			}
>  		}
>  	} else
>  		memcpy(&pf->sp_die, sc_die, sizeof(Dwarf_Die));
> 


-- 
Masami HIRAMATSU
Linux Technology Research Center, System Productivity Research Dept.
Center for Technology Innovation - Systems Engineering
Hitachi, Ltd., Research & Development Group
E-mail: masami.hiramatsu.pt@hitachi.com