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[PATCH RFC V1] perf tests: ignore symbols before kernel start

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  [PATCH RFC V1] perf tests: ignore symbols before kernel start Song Shan Gong <gongss@linux.vnet.ibm.com> - 2016-07-19 11:10 +0200
    [Question] for duplicate symbols, kallsyms and vmlinux may retain  different symbol Songshan Gong <gongss@linux.vnet.ibm.com> - 2016-07-19 13:20 +0200

#1446260 — [PATCH RFC V1] perf tests: ignore symbols before kernel start

perf gets kernel map start in function 'machine__get_running_kernel_start',
by finding the first no-zero value of symbol start value of '_text' or
'_stext'. Though kernel maybe start from a no-zero value, perf loads all
symbols into a red-black tree, even if one symbol starts from zero (for
example, '_text' is zero, '_stext' is kernel map start value, '_text' is
also added to the red-black tree).

In test function 'test__vmlinux_matches_kallsyms', perf traverses all
vmlinux symbols by enumerating its red-black tree, but finding the pair
symbol in kallsyms by 'machine__find_kernel_symbol'. For function
'machine__find_kernel_symbol', it will find the matched map firstly, then
find the pair symbol in the red-black tree corresponding to that map.

So when '_text' is zero, '_stext' is kernel map start(no-zero), we can
get '_text' symbol of vmlinux, but we can't find the pair '_text' symbol of
kallsyms, because there is no map starting from zero. This mismatch is
incorrrect.

Fix by just ignoring these symbols before kernel start.

Signed-off-by: Song Shan Gong <gongss@linux.vnet.ibm.com>
---
 tools/perf/tests/vmlinux-kallsyms.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/perf/tests/vmlinux-kallsyms.c b/tools/perf/tests/vmlinux-kallsyms.c
index e63abab..6766006 100644
--- a/tools/perf/tests/vmlinux-kallsyms.c
+++ b/tools/perf/tests/vmlinux-kallsyms.c
@@ -28,6 +28,7 @@ int test__vmlinux_matches_kallsyms(int subtest __maybe_unused)
 	enum map_type type = MAP__FUNCTION;
 	struct maps *maps = &vmlinux.kmaps.maps[type];
 	u64 mem_start, mem_end;
+	u64 kernel_start;
 
 	/*
 	 * Step 1:
@@ -75,6 +76,7 @@ int test__vmlinux_matches_kallsyms(int subtest __maybe_unused)
 	 * same value in the vmlinux file we load.
 	 */
 	kallsyms_map = machine__kernel_map(&kallsyms);
+	kernel_start = kallsyms_map->start;
 
 	/*
 	 * Step 5:
@@ -119,7 +121,7 @@ int test__vmlinux_matches_kallsyms(int subtest __maybe_unused)
 
 		sym  = rb_entry(nd, struct symbol, rb_node);
 
-		if (sym->start == sym->end)
+		if (sym->start == sym->end || sym->start < kernel_start)
 			continue;
 
 		mem_start = vmlinux_map->unmap_ip(vmlinux_map, sym->start);
-- 
2.3.0

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#1446343 — [Question] for duplicate symbols, kallsyms and vmlinux may retain different symbol

I try to fix the failure of testcase 'test__vmlinux_matches_kallsyms', 
and meet a kind of failure because of duplicate symbols.

For example,two functions:
kretprobe_trampoline_holder and kretprobe_trampoline

(1)They all start from the same addr,0x011b180;

(2)kretprobe_trampoline_holder is STB_LOCAL,
    kretprobe_trampoline is STB_GLOBAL;

(3) for kallsyms, because we cann't get the correct size of each symbol 
from /proc/kallsyms, before symbols__fixup_duplicate(), perf assumes 
each symbol's size is zero, so when choose_best_symbol(), we get 
kretprobe_trampoline because perf prefers 'a global symbol over a 
non-global one';

(4) for vmlinux, by readelf, I found that:
size of kretprobe_trampoline_holder is 4, not zero;
size of kretprobe_trampoline is zero;
So when choose_best_symbol(), we get pretprobe_trampoline_holder instead 
because perf prefers 'A symbol with non-zero length' and this condition 
is judged before 'prefer a global symbol';

I have a idea to fix this problem, but may be inappropriate:
Could we move the judge of symbol size to the bottom of the judge 
whether a symbol is global?

Anyone else have good idea?

Song Shan Gong

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