Messages in this thread | | | Date | Sat, 13 Mar 2010 07:35:41 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH] vsprintf.c: Use noinline_for_stack |
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On Fri, 12 Mar 2010, Andrew Morton wrote: > > -ENOTESTINGRESULTS. > > Before: > > akpm:/usr/src/25> objdump -d lib/vsprintf.o | perl scripts/checkstack.pl > 0x00000e82 pointer [vsprintf.o]: 344 > 0x0000198c pointer [vsprintf.o]: 344 > 0x000025d6 scnprintf [vsprintf.o]: 216 > 0x00002648 scnprintf [vsprintf.o]: 216 > 0x00002565 snprintf [vsprintf.o]: 208 > 0x0000267c sprintf [vsprintf.o]: 208 > 0x000030a3 bprintf [vsprintf.o]: 208 > 0x00003b1e sscanf [vsprintf.o]: 208 > 0x00000608 number [vsprintf.o]: 136 > 0x00000937 number [vsprintf.o]: 136 > > After: > > akpm:/usr/src/25> objdump -d lib/vsprintf.o | perl scripts/checkstack.pl > 0x00000a7c symbol_string [vsprintf.o]: 248 > 0x00000ae8 symbol_string [vsprintf.o]: 248 > 0x00002310 scnprintf [vsprintf.o]: 216 > 0x00002382 scnprintf [vsprintf.o]: 216 > 0x0000229f snprintf [vsprintf.o]: 208 > 0x000023b6 sprintf [vsprintf.o]: 208 > 0x00002ddd bprintf [vsprintf.o]: 208 > 0x00003858 sscanf [vsprintf.o]: 208 > 0x00000625 number [vsprintf.o]: 136 > 0x00000954 number [vsprintf.o]: 136 > > nice.
Note that the fact that the numbers are smaller is to some degree less important than _where_ the numbers are.
In the "before" side, it's the "pointer()" function that has a big stack depth. And the recursion that is going to happen is very much about vsnprintf -> pointer -> vsnprintf, so that is bad.
Now it's the new non-inlined leaf functions that still have a big stack footprint, and that's much better, because they wouldn't be part of any recursive behavior.
Not that I think it's wonderful even now. Especially that whole 'symbol_string()' thing is not only a big stack user, it ends up calling down a fair number of other functions. Non-recursively, but still.
That, in turn, is due to this:
- include/linux/kallsyms.h: #define KSYM_NAME_LEN 128 #define KSYM_SYMBOL_LEN (sizeof("%s+%#lx/%#lx [%s]") + (KSYM_NAME_LEN - 1) + \
- symbol_string(): char sym[KSYM_SYMBOL_LEN];
ie we "need" about 150 bytes for just that silly symbol expansion (rounded up etc). Which is ridiculous, since we could/should limit it to something sane. But the kallsyms_lookup()/sprint_symbol() functions don't take a length parameter, so we have to do the worst-case thing (which itself has tons of unnecessary padding).
Gaah. We do _not_ want a kmalloc() or something like that in this path, since its' very much used for oopses (which in turn may be due to various slab bugs etc).
Linus
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