I sent this same message to LKML a while ago, but thought I'd get it a
more targeted audience:
=========================
Noticed today that the combination of 4KSTACKS and DEBUG_STACKOVERFLOW
config options is a bit deadly.
DEBUG_STACKOVERFLOW warns in do_IRQ if we're within THREAD_SIZE/8 of the
end of useable stack space, or 512 bytes on a 4k stack.
If we are, then it goes down the dump_stack path, which uses most, if
not all, of the remaining stack, thereby turning a well-intentioned
warning into a full-blown catastrophe.
The callchain from the warning looks something like this, with stack
usage shown as found on my x86 box:
4 dump_stack
4 show_trace
8 show_trace_log_lvl
4 dump_trace
print_context_stack
12 print_trace_address
print_symbol
232 __print_symbol
164 sprint_symbol
20 printk
___
448
448 bytes to tell us that we're within 512 bytes (or less) of certain
doom... and I think there's call overhead on top of that?
The large stack usage in those 2 functions is due to big char arrays, of
size KSYM_NAME_LEN (128 bytes) and KSYM_SYMBOL_LEN (223 bytes).
IOW, the stack warning effectively reduces useful stack left in our itty
bitty 4k stacks by over 10%.
...
=========================
In light of this, I'd like to propose that we turn off
DEBUG_STACKOVERFLOW in Fedora, at least on x86/4KSTACKS. I think it
does more harm than good; the warning is going to turn deadly most of
the time.
I also had sent a patch to LKML to print whether or not the stack was
overflowing, or had ever overflowed, on a kernel panic. It's not yet
been merged.
... any comments? I can file a bug but thought some discussion might be
in order.
Thanks,
-Eric
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