Marcelo,
just another one and that's all. Zachary Amsden found an unconditional
write to a debug register in the signal delivery path which is only
needed when we use a breakpoint. This is a very expensive operation on
x86, and doing it conditionnaly enhanced signal delivery speed by 33%
for him.
His patch got merged in 2.6.10, and I've merged it a month ago in my
local tree. Could we get it in 2.4.30, please ?
Thanks in advance,
Willy
--
I noticed an unneeded write to dr7 in the signal handling path for x86.
We only need to write to dr7 if there is a breakpoint to re-enable, and
MOVDR is a serializing instruction, which is expensive. Getting rid of
it gets a 33% faster signal delivery path (at least on Xeon - I didn't
test other CPUs, so your gain may vary).
Cheers,
Zachary Amsden
[email protected]
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Optimize away the unconditional write to debug registers on signal delivery
path. This is already done on x86_64. Measured delta TSC for three paths
on a 2.4GHz Xeon.
1) With unconditional write to dr7 : 800-1000 cycles
2) With conditional write to dr7 : 84-112 cycles
3) With unlikely write to dr7 : 84 cycles
Performance test using divzero microbenchmark (3 million divide by zeros):
With unconditional write:
7.445 real / 6.136 system
7.529 real / 6.482 system
7.541 real / 5.974 system
7.546 real / 6.217 system
7.445 real / 6.167 system
With unlikely write:
5.779 real / 4.518 system
5.783 real / 4.591 system
5.552 real / 4.569 system
5.790 real / 4.528 system
5.554 real / 4.382 system
That's about a 33% speedup - more than I expected; apparently getting rid
of the serializing instruction makes the do_signal path much faster.
Zachary Amsden ([email protected])
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[hand-edited line numbers to match 2.4]
--- linux-2.6.10-rc1/arch/i386/kernel/signal.c 2004-10-25 11:15:43.000000000 -0700
+++ linux-2.6.10-rc1-nsz/arch/i386/kernel/signal.c 2004-10-26 14:30:54.000000000 -0700
@@ -600,7 +600,9 @@
* have been cleared if the watchpoint triggered
* inside the kernel.
*/
- __asm__("movl %0,%%db7" : : "r" (current->thread.debugreg[7]));
+ if (unlikely(current->thread.debugreg[7])) {
+ __asm__("movl %0,%%db7" : : "r" (current->thread.debugreg[7]));
+ }
/* Whee! Actually deliver the signal. */
handle_signal(signr, &info, &ka, oldset, regs);
-
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