Messages in this thread | | | Subject | Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs() | From | Christophe Leroy <> | Date | Thu, 3 Sep 2020 09:20:35 +0200 |
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Le 02/09/2020 à 20:02, Linus Torvalds a écrit : > On Wed, Sep 2, 2020 at 8:17 AM Christophe Leroy > <christophe.leroy@csgroup.eu> wrote: >> >> >> With this fix, I get >> >> root@vgoippro:~# time dd if=/dev/zero of=/dev/null count=1M >> 536870912 bytes (512.0MB) copied, 6.776327 seconds, 75.6MB/s >> >> That's still far from the 91.7MB/s I get with 5.9-rc2, but better than >> the 65.8MB/s I got yesterday with your series. Still some way to go thought. > > I don't see why this change would make any difference. >
Neither do I.
Looks like nowadays, CONFIG_STACKPROTECTOR has become a default. I rebuilt the kernel without it, I now get a throughput of 99.8MB/s both without and with this series.
Looking at the generated code (GCC 10.1), a small change in a function seems to make large changes in the generated code when CONFIG_STACKPROTECTOR is set.
In addition to that, trivial functions which don't use the stack at all get a stack frame anyway when CONFIG_STACKPROTECTOR is set, allthough that's only -fstack-protector-strong. And there is no canary check.
Without CONFIG_STACKPROTECTOR:
c01572a0 <no_llseek>: c01572a0: 38 60 ff ff li r3,-1 c01572a4: 38 80 ff e3 li r4,-29 c01572a8: 4e 80 00 20 blr
With CONFIG_STACKPROTECTOR (regardless of CONFIG_STACKPROTECTOR_STRONG or not):
c0164e08 <no_llseek>: c0164e08: 94 21 ff f0 stwu r1,-16(r1) c0164e0c: 38 60 ff ff li r3,-1 c0164e10: 38 80 ff e3 li r4,-29 c0164e14: 38 21 00 10 addi r1,r1,16 c0164e18: 4e 80 00 20 blr
Wondering why CONFIG_STACKPROTECTOR has become the default. It seems to imply a 10% performance loss even in the best case (91.7MB/s versus 99.8MB/s)
Note that without CONFIG_STACKPROTECTOR_STRONG, I'm at 99.3MB/s, so that's really the _STRONG alternative that hurts.
Christophe
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