Re: [PATCH 10/10] powerpc: remove address space overrides using set_fs()

[Christophe Leroy]

Le 02/09/2020 à 20:02, Linus Torvalds a écrit :
> On Wed, Sep 2, 2020 at 8:17 AM Christophe Leroy
> <christophe.leroy@xxxxxxxxxx> 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