On 3/3/23, Linus Torvalds [off-list ref] wrote:

On Fri, Mar 3, 2023 at 9:39 AM Mateusz Guzik [off-list ref] wrote:

quoted

So the key is: memset is underperforming at least on x86-64 for
certain sizes and the init-on-alloc thingy makes it used significantly
more, exacerbating the problem

One reason that the kernel memset isn't as optimized as memcpy, is
simply because under normal circumstances it shouldn't be *used* that
much outside of page clearing and constant-sized structure
initialization.

I mentioned in the previous e-mail that memset is used a lot even
without the problematic opt and even have shown size distribution of
what's getting passed there.

You made me curious how usage compares to memcpy, so I checked 'make'
in kernel dir once again.

I stress the init-on-alloc opt is *OFF*:
# CONFIG_INIT_ON_ALLOC_DEFAULT_ON is not set
# CONFIG_INIT_ON_FREE_DEFAULT_ON is not set

# bpftrace -e 'kprobe:memset,kprobe:memcpy { @[probe] = count(); }'
[snip]
@[kprobe:memcpy]: 6948498
@[kprobe:memset]: 36150671

iow memset is used about 7 times as much as memcpy when building the
kernel. If it matters I'm building on top of
2eb29d59ddf02e39774abfb60b2030b0b7e27c1f (reasonably fresh master).

So this is literally a problem with pointless automated memset,
introduced by that hardening option. And hardening people generally
simply don't care about performance, and the people who _do _care
about performance usually don't enable the known-expensive crazy
stuff.

Honestly, I think the INIT_ONCE stuff is actively detrimental, and
only hides issues (and in this case adds its own). So I can't but help
to say "don't do that then". I think it's literally stupid to clear
allocations by default.

Questioning usability of the entire thing was on the menu in what I
intended to write, but I did not read entire public discussion so
perhaps I missed a crucial insight.

I'm not opposed to improving memset, but honestly, if the argument is
based on the stupid hardening behavior, I really think *that* needs to
be fixed first.

It is not. The argument is that this is a core primitive, used a lot
with sizes where rep stos is detrimental to its performance. There is
no urgency, but eventually someone(tm) should sort it out. For
$reasons I don't want to do it myself.

I do bring it up in the context of the init-on-alloc machinery because
it disfigures whatever results are obtained testing on x86-64 -- they
can get exactly the same effect for much smaller cost, consequently
they should have interest in sorting this out.

General point though was that the work should have sanity-checked
performance of the primitive it relies on, instead of assuming it is
~optimal. I'm guilty of this mistake myself, so not going to throw
stones.

-- 
Mateusz Guzik <mjguzik gmail.com>