12:26 a.m.
On Thu, Aug 6, 2020 at 8:35 AM Ingo Molnar <mingo(a)kernel.org> wrote:
* Dan Williams <dan.j.williams(a)intel.com> wrote:
> On Thu, Aug 6, 2020 at 6:35 AM Ingo Molnar <mingo(a)kernel.org> wrote:
> >
> >
> > * kernel test robot <rong.a.chen(a)intel.com> wrote:
> >
> > > Greeting,
> > >
> > > FYI, we noticed a -43.3% regression of fio.read_iops due to commit:
> > >
> > >
> > > commit: a0ac629ebe7b3d248cb93807782a00d9142fdb98 ("x86/copy_mc:
Introduce copy_mc_generic()")
> > > url:
https://github.com/0day-ci/linux/commits/Dan-Williams/Renovate-memcpy_mcs...
> > >
> > >
> > > in testcase: fio-basic
> > > on test machine: 96 threads Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz with
256G memory
> > > with following parameters:
> >
> > So this performance regression, if it isn't a spurious result, looks
> > concerning. Is this expected?
>
> This is not expected and I think delays these patches until I'm back
> from leave in a few weeks. I know that we might lose some inlining
> effect due to replacing native memcpy, but I did not expect it would
> have an impact like this. In my testing I was seeing a performance
> improvement from replacing the careful / open-coded copy with rep;
> mov;, which increases the surprise of this result.
It would be nice to double check this on the kernel-test-robot side as
well, to make sure it's not a false positive.
Circling back to this, I found the bug. This incremental patch nearly
doubles the iops in the case when copy_mc_fragile() is enabled because
it was turning around and redoing the copy with copy_mc_generic(). So
this would have been a regression for existing systems that indicate
that "carefu/fragilel" copying can avoid some PCC=1 machine checks. My
performance checkout was comparing copy_mc_fragile() and
copy_mc_generic() in isolation. Refreshed patches inbound.
diff --git a/arch/x86/lib/copy_mc.c b/arch/x86/lib/copy_mc.c
index 9e6fac1ab72e..afac844c8f45 100644
--- a/arch/x86/lib/copy_mc.c
+++ b/arch/x86/lib/copy_mc.c
@@ -58,7 +58,8 @@ copy_mc_to_user(void *to, const void *from, unsigned len)
__uaccess_begin();
if (static_branch_unlikely(©_mc_fragile_key))
ret = copy_mc_fragile(to, from, len);
- ret = copy_mc_generic(to, from, len);
+ else
+ ret = copy_mc_generic(to, from, len);
__uaccess_end();
return ret;
}