Re: remove the last set_fs() in common code, and remove it for x86 and powerpc

[Christophe Leroy]

Le 18/08/2020 à 20:23, Christophe Leroy a écrit :
> Le 18/08/2020 à 20:05, Christoph Hellwig a écrit :
> > On Tue, Aug 18, 2020 at 07:46:22PM +0200, Christophe Leroy wrote:
> > > I gave it a go on my powerpc mpc832x. I tested it on top of my newest
> > > series that reworks the 32 bits signal handlers (see
> > > https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=196278) with 
> > >
> > > the microbenchmark test used is that series.
> > >
> > > With KUAP activated, on top of signal32 rework, performance is 
> > > boosted as
> > > system time for the microbenchmark goes from 1.73s down to 1.56s, 
> > > that is
> > > 10% quicker
> > >
> > > Surprisingly, with the kernel as is today without my signal's series, 
> > > your
> > > series degrades performance slightly (from 2.55s to 2.64s ie 3.5% 
> > > slower).
> > >
> > >
> > > I also observe, in both cases, a degradation on
> > >
> > >     dd if=/dev/zero of=/dev/null count=1M
> > >
> > > Without your series, it runs in 5.29 seconds.
> > > With your series, it runs in 5.82 seconds, that is 10% more time.
> >
> > That's pretty strage, I wonder if some kernel text cache line
> > effects come into play here?
> >
> > The kernel access side is only used in slow path code, so it should
> > not make a difference, and the uaccess code is simplified and should be
> > (marginally) faster.
> >
> > Btw, was this with the __{get,put}_user_allowed cockup that you noticed
> > fixed?
>
> Yes it is with the __get_user_size() replaced by __get_user_size_allowed().

I made a test with only the first patch of your series: That's 
definitely the culprit. With only that patch applies, the duration is 
6.64 seconds, that's a 25% degradation.

A perf record provides the following without the patch:
    41.91%  dd       [kernel.kallsyms]  [k] __arch_clear_user
     7.02%  dd       [kernel.kallsyms]  [k] vfs_read
     6.86%  dd       [kernel.kallsyms]  [k] new_sync_read
     6.68%  dd       [kernel.kallsyms]  [k] iov_iter_zero
     6.03%  dd       [kernel.kallsyms]  [k] transfer_to_syscall
     3.39%  dd       [kernel.kallsyms]  [k] memset
     3.07%  dd       [kernel.kallsyms]  [k] __fsnotify_parent
     2.68%  dd       [kernel.kallsyms]  [k] ksys_read
     2.09%  dd       [kernel.kallsyms]  [k] read_iter_zero
     2.01%  dd       [kernel.kallsyms]  [k] __fget_light
     1.84%  dd       [kernel.kallsyms]  [k] __fdget_pos
     1.35%  dd       [kernel.kallsyms]  [k] rw_verify_area
     1.32%  dd       libc-2.23.so       [.] __GI___libc_write
     1.21%  dd       [kernel.kallsyms]  [k] vfs_write
...
     0.03%  dd       [kernel.kallsyms]  [k] write_null

And the following with the patch:

    15.54%  dd       [kernel.kallsyms]  [k] __arch_clear_user
     9.17%  dd       [kernel.kallsyms]  [k] vfs_read
     6.54%  dd       [kernel.kallsyms]  [k] new_sync_write
     6.31%  dd       [kernel.kallsyms]  [k] transfer_to_syscall
     6.29%  dd       [kernel.kallsyms]  [k] __fsnotify_parent
     6.20%  dd       [kernel.kallsyms]  [k] new_sync_read
     5.47%  dd       [kernel.kallsyms]  [k] memset
     5.13%  dd       [kernel.kallsyms]  [k] vfs_write
     4.44%  dd       [kernel.kallsyms]  [k] iov_iter_zero
     2.95%  dd       [kernel.kallsyms]  [k] write_iter_null
     2.82%  dd       [kernel.kallsyms]  [k] ksys_read
     2.46%  dd       [kernel.kallsyms]  [k] __fget_light
     2.34%  dd       libc-2.23.so       [.] __GI___libc_read
     1.89%  dd       [kernel.kallsyms]  [k] iov_iter_advance
     1.76%  dd       [kernel.kallsyms]  [k] __fdget_pos
     1.65%  dd       [kernel.kallsyms]  [k] rw_verify_area
     1.63%  dd       [kernel.kallsyms]  [k] read_iter_zero
     1.60%  dd       [kernel.kallsyms]  [k] iov_iter_init
     1.22%  dd       [kernel.kallsyms]  [k] ksys_write
     1.14%  dd       libc-2.23.so       [.] __GI___libc_write

Christophe

> Christophe