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

Christophe Leroy
2020-08-18T18:23:28Z

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().

Christophe