linux-kernel - Re: [LKP] [x86_64,entry] f04e05b81e4: -62.7% time.user

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Date:	Mon, 10 Nov 2014 11:39:56 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	LKP <lkp@...org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [LKP] [x86_64,entry] f04e05b81e4: -62.7% time.user_time

On Thu, Nov 6, 2014 at 10:40 PM, Yuanhan Liu
<yuanhan.liu@...ux.intel.com> wrote:
> On Thu, Nov 06, 2014 at 10:26:23PM -0800, Andy Lutomirski wrote:
>> On Thu, Nov 6, 2014 at 10:00 PM, LKP <lkp@...org> wrote:
>> > FYI, we noticed the below changes on
>> >
>> > git://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git x86/entry
>> > commit f04e05b81e4d9ae88bee379f12176f551a24394a ("x86_64,entry: Use sysret to return to userspace when possible")
>> >
>> >
>> > f2ee9bebb99dd4a4  f04e05b81e4d9ae88bee379f12  testbox/testcase/testparams
>> > ----------------  --------------------------
>> >          %stddev     %change         %stddev
>> >              \          |                \
>> >      24.09 ą  4%     -62.7%       8.99 ą  3%  lkp-wsx02/aim9/performance-300s-creat-clo
>> >      24.09           -62.7%       8.99        GEO-MEAN time.user_time
>> >
>>
>> I don't really know how to interpret this.  Is this a good thing or a bad thing?
>
> TBH, I don't, either. Here we just list the changes we captured, which
> is then for your information to decide it's good or bad.
>
>>
>> I actually expect this change to be a dramatic speedup for some
>> workloads.
>
> Would you like to share your workload? If so, I can add them to our
> testing system, and give feedback to you when datat is available.

My workload is just syscall performance under different conditions.
There's no obvious interesting benchmark, although I may write one at
some point.

>
>> Any chance you can send me the .config, kernel command
>> line, and whether you have any tracing features enabled?
>
> .config and dmesg are attached.

Hmm.  I'm not entirely sure what's going on.  I think that you have
NO_HZ_FULL active, but I can't see what's actually activating it.  My
initial attempt to replicate this failed, but I see a big performance
gain if CONFIG_CONTEXT_TRACKING_FORCE=y, so I suspect that you've just
managed to activate context tracking without forcing it.

--Andy
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