Re: [LKP] [lkp-robot] [sched/fair] d519329f72: unixbench.score -9.9% regression

[Aaron Lu]

On Wed, Oct 24, 2018 at 06:01:37PM +0100, Patrick Bellasi wrote:
> On 24-Oct 14:41, Aaron Lu wrote:
> > On Mon, Apr 02, 2018 at 11:20:00AM +0800, Ye, Xiaolong wrote:
> > > 
> > > Greeting,
> > > 
> > > FYI, we noticed a -9.9% regression of unixbench.score due to commit:
> > > 
> > > 
> > > commit: d519329f72a6f36bc4f2b85452640cfe583b4f81 ("sched/fair: Update util_est only on util_avg updates")
> > > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> > > 
> > > in testcase: unixbench
> > > on test machine: 8 threads Intel(R) Core(TM) i7 CPU 870 @ 2.93GHz with 6G memory
> > > with following parameters:
> > > 
> > >         runtime: 300s
> > >         nr_task: 100%
> > >         test: execl
> > > 
> > > test-description: UnixBench is the original BYTE UNIX benchmark suite aims to test performance of Unix-like system.
> > > test-url: https://github.com/kdlucas/byte-unixbench
> 
> Hi Aaron,
>  
> > I tested this workload on different machines with this commit
> > d519329f72a6f36bc4f2b85452 and its parent a07630b8b2c16f82, I also
> > tested with v4.19-rc8 to see if the regression is gone -
> > the performance drop is there with v4.19-rc8 and with different
> > machines so I assume this regression is not solved yet.
> >
> > Here are detailed data:
> > 
> > cmdline used to run this workload:
> > ./Run execl -c $nr_cpu -i 30
> 
> I had a better look into this issue and found that something like this
> could be the cure for the execl throughput regression:

Good news, yes they are!

> ---8<---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 908c9cdae2f0..c34d41b542fc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6258,8 +6258,17 @@ static unsigned long cpu_util_wake(int cpu, struct task_struct *p)
>  	 * covered by the following code when estimated utilization is
>  	 * enabled.
>  	 */
> -	if (sched_feat(UTIL_EST))
> -		util = max(util, READ_ONCE(cfs_rq->avg.util_est.enqueued));
> +	if (sched_feat(UTIL_EST)) {
> +		unsigned int estimated =
> +			READ_ONCE(cfs_rq->avg.util_est.enqueued);
> +
> +		if (unlikely(current == p)) {
> +			estimated -= min_t(unsigned int, estimated,
> +				(_task_util_est(p) | UTIL_AVG_UNCHANGED));
> +		}
> +
> +		util = max(util, estimated);
> +	}
> 
>  	/*
>  	 * Utilization (estimated) can exceed the CPU capacity, thus let's
> ---8<---
> 
> I'll test this better on a machine on my side and send out a proper
> patch by tomorrow.
> 
> > Please let me know if you need other information, thanks.
> 
> Would be nice if you can test the above on your side too.
> 

commit cbcb74a95c5af32f9127a102feca323139ba2c49 is the commit I made
from your diff and it restored performance for the two desktops. the
result on the skylake server isn't quite stable so I think the
performance gap is due to noise.

lkp-ivb-d04:
cbcb74a95c5af32f9127a102feca323139ba2c49/avg.json:  "unixbench.score": 2946.0,
d519329f72a6f36bc4f2b85452640cfe583b4f81/avg.json:  "unixbench.score": 2669.5333333333333,
a07630b8b2c16f82fd5b71d890079f4dd7599c1d/avg.json:  "unixbench.score": 2924.3333333333335,

lkp-hsw-d01:
cbcb74a95c5af32f9127a102feca323139ba2c49/avg.json:  "unixbench.score": 7013.533333333333,
d519329f72a6f36bc4f2b85452640cfe583b4f81/avg.json:  "unixbench.score": 6421.233333333333,
a07630b8b2c16f82fd5b71d890079f4dd7599c1d/avg.json:  "unixbench.score": 7090.400000000001,

lkp-skl-2sp2:
cbcb74a95c5af32f9127a102feca323139ba2c49/avg.json:  "unixbench.score": 9347.02,
d519329f72a6f36bc4f2b85452640cfe583b4f81/avg.json:  "unixbench.score": 9362.76,
a07630b8b2c16f82fd5b71d890079f4dd7599c1d/avg.json:  "unixbench.score": 9520.86,