Re: [performance regression, bisected] scheduler:should_we_balance() kills filesystem performance

[Joonsoo Kim]

On Tue, Sep 10, 2013 at 02:02:54PM +1000, Dave Chinner wrote:
> Hi folks,
> 
> I just updated my performance test VM to the current 3.12-git
> tree after the XFS dev branch was merged. The first test I ran
> which was a 16-way concurrent fsmark test to create lots of files
> gave me a number about 30% lower than I expected - ~180k files/s
> when I was expecting somewhere around 250k files/s.
> 
> I did a bisect, and the bisect landed on this commit:
> 
> commit 23f0d2093c789e612185180c468fa09063834e87
> Author: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Date:   Tue Aug 6 17:36:42 2013 +0900
> 
>     sched: Factor out code to should_we_balance()
>     
>     Now checking whether this cpu is appropriate to balance or not
>     is embedded into update_sg_lb_stats() and this checking has no direct
>     relationship to this function. There is not enough reason to place
>     this checking at update_sg_lb_stats(), except saving one iteration
>     for sched_group_cpus.
> ....
> 
> Now, i couldn't revert that patch by itself, but I reverted the
> series of about 10 scheduler patches in that series total from a
> current TOT and the regression went away. Hence I'm pretty confident
> that the this is the patch causing the issue as i've verified it in
> more than one way and the difference between "good" and "bad" was
> signficantlt greater than the variance of the test (1.5-2 stddev
> difference).
> 
> In more detail:
> 
> 			v4 filesystem		v5 filesystem
> 3.11+xfsdev:		220k files/s		225k files/s
> 3.12-git		180k files/s		185k files/s
> 3.12-git-revert		245k files/s		247k files/s
> 
> The test vm is a 16p/16GB RAM VM, with a sparse 100TB filesystem
> image sitting on a 4-way RAID0 SSD array formatted with XFS and the
> image file is accessed by virtio+direct IO. The fsmark command line
> is:
> 
> time ./fs_mark  -D  10000  -S0  -n  100000  -s  0  -L  32 \
>         -d  /mnt/scratch/0  -d  /mnt/scratch/1 \
>         -d  /mnt/scratch/2  -d  /mnt/scratch/3 \
>         -d  /mnt/scratch/4  -d  /mnt/scratch/5 \
>         -d  /mnt/scratch/6  -d  /mnt/scratch/7 \
>         -d  /mnt/scratch/8  -d  /mnt/scratch/9 \
>         -d  /mnt/scratch/10  -d  /mnt/scratch/11 \
>         -d  /mnt/scratch/12  -d  /mnt/scratch/13 \
>         -d  /mnt/scratch/14  -d  /mnt/scratch/15 \
>         | tee >(stats --trim-outliers | tail -1 1>&2)
> 
> The workload on XFS runs to almost being CPU bound - the effect of
> the above patch was that there was a lot of idle time left in the
> system. The workload consumed the same amount of user and system
> CPU, just instantaneous CPU usage was reduced by 20-30% and the
> elaspsed time was increased by 20-30%.

Hello, Dave.

Now, I look again this patch and find one mistake.
If we find that we are appropriate cpu for balancing, should_we_balance()
should return 1. But current code doesn't do so. This correspond with
your observation that a lot of idle time left.

Could you re-test your benchmark with below?

Thanks.

------------------->8-------------------------
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7f0a5e6..9b3fe1c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5151,7 +5151,7 @@ static int should_we_balance(struct lb_env *env)
         * First idle cpu or the first cpu(busiest) in this sched group
         * is eligible for doing load balancing at this and above domains.
         */
-       return balance_cpu != env->dst_cpu;
+       return balance_cpu == env->dst_cpu;
 }
 
 /*
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