The patch titled
sched: fix over-scheduling performance regression
has been added to the -mm tree. Its filename is
sched-fix-over-scheduling-performance-regression.patch
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------------------------------------------------------
Subject: sched: fix over-scheduling performance regression
From: "Alex,Shi" <alex.shi@xxxxxxxxx>
commit e709715915d6 ("sched: Optimize unused cgroup configuration")
introduced an imbalance schedule issue. If we do not use CGROUP, function
update_h_load won't want to update h_load. When the system has a large
number of tasks far more than logical CPU number, the incorrect
cfs_rq[cpu]->h_load value will cause load_balance() to pull too many tasks
to local CPU from the busiest CPU. So the busiest CPU keeps being in a
round robin. That will hurt performance.
The issue was found originally by a scientific calculation workload that
developed by Yanmin. with the commit, the workload performance drops
about 40% from this commit. We can be reproduced by a short program as
following.
# gcc -o sl sched-loop.c -lpthread
# ./sl -n 100 -t 100 &
# cat /proc/sched_debug &> sd1
# grep -A 1 cpu# sd1
sd1:cpu#0, 2533.008 MHz
sd1- .nr_running : 2
--
sd1:cpu#1, 2533.008 MHz
sd1- .nr_running : 1
--
sd1:cpu#2, 2533.008 MHz
sd1- .nr_running : 11
--
sd1:cpu#3, 2533.008 MHz
sd1- .nr_running : 12
--
sd1:cpu#4, 2533.008 MHz
sd1- .nr_running : 6
--
sd1:cpu#5, 2533.008 MHz
sd1- .nr_running : 11
--
sd1:cpu#6, 2533.008 MHz
sd1- .nr_running : 10
--
sd1:cpu#7, 2533.008 MHz
sd1- .nr_running : 12
--
sd1:cpu#8, 2533.008 MHz
sd1- .nr_running : 11
--
sd1:cpu#9, 2533.008 MHz
sd1- .nr_running : 12
--
sd1:cpu#10, 2533.008 MHz
sd1- .nr_running : 1
--
sd1:cpu#11, 2533.008 MHz
sd1- .nr_running : 1
--
sd1:cpu#12, 2533.008 MHz
sd1- .nr_running : 6
--
sd1:cpu#13, 2533.008 MHz
sd1- .nr_running : 2
--
sd1:cpu#14, 2533.008 MHz
sd1- .nr_running : 2
--
sd1:cpu#15, 2533.008 MHz
sd1- .nr_running : 1
After apply the fixing patch, cfs_rq get balance.
sd1:cpu#0, 2533.479 MHz
sd1- .nr_running : 7
--
sd1:cpu#1, 2533.479 MHz
sd1- .nr_running : 7
--
sd1:cpu#2, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#3, 2533.479 MHz
sd1- .nr_running : 7
--
sd1:cpu#4, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#5, 2533.479 MHz
sd1- .nr_running : 7
--
sd1:cpu#6, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#7, 2533.479 MHz
sd1- .nr_running : 7
--
sd1:cpu#8, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#9, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#10, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#11, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#12, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#13, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#14, 2533.479 MHz
sd1- .nr_running : 6
--
sd1:cpu#15, 2533.479 MHz
sd1- .nr_running : 6
---
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
volatile int * exiting;
void *idle_loop(){
volatile int calc01 = 100;
while(*exiting !=1)
calc01++;
}
int main(int argc, char *argv[]){
int i, t, c, er=0, num=8;
static char optstr[] = "n:t:";
pthread_t ptid[1024];
while ((c = getopt(argc, argv, optstr)) != EOF)
switch (c) {
case 'n':
num = atoi(optarg);
break;
case 't':
t = atoi(optarg);
break;
case '?':
er = 1;
break;
}
if (er) {
printf("usage: %s %s\n", argv[0], optstr);
exit(1);
}
exiting = malloc(sizeof(int));
*exiting = 0;
for(i=0; i<num ; i++)
pthread_create(&ptid[i], NULL, idle_loop, NULL);
sleep(t);
*exiting = 1;
for (i=0; i<num; i++)
pthread_join(ptid[i], NULL);
exit(0);
}
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxx>
Reviewed-by: Yanmin zhang <yanmin.zhang@xxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Cc: "Siddha, Suresh B" <suresh.b.siddha@xxxxxxxxx>
Cc: "Zhang, Yanmin" <yanmin_zhang@xxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/sched.c | 3 ---
1 file changed, 3 deletions(-)
diff -puN kernel/sched.c~sched-fix-over-scheduling-performance-regression kernel/sched.c
--- a/kernel/sched.c~sched-fix-over-scheduling-performance-regression
+++ a/kernel/sched.c
@@ -1666,9 +1666,6 @@ static void update_shares(struct sched_d
static void update_h_load(long cpu)
{
- if (root_task_group_empty())
- return;
-
walk_tg_tree(tg_load_down, tg_nop, (void *)cpu);
}
_
Patches currently in -mm which might be from alex.shi@xxxxxxxxx are
sched-fix-over-scheduling-performance-regression.patch
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