Re: [patch] CFS scheduler, -v8

Balbir Singh
2007-05-02T11:00:07Z

Ingo Molnar wrote:
> * Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
>
>> With -v7 I would run the n/n+1 test. Basically on a system with n
>> cpus, I would run n+1 tasks and see how their load is distributed. I
>> usually find that the last two tasks would get stuck on one CPU on the
>> system and would get half the cpu time as their other peers. I think
>> this issue has been around for long even before CFS. But while I was
>> investigating that, I found that with -v8, all the n+1 tasks are stuck
>> on the same cpu.
>
> i believe this problem is specific to powerpc - load is distributed fine
> on i686/x86_64 and your sched_debug shows a cpu_load[0] == 0 on CPU#2
> which is 'impossible'. (I sent a few suggestions off-Cc about how to
> debug this.)
>
> Ingo

Hi, Ingo

The suggestions helped, here is a fix tested on PowerPC only.

Patch and Description
=====================

Load balancing on PowerPC is broken. Running 5 tasks on a 4 cpu system
results in all 5 tasks running on the same CPU. Based on Ingo's feedback,
I instrumented and debugged update_load_fair().

The problem is with comparing a s64 values with (s64)ULONG_MAX, which
evaluates to -1. Then we check if exec_delta64 and fair_delta64 are greater
than (s64)ULONG_MAX (-1), if so we assign (s64)ULONG_MAX to the respective
values.

The fix is to compare these values against (s64)LONG_MAX and assign
(s64)LONG_MAX to exec_delta64 and fair_delta64 if they are greater than
(s64)LONG_MAX.

Tested on PowerPC, the regression is gone, tasks are load balanced as they
were in v7.

Output of top

5614 root 20 0 4912 784 252 R 52 0.0 3:27.49 3 bash

5620 root 20 0 4912 784 252 R 47 0.0 3:07.38 2 bash

5617 root 20 0 4912 784 252 R 47 0.0 3:08.18 0 bash

5624 root 20 0 4912 784 252 R 26 0.0 1:42.97 1 bash

5621 root 20 0 4912 784 252 R 26 0.0 1:43.14 1 bash

Tasks 5624 and 5621 getting half of their peer values is a separate issue
altogether.

Signed-off-by: Balbir Singh <balbir@linux.vnet.ibm.com>
---

kernel/sched.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)

diff -puN kernel/sched.c~cfs-fix-load-balancing-arith kernel/sched.c
--- linux-2.6.21/kernel/sched.c~cfs-fix-load-balancing-arith 2007-05-02
16:16:20.000000000 +0530
+++ linux-2.6.21-balbir/kernel/sched.c 2007-05-02 16:16:47.000000000 +0530
@@ -1533,19 +1533,19 @@ static void update_load_fair(struct rq *
this_rq->prev_exec_clock = this_rq->exec_clock;
WARN_ON_ONCE(exec_delta64 <= 0);

- if (fair_delta64 > (s64)ULONG_MAX)
- fair_delta64 = (s64)ULONG_MAX;
+ if (fair_delta64 > (s64)LONG_MAX)
+ fair_delta64 = (s64)LONG_MAX;
fair_delta = (unsigned long)fair_delta64;

- if (exec_delta64 > (s64)ULONG_MAX)
- exec_delta64 = (s64)ULONG_MAX;
+ if (exec_delta64 > (s64)LONG_MAX)
+ exec_delta64 = (s64)LONG_MAX;
exec_delta = (unsigned long)exec_delta64;
if (exec_delta > TICK_NSEC)
exec_delta = TICK_NSEC;

idle_delta = TICK_NSEC - exec_delta;

- tmp = SCHED_LOAD_SCALE * exec_delta / fair_delta;
+ tmp = (SCHED_LOAD_SCALE * exec_delta) / fair_delta;
tmp64 = (u64)tmp * (u64)exec_delta;
do_div(tmp64, TICK_NSEC);
this_load = (unsigned long)tmp64;
_

--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL