![]() | |||||||||||
Messages in this thread Patch in this message |
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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||
| Last update: 2007-05-02 13:03 [from the cache] ©2003-2008 | |||||||||||