Messages in this thread | | | Date | Fri, 19 Aug 2022 18:54:06 +0800 | Subject | Re: [PATCH v2] sched/fair: Introduce priority load balance to reduce interference from IDLE tasks | From | "zhangsong (J)" <> |
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On 2022/8/18 16:31, Vincent Guittot wrote: > Le jeudi 18 août 2022 à 10:46:55 (+0800), Abel Wu a écrit : >> On 8/17/22 8:58 PM, Vincent Guittot Wrote: >>> On Tue, 16 Aug 2022 at 04:53, zhangsong (J) <zhangsong34@huawei.com> wrote: >>>> > ... > >>>> Yes, this is usually a corner case, but suppose that some non-idle tasks bounds to CPU 1-2 >>>> >>>> and idle tasks bounds to CPU 0-1, so CPU 1 may has many idle tasks and some non-idle >>>> >>>> tasks while idle tasks on CPU 1 can not be pulled to CPU 2, when trigger load balance if >>>> >>>> CPU 2 should pull some tasks from CPU 1, the bad result is idle tasks of CPU 1 cannot be >>>> >>>> migrated and non-idle tasks also cannot be migrated in case of env->loop_max constraint. >>> env->loop_max adds a break but load_balance will continue with next >>> tasks so it also tries to pull your non idle task at the end after >>> several breaks. >> Loop will be terminated without LBF_NEED_BREAK if exceeds loop_max :) > Argh yes, my brain is not yet back from vacation > I have been confused by loop_max and loop_break being set to the same value 32 > > Zhang Song, Could you try the patch below ? If it works, I will prepare a > clean patch with all tags > > > > sched/fair: make sure to try to detach at least one movable task > > During load balance we try at most env->loop_max time to move a task. But > it can happen that the LRU tasks (ie tail of the cfs_tasks list) can't > be moved to dst_cpu because of affinity. In this case, loop in the list > until we found at least one. > > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > --- > kernel/sched/fair.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index da388657d5ac..02b7b808e186 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8052,8 +8052,12 @@ static int detach_tasks(struct lb_env *env) > p = list_last_entry(tasks, struct task_struct, se.group_node); > > env->loop++; > - /* We've more or less seen every task there is, call it quits */ > - if (env->loop > env->loop_max) > + /* > + * We've more or less seen every task there is, call it quits > + * unless we haven't found any movable task yet. > + */ > + if (env->loop > env->loop_max && > + !(env->flags & LBF_ALL_PINNED)) > break; > > /* take a breather every nr_migrate tasks */ > @@ -10182,7 +10186,9 @@ static int load_balance(int this_cpu, struct rq *this_rq, > > if (env.flags & LBF_NEED_BREAK) { > env.flags &= ~LBF_NEED_BREAK; > - goto more_balance; > + /* Stop if we tried all running tasks */ > + if (env.loop < busiest->nr_running) > + goto more_balance; > } > > /* > -- > 2.17.1
Thanks for your reply. I have tried your patch and run test compared with it, it seems that the patch you provide makes no sense. The test result is below(1000 idle tasks bounds to CPU 0-1 and 10 normal tasks bounds to CPU 1-2):
=================================================================
Without patch:
6,777.37 msec cpu-clock # 1.355 CPUs utilized 20,812 context-switches # 0.003 M/sec 0 cpu-migrations # 0.000 K/sec 0 page-faults # 0.000 K/sec 13,333,983,148 cycles # 1.967 GHz 6,457,930,305 instructions # 0.48 insn per cycle 2,125,644,649 branches # 313.639 M/sec 1,690,587 branch-misses # 0.08% of all branches 5.001931983 seconds time elapsed
With your patch:
6,791.46 msec cpu-clock # 1.358 CPUs utilized 20,996 context-switches # 0.003 M/sec 0 cpu-migrations # 0.000 K/sec 0 page-faults # 0.000 K/sec 13,467,573,052 cycles # 1.983 GHz 6,516,989,062 instructions # 0.48 insn per cycle 2,145,139,220 branches # 315.858 M/sec 1,751,454 branch-misses # 0.08% of all branches
5.002274267 seconds time elapsed
With my patch:
7,495.14 msec cpu-clock # 1.499 CPUs utilized 23,176 context-switches # 0.003 M/sec 309 cpu-migrations # 0.041 K/sec 0 page-faults # 0.000 K/sec 14,849,083,489 cycles # 1.981 GHz 7,180,832,268 instructions # 0.48 insn per cycle 2,363,300,644 branches # 315.311 M/sec 1,964,169 branch-misses # 0.08% of all branches
5.001713352 seconds time elapsed ===============================================================
Obviously, when your patch is applied, the cpu-migrations of normal tasks is still 0 and the CPU ulization of normal tasks have no improvement compared with no patch applied. When apply my patch, the cpu-migrations and CPU ulization of normal tasks can both improve. I cannot explain the result with your patch, you also can test it by yourself.
Best, Zhang Song
> >>>> This will cause non-idle tasks cannot achieve more CPU utilization. >>> Your problem is not linked to IDLE vs NORMAL tasks but to the large >>> number of pinned tasks that can't migrate on CPU2. You can end with >>> the same behavior without using IDLE tasks but only NORMAL tasks. >> I feel the same thing. >> >> Best, >> Abel > .
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