Messages in this thread | | | From | Vincent Guittot <> | Date | Fri, 19 Aug 2022 14:35:37 +0200 | Subject | Re: [PATCH v2] sched/fair: Introduce priority load balance to reduce interference from IDLE tasks |
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Hi Zhang,
On Fri, 19 Aug 2022 at 12:54, zhangsong (J) <zhangsong34@huawei.com> wrote: > > > 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.
Do you have more details about the test that your are running ?
Do cpu0-2 share their cache ? Which kingd of task are the normal and idle tasks ? always running tasks ?
I'm going to try to reproduce your problem locally
Regards, Vincent
> > 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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