Hi,
> On Jul 24, 2020, at 10:05 AM, Li, Aubrey <aubrey...@linux.intel.com> wrote:
>
> On 2020/7/24 9:26, benbjiang(蒋彪) wrote:
>> Hi,
>>
>>> On Jul 24, 2020, at 7:43 AM, Aubrey Li <aubrey.in...@gmail.com> wrote:
>>>
>>> On Thu, Jul 23, 2020 at 4:28 PM benbjiang(蒋彪) <benbji...@tencent.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>>> On Jul 23, 2020, at 4:06 PM, Li, Aubrey <aubrey...@linux.intel.com> wrote:
>>>>>
>>>>> On 2020/7/23 15:47, benbjiang(蒋彪) wrote:
>>>>>> Hi,
>>>>>>
>>>>>>> On Jul 23, 2020, at 1:39 PM, Li, Aubrey <aubrey...@linux.intel.com>
>>>>>>> wrote:
>>>>>>>
>>>>>>> On 2020/7/23 12:23, benbjiang(蒋彪) wrote:
>>>>>>>> Hi,
>>>>>>>>> On Jul 23, 2020, at 11:35 AM, Li, Aubrey <aubrey...@linux.intel.com>
>>>>>>>>> wrote:
>>>>>>>>>
>>>>>>>>> On 2020/7/23 10:42, benbjiang(蒋彪) wrote:
>>>>>>>>>> Hi,
>>>>>>>>>>
>>>>>>>>>>> On Jul 23, 2020, at 9:57 AM, Li, Aubrey <aubrey...@linux.intel.com>
>>>>>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>> On 2020/7/22 22:32, benbjiang(蒋彪) wrote:
>>>>>>>>>>>> Hi,
>>>>>>>>>>>>
>>>>>>>>>>>>> On Jul 22, 2020, at 8:13 PM, Li, Aubrey
>>>>>>>>>>>>> <aubrey...@linux.intel.com> wrote:
>>>>>>>>>>>>>
>>>>>>>>>>>>> On 2020/7/22 16:54, benbjiang(蒋彪) wrote:
>>>>>>>>>>>>>> Hi, Aubrey,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> On Jul 1, 2020, at 5:32 AM, Vineeth Remanan Pillai
>>>>>>>>>>>>>>> <vpil...@digitalocean.com> wrote:
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> From: Aubrey Li <aubrey...@intel.com>
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> - Don't migrate if there is a cookie mismatch
>>>>>>>>>>>>>>> Load balance tries to move task from busiest CPU to the
>>>>>>>>>>>>>>> destination CPU. When core scheduling is enabled, if the
>>>>>>>>>>>>>>> task's cookie does not match with the destination CPU's
>>>>>>>>>>>>>>> core cookie, this task will be skipped by this CPU. This
>>>>>>>>>>>>>>> mitigates the forced idle time on the destination CPU.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> - Select cookie matched idle CPU
>>>>>>>>>>>>>>> In the fast path of task wakeup, select the first cookie matched
>>>>>>>>>>>>>>> idle CPU instead of the first idle CPU.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> - Find cookie matched idlest CPU
>>>>>>>>>>>>>>> In the slow path of task wakeup, find the idlest CPU whose core
>>>>>>>>>>>>>>> cookie matches with task's cookie
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> - Don't migrate task if cookie not match
>>>>>>>>>>>>>>> For the NUMA load balance, don't migrate task to the CPU whose
>>>>>>>>>>>>>>> core cookie does not match with task's cookie
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> Signed-off-by: Aubrey Li <aubrey...@linux.intel.com>
>>>>>>>>>>>>>>> Signed-off-by: Tim Chen <tim.c.c...@linux.intel.com>
>>>>>>>>>>>>>>> Signed-off-by: Vineeth Remanan Pillai <vpil...@digitalocean.com>
>>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>>> kernel/sched/fair.c | 64
>>>>>>>>>>>>>>> ++++++++++++++++++++++++++++++++++++++++----
>>>>>>>>>>>>>>> kernel/sched/sched.h | 29 ++++++++++++++++++++
>>>>>>>>>>>>>>> 2 files changed, 88 insertions(+), 5 deletions(-)
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>>>>>>>>>>>> index d16939766361..33dc4bf01817 100644
>>>>>>>>>>>>>>> --- a/kernel/sched/fair.c
>>>>>>>>>>>>>>> +++ b/kernel/sched/fair.c
>>>>>>>>>>>>>>> @@ -2051,6 +2051,15 @@ static void task_numa_find_cpu(struct
>>>>>>>>>>>>>>> task_numa_env *env,
>>>>>>>>>>>>>>> if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
>>>>>>>>>>>>>>> continue;
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> +#ifdef CONFIG_SCHED_CORE
>>>>>>>>>>>>>>> + /*
>>>>>>>>>>>>>>> + * Skip this cpu if source task's cookie does not
>>>>>>>>>>>>>>> match
>>>>>>>>>>>>>>> + * with CPU's core cookie.
>>>>>>>>>>>>>>> + */
>>>>>>>>>>>>>>> + if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
>>>>>>>>>>>>>>> + continue;
>>>>>>>>>>>>>>> +#endif
>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>> env->dst_cpu = cpu;
>>>>>>>>>>>>>>> if (task_numa_compare(env, taskimp, groupimp,
>>>>>>>>>>>>>>> maymove))
>>>>>>>>>>>>>>> break;
>>>>>>>>>>>>>>> @@ -5963,11 +5972,17 @@ find_idlest_group_cpu(struct
>>>>>>>>>>>>>>> sched_group *group, struct task_struct *p, int this
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> /* Traverse only the allowed CPUs */
>>>>>>>>>>>>>>> for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
>>>>>>>>>>>>>>> + struct rq *rq = cpu_rq(i);
>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>> +#ifdef CONFIG_SCHED_CORE
>>>>>>>>>>>>>>> + if (!sched_core_cookie_match(rq, p))
>>>>>>>>>>>>>>> + continue;
>>>>>>>>>>>>>>> +#endif
>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>> if (sched_idle_cpu(i))
>>>>>>>>>>>>>>> return i;
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> if (available_idle_cpu(i)) {
>>>>>>>>>>>>>>> - struct rq *rq = cpu_rq(i);
>>>>>>>>>>>>>>> struct cpuidle_state *idle =
>>>>>>>>>>>>>>> idle_get_state(rq);
>>>>>>>>>>>>>>> if (idle && idle->exit_latency <
>>>>>>>>>>>>>>> min_exit_latency) {
>>>>>>>>>>>>>>> /*
>>>>>>>>>>>>>>> @@ -6224,8 +6239,18 @@ static int select_idle_cpu(struct
>>>>>>>>>>>>>>> task_struct *p, struct sched_domain *sd, int t
>>>>>>>>>>>>>>> for_each_cpu_wrap(cpu, cpus, target) {
>>>>>>>>>>>>>>> if (!--nr)
>>>>>>>>>>>>>>> return -1;
>>>>>>>>>>>>>>> - if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
>>>>>>>>>>>>>>> - break;
>>>>>>>>>>>>>>> +
>>>>>>>>>>>>>>> + if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
>>>>>>>>>>>>>>> {
>>>>>>>>>>>>>>> +#ifdef CONFIG_SCHED_CORE
>>>>>>>>>>>>>>> + /*
>>>>>>>>>>>>>>> + * If Core Scheduling is enabled, select
>>>>>>>>>>>>>>> this cpu
>>>>>>>>>>>>>>> + * only if the process cookie matches core
>>>>>>>>>>>>>>> cookie.
>>>>>>>>>>>>>>> + */
>>>>>>>>>>>>>>> + if (sched_core_enabled(cpu_rq(cpu)) &&
>>>>>>>>>>>>>>> + p->core_cookie ==
>>>>>>>>>>>>>>> cpu_rq(cpu)->core->core_cookie)
>>>>>>>>>>>>>> Why not also add similar logic in select_idle_smt to reduce
>>>>>>>>>>>>>> forced-idle? :)
>>>>>>>>>>>>> We hit select_idle_smt after we scaned the entire LLC domain for
>>>>>>>>>>>>> idle cores
>>>>>>>>>>>>> and idle cpus and failed,so IMHO, an idle smt is probably a good
>>>>>>>>>>>>> choice under
>>>>>>>>>>>>> this scenario.
>>>>>>>>>>>>
>>>>>>>>>>>> AFAIC, selecting idle sibling with unmatched cookie will cause
>>>>>>>>>>>> unnecessary fored-idle, unfairness and latency, compared to
>>>>>>>>>>>> choosing *target* cpu.
>>>>>>>>>>> Choosing target cpu could increase the runnable task number on the
>>>>>>>>>>> target runqueue, this
>>>>>>>>>>> could trigger busiest->nr_running > 1 logic and makes the idle
>>>>>>>>>>> sibling trying to pull but
>>>>>>>>>>> not success(due to cookie not match). Putting task to the idle
>>>>>>>>>>> sibling is relatively stable IMHO.
>>>>>>>>>>
>>>>>>>>>> I’m afraid that *unsuccessful* pullings between smts would not
>>>>>>>>>> result in unstableness, because
>>>>>>>>>> the load-balance always do periodicly , and unsuccess means nothing
>>>>>>>>>> happen.
>>>>>>>>> unsuccess pulling means more unnecessary overhead in load balance.
>>>>>>>>>
>>>>>>>>>> On the contrary, unmatched sibling tasks running concurrently could
>>>>>>>>>> bring forced-idle to each other repeatedly,
>>>>>>>>>> Which is more unstable, and more costly when pick_next_task for all
>>>>>>>>>> siblings.
>>>>>>>>> Not worse than two tasks ping-pong on the same target run queue I
>>>>>>>>> guess, and better if
>>>>>>>>> - task1(cookie A) is running on the target, and task2(cookie B) in
>>>>>>>>> the runqueue,
>>>>>>>>> - task3(cookie B) coming
>>>>>>>>>
>>>>>>>>> If task3 chooses target's sibling, it could have a chance to run
>>>>>>>>> concurrently with task2.
>>>>>>>>> But if task3 chooses target, it will wait for next pulling luck of
>>>>>>>>> load balancer
>>>>>>>> That’s more interesting. :)
>>>>>>>> Distributing different cookie tasks onto different cpus(or cpusets)
>>>>>>>> could be the *ideal stable status* we want, as I understood.
>>>>>>>> Different cookie tasks running on sibling smts could hurt performance,
>>>>>>>> and that should be avoided with best effort.
>>>>>>> We already tried to avoid when we scan idle cores and idle cpus in llc
>>>>>>> domain.
>>>>>>
>>>>>> I’m afraid that’s not enough either, :)
>>>>>> 1. Scanning Idle cpus is not a full scan, there is limit according to
>>>>>> scan cost.
>>>>>> 2. That's only trying at the *core/cpu* level, *SMT* level should be
>>>>>> considered too.
>>>>>>
>>>>>>>
>>>>>>>> For above case, selecting idle sibling cpu can improve the concurrency
>>>>>>>> indeed, but it decrease the imbalance for load-balancer.
>>>>>>>> In that case, load-balancer could not notice the imbalance, and would
>>>>>>>> do nothing to improve the unmatched situation.
>>>>>>>> On the contrary, choosing the *target* cpu could enhance the
>>>>>>>> imbalance, and load-balancer could try to pull unmatched task away,
>>>>>>> Pulling away to where needs another bunch of elaboration.
>>>>>>
>>>>>> Still with the SMT2+3tasks case,
>>>>>> if *idle sibling* chosen,
>>>>>> Smt1’s load = task1+task2, smt2’s load = task3. Task3 will run
>>>>>> intermittently because of forced-idle,
>>>>>> so smt2’s real load could low enough, that it could not be pulled away
>>>>>> forever. That’s indeed a stable state,
>>>>>> but with performance at a discount.
>>>>>>
>>>>>> If *target sibling* chose,
>>>>>> Smt1’s load = task1+task2+task3, smt2’s load=0. It’s a obvious
>>>>>> imbalance, and load-balancer will pick a task to pull,
>>>>>> 1. If task1(cookie A) picked, that’s done for good.
>>>>>> 2. If task2(cookie B) or task3(cookie B) picked, that’s ok too, the rest
>>>>>> task(cookie B) could be pulled away at next balance(maybe need to
>>>>>> improve the pulling to tend to pull matched task more aggressively).
>>>>>> And then, we may reach a more stable state *globally* without
>>>>>> performance discount.
>>>>>
>>>>> I'm not sure what you mean pulled away,
>>>> I mean pulled away by other cpus, may be triggered by idle balance or
>>>> periodic balance on other cpus.
>>>>
>>>>> - if you mean pulled away from this core, cookieA in idle sibling case
>>>>> can be
>>>>> pulled away too.
>>>> Yep, cookieA(task1) in idle sibling case could be pulled away, but
>>>> cookieB(task3) on the smt2 could never get the chance being pulled
>>>> away(unless being waken up).
>>>> If cookieA(task1) failed being pulled(cookieB(task2) on smt1 may be pulled,
>>>> 50% chance), cookieA(task1) and cookieB(task3) would reach the stable state
>>>> with performance discount.
>>>>
>>> If you meant pulled away from this core, I didn't see how two cases are
>>> different either. For example, when task2(cookieB) runs on SMT1, task3
>>> cookieb can be pulled to SMT2. and when task1(cookieA) switch onto SMT1,
>>> task2(cookieB) can be pulled away by other cpus, too.
>> That’s the case only if SMT2’s pulling happens when task2(cookieB) is running
>> on SMT1, which depends on,
>> 1. Smt2 not entering tickless or nohz_balancer_kick picks smt2 before other
>> cpu’s pulling, may be unlikely. :)
>> 2. Task1(cookieA) is not running on SMT1.
>> otherwise it would be the case I described above.
>>
>> Besides, for other cases, like smt2+2task(CookieA+CookieB), picking *target*
>> cpu instead of *idle sibling* could be more helpful to reach the global
>> stable
>> status(distribute different cookies onto different cores).
>> If the task number of two cookies has a big difference, then distributing
> different cookies onto different cores leads to a big imbalance, that state
> may
> be stable but not an optimal state, I guess that's why core run queue does
> not refuse different cookies onto its rb tree.
That’s the overcommit case, in which case distributing is not possible, and
would
fallback to the *local stable status*(as what current implementation does) too.
If the total task number is not overcommit, distributing could work. :)
>
> I think I understand your concern but IMHO I'm not convinced adding cookie
> match
> in idle SMT selection is a best choice, if you have some performance data of
> your
> workload, that will be very helpful to understand the case.
Got it, I’ll try later.
>
> If distributing different cookies onto different cores is a hard requirement
> from
> your side, you are welcome to submit a patch to see others opinion.
Thanks for your patience. If possible, could you please loop me in for future
discussions about core-scheduling.
Thx.
Regard,
Jiang
>
> Thanks,
> -Aubrey
