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Date: Thu, 23 Jul 2020 11:35:56 +0800
From: "Li, Aubrey" <aubrey.li@...ux.intel.com>
To: benbjiang(蒋彪) <benbjiang@...cent.com>
Cc: Vineeth Remanan Pillai <vpillai@...italocean.com>,
Nishanth Aravamudan <naravamudan@...italocean.com>,
Julien Desfossez <jdesfossez@...italocean.com>,
Peter Zijlstra <peterz@...radead.org>,
Tim Chen <tim.c.chen@...ux.intel.com>,
"mingo@...nel.org" <mingo@...nel.org>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"pjt@...gle.com" <pjt@...gle.com>,
"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
Aubrey Li <aubrey.li@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"subhra.mazumdar@...cle.com" <subhra.mazumdar@...cle.com>,
"fweisbec@...il.com" <fweisbec@...il.com>,
"keescook@...omium.org" <keescook@...omium.org>,
"kerrnel@...gle.com" <kerrnel@...gle.com>,
Phil Auld <pauld@...hat.com>, Aaron Lu <aaron.lwe@...il.com>,
Aubrey Li <aubrey.intel@...il.com>,
Valentin Schneider <valentin.schneider@....com>,
Mel Gorman <mgorman@...hsingularity.net>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Joel Fernandes <joelaf@...gle.com>,
"joel@...lfernandes.org" <joel@...lfernandes.org>,
"vineethrp@...il.com" <vineethrp@...il.com>,
Chen Yu <yu.c.chen@...el.com>,
Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [RFC PATCH 11/16] sched: migration changes for core
scheduling(Internet mail)
On 2020/7/23 10:42, benbjiang(蒋彪) wrote:
> Hi,
>
>> On Jul 23, 2020, at 9:57 AM, Li, Aubrey <aubrey.li@...ux.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.li@...ux.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 <vpillai@...italocean.com> wrote:
>>>>>>
>>>>>> From: Aubrey Li <aubrey.li@...el.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.li@...ux.intel.com>
>>>>>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>>>>>> Signed-off-by: Vineeth Remanan Pillai <vpillai@...italocean.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
Thanks,
-Aubrey
> In consideration of currently load-balance being not fully aware of core-scheduling, and can not improve
> the *unmatched sibling* case, the *find_idlest_** entry should try its best to avoid the case, IMHO.
> Also, just an advice and an option. :)
>
> Thx.
> Regards,
> Jiang
>
>>
>>> Besides, choosing *target* cpu may be more cache friendly. So IMHO, *target* cpu may be a better choice if cookie not match, instead of idle sibling.
>> I'm not sure if it's more cache friendly as the target is busy, and the coming task
>> is a cookie unmatched task.
>>
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