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Message-ID: <897E5117-8A78-4CE3-8514-3577C4474775@tencent.com>
Date: Thu, 23 Jul 2020 02:42:58 +0000
From: benbjiang(蒋彪) <benbjiang@...cent.com>
To: "Li, Aubrey" <aubrey.li@...ux.intel.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)
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.
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.
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.
>
>>
>>>
>>>>
>>>>> +#endif
>>>>> + break;
>>>>> + }
>>>>> }
>>>>>
>>>>> time = cpu_clock(this) - time;
>>>>> @@ -7609,8 +7634,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>>> * We do not migrate tasks that are:
>>>>> * 1) throttled_lb_pair, or
>>>>> * 2) cannot be migrated to this CPU due to cpus_ptr, or
>>>>> - * 3) running (obviously), or
>>>>> - * 4) are cache-hot on their current CPU.
>>>>> + * 3) task's cookie does not match with this CPU's core cookie
>>>>> + * 4) running (obviously), or
>>>>> + * 5) are cache-hot on their current CPU.
>>>>> */
>>>>> if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>>>>> return 0;
>>>>> @@ -7645,6 +7671,15 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>>> return 0;
>>>>> }
>>>>>
>>>>> +#ifdef CONFIG_SCHED_CORE
>>>>> + /*
>>>>> + * Don't migrate task if the task's cookie does not match
>>>>> + * with the destination CPU's core cookie.
>>>>> + */
>>>>> + if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
>>>>> + return 0;
>>>>> +#endif
>>>>> +
>>>>> /* Record that we found atleast one task that could run on dst_cpu */
>>>>> env->flags &= ~LBF_ALL_PINNED;
>>>>>
>>>>> @@ -8857,6 +8892,25 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>>>> p->cpus_ptr))
>>>>> continue;
>>>>>
>>>>> +#ifdef CONFIG_SCHED_CORE
>>>>> + if (sched_core_enabled(cpu_rq(this_cpu))) {
>>>>> + int i = 0;
>>>>> + bool cookie_match = false;
>>>>> +
>>>>> + for_each_cpu(i, sched_group_span(group)) {
>>>> Should we consider the p->cpus_ptr here? like,
>>>> for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr ) {
>>>
>>> This is already considered just above #ifdef CONFIG_SCHED_CORE, but not included
>>> in the patch file.
>>>
>>> Thanks,
>>> -Aubrey
>>
>> The above consideration is,
>> 8893 /* Skip over this group if it has no CPUs allowed */
>> 8894 if (!cpumask_intersects(sched_group_span(group),
>> 8895 p->cpus_ptr))
>> 8896 continue;
>> 8897
>> It only considers the case of *p is not allowed for the whole group*, which is not enough.
>> If( cpumask_subset(p->cpus_ptr, sched_group_span(group)), the following sched_core_cookie_match() may choose a *wrong(not allowed)* cpu to match cookie. In that case, the matching result could be confusing and lead to wrong result.
>> On the other hand, considering p->cpus_ptr here could reduce the loop times and cost, if cpumask_and(p->cpus_ptr, sched_group_span(group)) is the subset of sched_group_span(group).
>
> Though find_idlest_group_cpu() will check p->cpus_ptr again, I believe this is a good catch and
> should be fixed in the next iteration.
>
> Thanks,
> -Aubrey
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