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SubjectRe: [PATCH 1/6] sched: migration changes for core scheduling
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Hi Peter,

On 2021/3/20 23:34, Peter Zijlstra wrote:
> On Fri, Mar 19, 2021 at 04:32:48PM -0400, Joel Fernandes (Google) wrote:
>> @@ -7530,8 +7543,9 @@ 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;
>> @@ -7566,6 +7580,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>> return 0;
>> }
>>
>> + /*
>> + * 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;
>> +
>> /* Record that we found atleast one task that could run on dst_cpu */
>> env->flags &= ~LBF_ALL_PINNED;
>>
>
> This one is too strong.. persistent imbalance should be able to override
> it.
>

IIRC, this change can avoid the following scenario:

One sysbench cpu thread(cookieA) and sysbench mysql thread(cookieB) running
on the two siblings of core_1, the other sysbench cpu thread(cookieA) and
sysbench mysql thread(cookieB) running on the two siblings of core2, which
causes 50% force idle.

This is not an imbalance case.

Thanks,
-Aubrey

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