Re: [PATCH] sched/fair: Fix wrong cpu selecting from isolated domain

[xunlei <xlpang@linux.alibaba.com>]

On 2020/8/24 PM9:38, Srikar Dronamraju wrote:
> * Xunlei Pang <xlpang@linux.alibaba.com> [2020-08-24 20:30:19]:
> 
>> We've met problems that occasionally tasks with full cpumask
>> (e.g. by putting it into a cpuset or setting to full affinity)
>> were migrated to our isolated cpus in production environment.
>>
>> After some analysis, we found that it is due to the current
>> select_idle_smt() not considering the sched_domain mask.
>>
>> Fix it by checking the valid domain mask in select_idle_smt().
>>
>> Fixes: 10e2f1acd010 ("sched/core: Rewrite and improve select_idle_siblings())
>> Reported-by: Wetp Zhang <wetp.zy@linux.alibaba.com>
>> Signed-off-by: Xunlei Pang <xlpang@linux.alibaba.com>
>> ---
>>  kernel/sched/fair.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 1a68a05..fa942c4 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -6075,7 +6075,7 @@ static int select_idle_core(struct task_struct *p, struct sched_domain *sd, int
>>  /*
>>   * Scan the local SMT mask for idle CPUs.
>>   */
>> -static int select_idle_smt(struct task_struct *p, int target)
>> +static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
>>  {
>>  	int cpu;
>>  
>> @@ -6083,7 +6083,8 @@ static int select_idle_smt(struct task_struct *p, int target)
>>  		return -1;
>>  
>>  	for_each_cpu(cpu, cpu_smt_mask(target)) {
>> -		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
>> +		if (!cpumask_test_cpu(cpu, p->cpus_ptr) ||
>> +		    !cpumask_test_cpu(cpu, sched_domain_span(sd)))
>>  			continue;
> 
> Don't think this is right thing to do.  What if this task had set a cpumask
> that doesn't cover all the cpus in this sched_domain_span(sd)

It doesn't matter, without this patch, it selects an idle cpu from:
"cpu_smt_mask(target) and p->cpus_ptr"

with this patch, it selects an idle cpu from:
"cpu_smt_mask(target) and p->cpus_ptr and sched_domain_span(sd)"

> 
> cpu_smt_mask(target) would already limit to the sched_domain_span(sd) so I
> am not sure how this can help?
> 
> 

Here is an example:
CPU0 and CPU16 are hyper-thread pair, CPU16 is domain isolated. So its
sd_llc doesn't contain CPU16, and cpu_smt_mask(0) is 0 and 16.

Then we have @target is 0, select_idle_smt() may return the isolated(and
idle) CPU16 without this patch.