Re: [PATCH v10 3/3] sched/fair: Use candidate prev/recent_used CPU if scanning failed for cluster wakeup

[Yicong Yang]

Hi Vincent,

On 2023/10/13 23:04, Vincent Guittot wrote:
> On Thu, 12 Oct 2023 at 14:19, Yicong Yang <yangyicong@xxxxxxxxxx> wrote:
>>
>> From: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>>
>> Chen Yu reports a hackbench regression of cluster wakeup when
>> hackbench threads equal to the CPU number [1]. Analysis shows
>> it's because we wake up more on the target CPU even if the
>> prev_cpu is a good wakeup candidate and leads to the decrease
>> of the CPU utilization.
>>
>> Generally if the task's prev_cpu is idle we'll wake up the task
>> on it without scanning. On cluster machines we'll try to wake up
>> the task in the same cluster of the target for better cache
>> affinity, so if the prev_cpu is idle but not sharing the same
>> cluster with the target we'll still try to find an idle CPU within
>> the cluster. This will improve the performance at low loads on
>> cluster machines. But in the issue above, if the prev_cpu is idle
>> but not in the cluster with the target CPU, we'll try to scan an
>> idle one in the cluster. But since the system is busy, we're
>> likely to fail the scanning and use target instead, even if
>> the prev_cpu is idle. Then leads to the regression.
>>
>> This patch solves this in 2 steps:
>> o record the prev_cpu/recent_used_cpu if they're good wakeup
>>   candidates but not sharing the cluster with the target.
>> o on scanning failure use the prev_cpu/recent_used_cpu if
>>   they're still idle
>>
>> [1] https://lore.kernel.org/all/ZGzDLuVaHR1PAYDt@chenyu5-mobl1/
>> Reported-by: Chen Yu <yu.c.chen@xxxxxxxxx>
>> Signed-off-by: Yicong Yang <yangyicong@xxxxxxxxxxxxx>
>> ---
>>  kernel/sched/fair.c | 19 ++++++++++++++++++-
>>  1 file changed, 18 insertions(+), 1 deletion(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 4039f9b348ec..f1d94668bd71 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -7392,7 +7392,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>         bool has_idle_core = false;
>>         struct sched_domain *sd;
>>         unsigned long task_util, util_min, util_max;
>> -       int i, recent_used_cpu;
>> +       int i, recent_used_cpu, prev_aff = -1;
>>
>>         /*
>>          * On asymmetric system, update task utilization because we will check
>> @@ -7425,6 +7425,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>
>>                 if (cpus_share_resources(prev, target))
>>                         return prev;
>> +
>> +               prev_aff = prev;
>>         }
>>
>>         /*
>> @@ -7457,6 +7459,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>
>>                 if (cpus_share_resources(recent_used_cpu, target))
>>                         return recent_used_cpu;
>> +       } else {
>> +               recent_used_cpu = -1;
>>         }
>>
>>         /*
>> @@ -7497,6 +7501,19 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>>         if ((unsigned)i < nr_cpumask_bits)
>>                 return i;
>>
>> +       /*
>> +        * For cluster machines which have lower sharing cache like L2 or
>> +        * LLC Tag, we tend to find an idle CPU in the target's cluster
>> +        * first. But prev_cpu or recent_used_cpu may also be a good candidate,
>> +        * use them if possible when no idle CPU found in select_idle_cpu().
>> +        */
>> +       if ((unsigned int)prev_aff < nr_cpumask_bits &&
>> +           (available_idle_cpu(prev_aff) || sched_idle_cpu(prev_aff)))
> 
> Hasn't prev_aff (i.e. prev) been already tested as idle ?
> 
>> +               return prev_aff;
>> +       if ((unsigned int)recent_used_cpu < nr_cpumask_bits &&
>> +           (available_idle_cpu(recent_used_cpu) || sched_idle_cpu(recent_used_cpu)))
>> +               return recent_used_cpu;
> 
> same here
> 

It was thought that there maybe a small potential race window here that the prev/recent_used
CPU becoming non-idle after scanning, discussed in [1]. I think the check here won't be
expensive so added it here. It should be redundant and can be removed.

[1] https://lore.kernel.org/all/ZIams6s+qShFWhfQ@xxxxxxxxxxxxxxxxxxxxxx/

Thanks.

> 
>> +
>>         return target;
>>  }
>>
>> --
>> 2.24.0
>>
>