linux-kernel - Re: [PATCH] sched: Skip useless sched_balance_running acquisition if load balance is not due

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Message-ID: <6fe46df2-2c80-4e2f-89a4-43f79e554f65@linux.ibm.com>
Date: Wed, 16 Apr 2025 19:44:45 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: "Chen, Yu C" <yu.c.chen@...el.com>, Tim Chen
 <tim.c.chen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Doug Nelson <doug.nelson@...el.com>,
        Mohini Narkhede <mohini.narkhede@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Skip useless sched_balance_running acquisition if
 load balance is not due



On 4/16/25 15:17, Vincent Guittot wrote:
> On Wed, 16 Apr 2025 at 11:29, Shrikanth Hegde <sshegde@...ux.ibm.com> wrote:
>>
>>
>>
>> On 4/16/25 14:46, Shrikanth Hegde wrote:
>>>
>>>
>>> On 4/16/25 11:58, Chen, Yu C wrote:
>>>> Hi Shrikanth,
>>>>
>>>> On 4/16/2025 1:30 PM, Shrikanth Hegde wrote:
>>>>>
>>>>>
>>>>> On 4/16/25 09:28, Tim Chen wrote:
>>>>>> At load balance time, balance of last level cache domains and
>>>>>> above needs to be serialized. The scheduler checks the atomic var
>>>>>> sched_balance_running first and then see if time is due for a load
>>>>>> balance. This is an expensive operation as multiple CPUs can attempt
>>>>>> sched_balance_running acquisition at the same time.
>>>>>>
>>>>>> On a 2 socket Granite Rapid systems enabling sub-numa cluster and
>>>>>> running OLTP workloads, 7.6% of cpu cycles are spent on cmpxchg of
>>>>>> sched_balance_running.  Most of the time, a balance attempt is aborted
>>>>>> immediately after acquiring sched_balance_running as load balance time
>>>>>> is not due.
>>>>>>
>>>>>> Instead, check balance due time first before acquiring
>>>>>> sched_balance_running. This skips many useless acquisitions
>>>>>> of sched_balance_running and knocks the 7.6% CPU overhead on
>>>>>> sched_balance_domain() down to 0.05%.  Throughput of the OLTP workload
>>>>>> improved by 11%.
>>>>>>
>>>>>
>>>>> Hi Tim.
>>>>>
>>>>> Time check makes sense specially on large systems mainly due to
>>>>> NEWIDLE balance.
>>>
>>> scratch the NEWLY_IDLE part from that comment.
>>>
>>>>>
>>>>
>>>> Could you elaborate a little on this statement? There is no timeout
>>>> mechanism like periodic load balancer for the NEWLY_IDLE, right?
>>>
>>> Yes. NEWLY_IDLE is very opportunistic.
>>>
>>>>
>>>>> One more point to add, A lot of time, the CPU which acquired
>>>>> sched_balance_running,
>>>>> need not end up doing the load balance, since it not the CPU meant to
>>>>> do the load balance.
>>>>>
>>>>> This thread.
>>>>> https://lore.kernel.org/all/1e43e783-55e7-417f-
>>>>> a1a7-503229eb163a@...ux.ibm.com/
>>>>>
>>>>>
>>>>> Best thing probably is to acquire it if this CPU has passed the time
>>>>> check and as well it is
>>>>> actually going to do load balance.
>>>>>
>>>>>
>>>>
>>>> This is a good point, and we might only want to deal with periodic load
>>>> balancer rather than NEWLY_IDLE balance. Because the latter is too
>>>> frequent and contention on the sched_balance_running might introduce
>>>> high cache contention.
>>>>
>>>
>>> But NEWLY_IDLE doesn't serialize using sched_balance_running and can
>>> endup consuming a lot of cycles. But if we serialize using
>>> sched_balance_running, it would definitely cause a lot contention as is.
>>>
>>>
>>> The point was, before acquiring it, it would be better if this CPU is
>>> definite to do the load balance. Else there are chances to miss the
>>> actual load balance.
>>>
>>>
>>
>> Sorry, forgot to add.
>>
>> Do we really need newidle running all the way till NUMA? or if it runs till PKG is it enough?
>> the regular (idle) can take care for NUMA by serializing it?
>>
>> -               if (sd->flags & SD_BALANCE_NEWIDLE) {
>> +               if (sd->flags & SD_BALANCE_NEWIDLE && !(sd->flags & SD_SERIALIZE)) {
> 
> Why not just clearing SD_BALANCE_NEWIDLE in your sched domain when you
> set SD_SERIALIZE

Hi Vincent.

There is even kernel parameter "relax_domain_level" which one can make use of.
concern was newidle does this without acquiring the sched_balance_running,
while busy,idle try to acquire this for NUMA.



Slightly different topic: It(kernel parameter) also resets SHCED_BALANCE_WAKE. But is it being used?
I couldn't find out how it is used.

> 
>>
>>                           pulled_task = sched_balance_rq(this_cpu, this_rq,
>>                                                      sd, CPU_NEWLY_IDLE,
>>
>>
>> Anyways, having a policy around this SD_SERIALIZE would be a good thing.
>>
>>>> thanks,
>>>> Chenyu
>>>>
>>>>>> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
>>>>>> Reported-by: Mohini Narkhede <mohini.narkhede@...el.com>
>>>>>> Tested-by: Mohini Narkhede <mohini.narkhede@...el.com>
>>>>>> ---
>>>>>>    kernel/sched/fair.c | 16 ++++++++--------
>>>>>>    1 file changed, 8 insertions(+), 8 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>>> index e43993a4e580..5e5f7a770b2f 100644
>>>>>> --- a/kernel/sched/fair.c
>>>>>> +++ b/kernel/sched/fair.c
>>>>>> @@ -12220,13 +12220,13 @@ static void sched_balance_domains(struct
>>>>>> rq *rq, enum cpu_idle_type idle)
>>>>>>            interval = get_sd_balance_interval(sd, busy);
>>>>>> -        need_serialize = sd->flags & SD_SERIALIZE;
>>>>>> -        if (need_serialize) {
>>>>>> -            if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
>>>>>> -                goto out;
>>>>>> -        }
>>>>>> -
>>>>>>            if (time_after_eq(jiffies, sd->last_balance + interval)) {
>>>>>> +            need_serialize = sd->flags & SD_SERIALIZE;
>>>>>> +            if (need_serialize) {
>>>>>> +                if (atomic_cmpxchg_acquire(&sched_balance_running,
>>>>>> 0, 1))
>>>>>> +                    goto out;
>>>>>> +            }
>>>>>> +
>>>>>>                if (sched_balance_rq(cpu, rq, sd, idle,
>>>>>> &continue_balancing)) {
>>>>>>                    /*
>>>>>>                     * The LBF_DST_PINNED logic could have changed
>>>>>> @@ -12238,9 +12238,9 @@ static void sched_balance_domains(struct rq
>>>>>> *rq, enum cpu_idle_type idle)
>>>>>>                }
>>>>>>                sd->last_balance = jiffies;
>>>>>>                interval = get_sd_balance_interval(sd, busy);
>>>>>> +            if (need_serialize)
>>>>>> +                atomic_set_release(&sched_balance_running, 0);
>>>>>>            }
>>>>>> -        if (need_serialize)
>>>>>> -            atomic_set_release(&sched_balance_running, 0);
>>>>>>    out:
>>>>>>            if (time_after(next_balance, sd->last_balance + interval)) {
>>>>>>                next_balance = sd->last_balance + interval;
>>>>>
>>>
>>