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Message-ID: <e84de5a9-b498-4152-846a-c72e1ac66109@linux.ibm.com>
Date: Tue, 14 Oct 2025 15:03:41 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Peter Zijlstra <peterz@...radead.org>,
Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...nel.org>, Chen Yu <yu.c.chen@...el.com>,
Doug Nelson <doug.nelson@...el.com>,
Mohini Narkhede <mohini.narkhede@...el.com>,
linux-kernel@...r.kernel.org,
Vincent Guittot <vincent.guittot@...aro.org>,
K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg
when balance is not due
On 10/14/25 2:54 PM, Peter Zijlstra wrote:
> On Mon, Oct 13, 2025 at 02:54:19PM -0700, Tim Chen wrote:
>
>>> So I'm not sure I understand the situation, @continue_balancing should
>>> limit this concurrency to however many groups are on this domain -- your
>>> granite thing with SNC on would have something like 6 groups?
>>
>> That's a good point. But I think the contention is worse than
>> 6 CPUs.
>>
>> The hierarchy would be
>>
>> SMT
>> NUMA-level1
>> NUMA-level2
>> NUMA-level3
>> NUMA-level4
>
> Aren't you missing the LLC/NODE domain here? We should have at least one
> !SD_NUMA domain above SMT.
>
>> There would be multiple CPUs in that are first in the SMT group
>> with continue_balancing=1 going up in the hierachy and
>> attempting the cmpxchg in the first NUMA domain level,
>> before calling should_we_balance() and finding that they are
>> not the first in the NUMA domain and set continue_balancing=0
>> and abort. Those CPUS are in same L3.
>> But at the same time, there could be CPUs in other sockets
>> cmpxchg on sched_balance_running.
>
> Right, Yu Chen said something like that as well, should_we_balance() is
> too late.
>
> Should we instead move the whole serialize thing inside
> sched_balance_rq() like so:
>
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bc0b7ce8a65d..e9f719ba17e1 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11722,6 +11722,22 @@ static void update_lb_imbalance_stat(struct lb_env *env, struct sched_domain *sd
> }
> }
>
> +
> +/*
> + * This flag serializes load-balancing passes over large domains
> + * (above the NODE topology level) - only one load-balancing instance
> + * may run at a time, to reduce overhead on very large systems with
> + * lots of CPUs and large NUMA distances.
> + *
> + * - Note that load-balancing passes triggered while another one
> + * is executing are skipped and not re-tried.
> + *
> + * - Also note that this does not serialize rebalance_domains()
> + * execution, as non-SD_SERIALIZE domains will still be
> + * load-balanced in parallel.
> + */
> +static atomic_t sched_balance_running = ATOMIC_INIT(0);
> +
> /*
> * Check this_cpu to ensure it is balanced within domain. Attempt to move
> * tasks if there is an imbalance.
> @@ -11747,6 +11763,7 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
> .fbq_type = all,
> .tasks = LIST_HEAD_INIT(env.tasks),
> };
> + int need_unlock = false;
>
> cpumask_and(cpus, sched_domain_span(sd), cpu_active_mask);
>
> @@ -11758,6 +11775,12 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
> goto out_balanced;
> }
>
> + if (idle != CPU_NEWLY_IDLE && (sd->flags & SD_SERIALIZE)) {
> + if (atomic_cmpxchg_acquire(&sched_balance_running, 0, 1))
> + goto out_balanced;
Maybe goto out instead of out_balanced ?
> + need_unlock = true;
> + }
> +
> group = sched_balance_find_src_group(&env);
> if (!group) {
> schedstat_inc(sd->lb_nobusyg[idle]);
> @@ -11998,6 +12021,9 @@ static int sched_balance_rq(int this_cpu, struct rq *this_rq,
> sd->balance_interval < sd->max_interval)
> sd->balance_interval *= 2;
> out:
> + if (need_unlock)
> + atomic_set_release(&sched_balance_running, 0);
> +
> return ld_moved;
> }
>
> @@ -12122,21 +12148,6 @@ static int active_load_balance_cpu_stop(void *data)
> return 0;
> }
>
> -/*
> - * This flag serializes load-balancing passes over large domains
> - * (above the NODE topology level) - only one load-balancing instance
> - * may run at a time, to reduce overhead on very large systems with
> - * lots of CPUs and large NUMA distances.
> - *
> - * - Note that load-balancing passes triggered while another one
> - * is executing are skipped and not re-tried.
> - *
> - * - Also note that this does not serialize rebalance_domains()
> - * execution, as non-SD_SERIALIZE domains will still be
> - * load-balanced in parallel.
> - */
> -static atomic_t sched_balance_running = ATOMIC_INIT(0);
> -
> /*
> * Scale the max sched_balance_rq interval with the number of CPUs in the system.
> * This trades load-balance latency on larger machines for less cross talk.
> @@ -12192,7 +12203,7 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
> /* Earliest time when we have to do rebalance again */
> unsigned long next_balance = jiffies + 60*HZ;
> int update_next_balance = 0;
> - int need_serialize, need_decay = 0;
> + int need_decay = 0;
> u64 max_cost = 0;
>
> rcu_read_lock();
> @@ -12216,13 +12227,6 @@ 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)) {
> if (sched_balance_rq(cpu, rq, sd, idle, &continue_balancing)) {
> /*
> @@ -12236,9 +12240,7 @@ 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);
> -out:
> +
> if (time_after(next_balance, sd->last_balance + interval)) {
> next_balance = sd->last_balance + interval;
> update_next_balance = 1;
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