Re: [RFC PATCH 5/6] sched/fair: Add task burst threshold for large-scale bursts

[Peter Zijlstra]

On Tue, Oct 20, 2026 at 04:37:45PM +0800, Li kunyu wrote:

> +#ifdef CONFIG_NO_HZ_COMMON
> +	/*
> +	 * In practice, bursty tasks can lead to uneven task distribution.
> +	 * For example, some CPUs may have kernel-mode utilization above 50%,
> +	 * while more than 10% of CPUs in the system remain idle.
> +	 * Therefore, we define a task burst threshold:
> +	 *
> +	 * 1. For a task to be woken up, the scheduler first follows the
> +	 * original wake-up logic (such as wake-up on the current CPU or
> +	 * affinity wake-up) to return a CPU. If no CPU is returned, it
> +	 * then checks the task burst threshold of new_cpu.
> +	 * 2. Newly created child processes or threads have fewer
> +	 * considerations; only the task burst threshold needs to be checked.
> +	 */
> +	if (sched_feat(SMT_BUDDY_ASSIST) && cpu_rq(new_cpu)->nr_running >=
> +			sysctl_smt_buddy_balance_task)
> +		for_each_cpu_wrap(nohz_idle_cpu,  nohz.idle_cpus_mask, new_cpu + 1) {
> +			if (!idle_cpu(nohz_idle_cpu))
> +				continue;
> +
> +			/*
> +			 * In general-purpose scenarios, the CPU can
> +			 * typically meet task requirements.
> +			 */
> +			if (!cpumask_test_cpu(nohz_idle_cpu, p->cpus_ptr))
> +				continue;
> +
> +		/*
> +		 * Simply returning nohz_idle_cpu is sufficient, since the
> +		 * subsequent NOHZ kick will clear the corresponding bit in
> +		 * nohz.idle_cpus_mask.
> +		 */
> +		return nohz_idle_cpu;
> +	}
> +#endif

I don't understand why this depends on NO_HZ. I also don't understand
why this only considers idle CPUs.

Also, static thresholds are horrible.


Someone that burst wakes a ton of tasks, even on a non-idle system, will
cause an uneven task distribution. Irrespective of there being idle CPUs
in the system or NOHZ whatever.

So how about we track an average rq-depth in the periodic load-balance
pass, in say the root domain or so. And then, when the the local
rq-depth reaches some multiple (3 sounds like a good number?) of that,
we'll fall back and do something utterly dumb like:

	cpu = cpumask_any_distribute(p->cpus_ptr);

Eg, simply start rotating new tasks around the system *and* reset the
periodic load balance period.