Re: [PATCH v2 2/9] sched/fair: Add rate-limiting and validation helpers

[Hillf Danton <hdanton@sina.com>]

Hi Wanpeng 

On Fri, 19 Dec 2025 11:53:26 +0800
> From: Wanpeng Li <wanpengli@tencent.com>
> 
> Implement core safety mechanisms for yield deboost operations.
> 
> Add yield_deboost_rate_limit() for high-frequency gating to prevent
> excessive overhead on compute-intensive workloads. The 6ms threshold
> balances responsiveness with overhead reduction.
> 
> Add yield_deboost_validate_tasks() for comprehensive validation ensuring
> both tasks are valid and distinct, both belong to fair_sched_class,
> target is on the same runqueue, and tasks are runnable.
> 
Given IPI in subsequent pacthes, why is same rq required?

> The rate limiter prevents pathological high-frequency cases while
> validation ensures only appropriate task pairs proceed. Both functions
> are static and will be integrated in subsequent patches.
> 
> v1 -> v2:
> - Remove unnecessary READ_ONCE/WRITE_ONCE for per-rq fields accessed
>   under rq->lock
> - Change rq->clock to rq_clock(rq) helper for consistency
> - Change yield_deboost_rate_limit() signature from (rq, now_ns) to (rq),
>   obtaining time internally via rq_clock()
> - Remove redundant sched_class check for p_yielding (already implied by
>   rq->donor being fair)
> - Simplify task_rq check to only verify p_target
> - Change rq->curr to rq->donor for correct EEVDF donor tracking
> - Move sysctl_sched_vcpu_debooster_enabled and NULL checks to caller
>   (yield_to_deboost) for early exit before update_rq_clock()
> - Simplify function signature by returning p_yielding directly instead
>   of using output pointer parameters
> - Add documentation explaining the 6ms rate limit threshold
> 
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
>  kernel/sched/fair.c | 62 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 62 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 87c30db2c853..2f327882bf4d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9040,6 +9040,68 @@ static void put_prev_task_fair(struct rq *rq, struct task_struct *prev, struct t
>  	}
>  }
>  
> +/*
> + * Rate-limit yield deboost operations to prevent excessive overhead.
> + * Returns true if the operation should be skipped due to rate limiting.
> + *
> + * The 6ms threshold balances responsiveness with overhead reduction:
> + * - Short enough to allow timely yield boosting for lock contention
> + * - Long enough to prevent pathological high-frequency penalty application
> + *
> + * Called under rq->lock, so direct field access is safe.
> + */
> +static bool yield_deboost_rate_limit(struct rq *rq)
> +{
> +	u64 now = rq_clock(rq);
> +	u64 last = rq->yield_deboost_last_time_ns;
> +
> +	if (last && (now - last) <= 6 * NSEC_PER_MSEC)
> +		return true;
> +
> +	rq->yield_deboost_last_time_ns = now;
> +	return false;
> +}
> +
> +/*
> + * Validate tasks for yield deboost operation.
> + * Returns the yielding task on success, NULL on validation failure.
> + *
> + * Checks: feature enabled, valid target, same runqueue, target is fair class,
> + * both on_rq. Called under rq->lock.
> + *
> + * Note: p_yielding (rq->donor) is guaranteed to be fair class by the caller
> + * (yield_to_task_fair is only called when curr->sched_class == p->sched_class).
> + */
> +static struct task_struct __maybe_unused *
> +yield_deboost_validate_tasks(struct rq *rq, struct task_struct *p_target)
> +{
> +	struct task_struct *p_yielding;
> +
> +	if (!sysctl_sched_vcpu_debooster_enabled)
> +		return NULL;
> +
> +	if (!p_target)
> +		return NULL;
> +
> +	if (yield_deboost_rate_limit(rq))
> +		return NULL;
> +
> +	p_yielding = rq->donor;
> +	if (!p_yielding || p_yielding == p_target)
> +		return NULL;
> +
> +	if (p_target->sched_class != &fair_sched_class)
> +		return NULL;
> +
> +	if (task_rq(p_target) != rq)
> +		return NULL;
> +
> +	if (!p_target->se.on_rq || !p_yielding->se.on_rq)
> +		return NULL;
> +
> +	return p_yielding;
> +}
> +
>  /*
>   * sched_yield() is very simple
>   */
> -- 
> 2.43.0