Re: [PATCH v4 2/6] sched_ext: Implement scx_rq_clock_update/stale()

[Andrea Righi <arighi@nvidia.com>]

On Mon, Dec 09, 2024 at 03:15:27PM +0900, Changwoo Min wrote:
> scx_rq_clock_update() and scx_rq_clock_stale() manage the status of an
> rq clock when sched_ext is enabled. scx_rq_clock_update() keeps the rq
> clock in memory and its status valid. scx_rq_clock_stale() invalidates
> the current rq clock not to use the cached rq clock.
> 
> Signed-off-by: Changwoo Min <changwoo@igalia.com>
> ---
>  kernel/sched/sched.h | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 440ecedf871b..7e71d8685fcc 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -754,6 +754,7 @@ enum scx_rq_flags {
>  	SCX_RQ_BAL_PENDING	= 1 << 2, /* balance hasn't run yet */
>  	SCX_RQ_BAL_KEEP		= 1 << 3, /* balance decided to keep current */
>  	SCX_RQ_BYPASSING	= 1 << 4,
> +	SCX_RQ_CLK_VALID	= 1 << 5, /* RQ clock is fresh and valid */
>  
>  	SCX_RQ_IN_WAKEUP	= 1 << 16,
>  	SCX_RQ_IN_BALANCE	= 1 << 17,
> @@ -766,9 +767,11 @@ struct scx_rq {
>  	unsigned long		ops_qseq;
>  	u64			extra_enq_flags;	/* see move_task_to_local_dsq() */
>  	u32			nr_running;
> -	u32			flags;
>  	u32			cpuperf_target;		/* [0, SCHED_CAPACITY_SCALE] */
>  	bool			cpu_released;
> +	u32			flags;
> +	u64			clock;			/* current per-rq clock -- see scx_bpf_now_ns() */
> +	u64			prev_clock;		/* previous per-rq clock -- see scx_bpf_now_ns() */

Since we're reordering this struct, we may want to move cpu_released all
the way to the bottom to get rid of the 3-bytes hole (and still have
flags, clock and prev_clock in the same cacheline).

>  	cpumask_var_t		cpus_to_kick;
>  	cpumask_var_t		cpus_to_kick_if_idle;
>  	cpumask_var_t		cpus_to_preempt;
> @@ -1725,9 +1728,28 @@ DECLARE_STATIC_KEY_FALSE(__scx_switched_all);	/* all fair class tasks on SCX */
>  
>  #define scx_enabled()		static_branch_unlikely(&__scx_ops_enabled)
>  #define scx_switched_all()	static_branch_unlikely(&__scx_switched_all)
> +
> +static inline void scx_rq_clock_update(struct rq *rq, u64 clock)
> +{
> +	if (scx_enabled()) {
> +		rq->scx.prev_clock = rq->scx.clock;
> +		rq->scx.clock = clock;
> +		rq->scx.flags |= SCX_RQ_CLK_VALID;
> +	}
> +}

Nit, this is just personal preference (feel free to ignore it):

	if (!scx_enabled())
		return;
	rq->scx.prev_clock = rq->scx.clock;
	rq->scx.clock = clock;
	rq->scx.flags |= SCX_RQ_CLK_VALID;

> +
> +static inline void scx_rq_clock_stale(struct rq *rq)
> +{
> +	if (scx_enabled())
> +		rq->scx.flags &= ~SCX_RQ_CLK_VALID;
> +}

I'm wondering if we need to invalidate the clock on all rqs when we call
scx_ops_enable() to prevent getting stale information from a previous
scx scheduler.

Probably it's not an issue, since scx_ops_disable_workfn() should make
sure that all tasks are going through rq_unpin_lock() before unloading
the current scheduler, maybe it could be helpful to add comment about
this scenario in scx_bpf_now_ns() (PATCH 4/6)?

> +
>  #else /* !CONFIG_SCHED_CLASS_EXT */
>  #define scx_enabled()		false
>  #define scx_switched_all()	false
> +
> +static inline void scx_rq_clock_update(struct rq *rq, u64 clock) {}
> +static inline void scx_rq_clock_stale(struct rq *rq) {}
>  #endif /* !CONFIG_SCHED_CLASS_EXT */
>  
>  /*
> -- 
> 2.47.1
> 

Thanks,
-Andrea