linux-kernel - Re: [PATCH 2/5] sched_ext: Manage the validity of scx_rq

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Message-ID: <20241116193235.GQ22801@noisy.programming.kicks-ass.net>
Date: Sat, 16 Nov 2024 20:32:35 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Changwoo Min <multics69@...il.com>
Cc: tj@...nel.org, void@...ifault.com, mingo@...hat.com,
	changwoo@...lia.com, kernel-dev@...lia.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] sched_ext: Manage the validity of scx_rq_clock

On Sun, Nov 17, 2024 at 01:01:23AM +0900, Changwoo Min wrote:
> An rq clock becomes valid when it is updated using update_rq_clock()
> and invalidated when the rq is unlocked using rq_unpin_lock(). Also,
> after long running operations -- ops.running() and ops.update_idle() --
> in a BPF scheduler, the sched_ext core invalidates the rq clock.
> 
> Signed-off-by: Changwoo Min <changwoo@...lia.com>
> ---
>  kernel/sched/core.c  | 6 +++++-
>  kernel/sched/ext.c   | 3 +++
>  kernel/sched/sched.h | 2 +-
>  3 files changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a910a5b4c274..d0eb58b6a2da 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -784,6 +784,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>  void update_rq_clock(struct rq *rq)
>  {
>  	s64 delta;
> +	u64 clock;
>  
>  	lockdep_assert_rq_held(rq);
>  
> @@ -795,11 +796,14 @@ void update_rq_clock(struct rq *rq)
>  		SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED);
>  	rq->clock_update_flags |= RQCF_UPDATED;
>  #endif
> +	clock = sched_clock_cpu(cpu_of(rq));
> +	scx_rq_clock_update(rq, clock);

It is not at all clear why you think you need to keep a second copy of
that value. You like cache misses?