linux-kernel - Re: [PATCH 1/2] sched_ext: Track currently locked rq

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Message-ID: <aAc2vdLzhR2BgL_q@gpd3>
Date: Tue, 22 Apr 2025 08:27:09 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched_ext: Track currently locked rq

On Mon, Apr 21, 2025 at 09:03:08AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Sun, Apr 20, 2025 at 09:30:21PM +0200, Andrea Righi wrote:
> ...
> > +static inline struct rq *scx_locked_rq(void)
> > +{
> > +	return __this_cpu_read(locked_rq);
> > +}
> > +
> > +#define SCX_CALL_OP(mask, rq, op, args...)					\
> >  do {										\
> > +	update_locked_rq(rq);							\
> 
> Minor but why not
> 
>         if (rq)
>                 update_locked_rq(rq);
> 
> here too to be symmetric?
> 
> >  	if (mask) {								\
> >  		scx_kf_allow(mask);						\
> >  		scx_ops.op(args);						\
> > @@ -1125,11 +1155,15 @@ do {										\
> >  	} else {								\
> >  		scx_ops.op(args);						\
> >  	}									\
> > +	if (rq)									\
> > +		update_locked_rq(NULL);						\
> 
> Or alternatively, drop `if (rq)` from both places. That's simpler and given
> that all the hot paths are called with rq locked, that may be *minutely*
> faster.

Ack, let's not complicate the code unnecessarily, that's a negligible
optimization (I tried to remove that `if (rq)` and I don't see any
measurable difference, as expected).

> 
> > @@ -2174,7 +2210,7 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
> >  	WARN_ON_ONCE(*ddsp_taskp);
> >  	*ddsp_taskp = p;
> >  
> > -	SCX_CALL_OP_TASK(SCX_KF_ENQUEUE, enqueue, p, enq_flags);
> > +	SCX_CALL_OP_TASK(SCX_KF_ENQUEUE, rq, enqueue, p, enq_flags);
> 
> Let's do SCX_CALL_OP_TASK(SCX_FK_ENQUEUE, enqueue, rq, p, enq_flags) so that
> the static parts of the invocation are grouped together and we usually have
> @rq and @p next to each other when they're used as parameters.

Ack. Will change this as well and send a v3.

Thanks,
-Andrea