linux-kernel - Re: sched: current instead rq->current

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Message-ID: <20160818112825.GB10153@twins.programming.kicks-ass.net>
Date:	Thu, 18 Aug 2016 13:28:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Colin Vidal <colin@...dal.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com
Subject: Re: sched: current instead rq->current

On Wed, Aug 17, 2016 at 09:39:41PM +0200, Colin Vidal wrote:
> On Mon, 2016-08-15 at 21:30 +0200, Colin Vidal wrote:
> > Hello,
> > 
> > At the beginning of __schedule (kernel/sched/core.c), the current
> > task
> > is get with rq->curr. I try to to understand why not directly using
> > current instead?

> > -       int cpu;
> >  
> > -       cpu = smp_processor_id();
> > -       rq = cpu_rq(cpu);
> > -       prev = rq->curr;
> > +       rq = cpu_rq(smp_processor_id());
> > +       prev = current;
> > 
> > and it seems to work (only tested on x86-64), but... To simple?

its more expensive, rq->curr is a simple dereference (and we need that
cacheline anyway), while current is a weird macro that expands to
potentially a lot of code, depending on the arch.