linux-kernel - Re: [PATCH] RFC: sched: Rework task_sched_runtime to avoid calling update_rq

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Date: Tue, 18 Jun 2024 09:04:12 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	Joel Fernandes <joel@...lfernandes.org>, kernel-team@...roid.com
Subject: Re: [PATCH] RFC: sched: Rework task_sched_runtime to avoid calling
 update_rq_clock

On Sun, Jun 16, 2024 at 11:36:16PM +0100, Qais Yousef wrote:

> > Which then gets me something like the (completely untested) below..
> > 
> > Hmm?
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 0935f9d4bb7b..36aed99d6a6c 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -724,7 +724,6 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
> >  
> >  	rq->prev_irq_time += irq_delta;
> >  	delta -= irq_delta;
> > -	psi_account_irqtime(rq->curr, irq_delta);
> >  	delayacct_irq(rq->curr, irq_delta);
> >  #endif
> >  #ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> > @@ -5459,6 +5458,8 @@ void sched_tick(void)
> >  
> >  	sched_clock_tick();
> >  
> > +	psi_account_irqtime(curr, NULL, &rq->psi_irq_time);
> > +
> >  	rq_lock(rq, &rf);
> >  
> >  	update_rq_clock(rq);
> > @@ -6521,6 +6524,7 @@ static void __sched notrace __schedule(unsigned int sched_mode)
> >  		++*switch_count;
> >  
> >  		migrate_disable_switch(rq, prev);
> > +		psi_account_irqtime(prev, next, &rq->psi_irq_time);
> 
> Hmm are prev and next swapped here? next == curr in my view if there's no
> subtly I missed

This is before context_switch() so prev == current at this point.
However, more importantly, the PSI thing accounts to its 'curr' group
and that should very much be the outgoing task's group in this case.

That is, we need to make sure the outgoing group is up-to-date before
switching to a new group.

Makes sense?

> >  		psi_sched_switch(prev, next, !task_on_rq_queued(prev));
> >  
> >  		trace_sched_switch(sched_mode & SM_MASK_PREEMPT, prev, next, prev_state);
> > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> > index 146baa91d104..65bba162408f 100644
> > --- a/kernel/sched/psi.c
> > +++ b/kernel/sched/psi.c
> > @@ -991,22 +991,31 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
> >  }
> >  
> >  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
> > -void psi_account_irqtime(struct task_struct *task, u32 delta)
> > +void psi_account_irqtime(struct task_struct *curr, struct task_struct *prev, u64 *time)
> >  {
> > -	int cpu = task_cpu(task);
> > +	int cpu = task_cpu(curr);
> >  	struct psi_group *group;
> >  	struct psi_group_cpu *groupc;
> > -	u64 now;
> > +	u64 now, irq;
> > +	s64 delta;
> >  
> >  	if (static_branch_likely(&psi_disabled))
> >  		return;
> >  
> > -	if (!task->pid)
> > +	if (!curr->pid)
> > +		return;
> > +
> > +	group = task_psi_group(curr);
> > +	if( prev && task_psi_group(prev) == group)
> 
> nit: whitespace misplaced

Ha!, is that's all and it all works in one go it's awesome :-)

I'm still trying to learn to type again after switching keyboards. I've
used a thinkpad keyboard (either on an actual laptop or the travel
version on my desktop for nearly 20 years... Now I've picked up a split
keyboard out of necessity (UHK 60 v2 for those interested) and muscle
memory is still cursing me every single day.

As a result, I now also cannot type on my laptop anymore, so lose-lose I
suppose ... urgh.

> LGTM otherwise.
> 
> Reviewed-by: Qais Yousef <qyousef@...alina.io>

Thanks!

John, can you write up a changelog with some pretty numbers and all
that? Also, when you re-post, can you make sure to Cc the PSI folks
(johannes and suren iirc, get_maintainers.pl seems to find them).