linux-kernel - Re: [PATCH] fs/dcache: resched/chill only if we make no progress

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Date:   Wed, 14 Sep 2016 19:14:16 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] fs/dcache: resched/chill only if we make no progress

On 2016-09-10 08:17:13 [+0200], Thomas Gleixner wrote:
> On Thu, 8 Sep 2016, Sebastian Andrzej Siewior wrote:
> > +		if (parent == dentry) {
> > +			/* the task with the highest priority won't schedule */
> > +			r = cond_resched();
> > +			if (!r && (rt_task(current) || dl_task(current)))
> 
> That's wrong because a non RT task will just spin until its time slice is
> up. Wasted cycles for nothing.
> 
> Ideally we make spinning conditional on 
> 
>     !on_cpu(dentry->lock->owner)
> 
> That would avoid going to sleep if the owner runs on another cpu. We have
> no mechanism for that, but I think it'd be worth to think about one.

something like

|#ifdef CONFIG_SMP
|                             task_running(task_rq(lock_owner), lock_owner))
|#else
|                             1)
|#endif
|                                 cpu_chill();

the lock_owner is tricky because we have two locks: inode->i_lock and
parent->d_lock. So we would have to do the same lookup, like
dentry_kill() is doing, and check both in worth case.

Sebastian