linux-kernel - Re: [PATCH] rcutorture: add random preemption

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Date:	Mon, 21 Jun 2010 19:06:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Lai Jiangshan <laijs@...fujitsu.com>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcutorture: add random preemption

On Mon, 2010-06-21 at 10:05 -0700, Paul E. McKenney wrote:
> On Mon, Jun 21, 2010 at 06:50:11PM +0200, Peter Zijlstra wrote:
> > On Mon, 2010-06-21 at 09:43 -0700, Paul E. McKenney wrote:
> > > > +#ifdef CONFIG_PREEMPT
> > > > +     if (!preempt_count() && !(rcu_random(rrsp) % (nrealreaders * 20000)))
> > > > +             preempt_schedule();
> > > > +#endif
> > > 
> > > This one scared me for a bit -- then I realized that preempt_schedule()
> > > won't actually schedule if preemption is in any way disabled.  So the
> > > above really is OK, because Classic RCU and RCU-bh disable preemption.
> > > 
> > > So, should we have a comment to this effect, or is my hypersensitivity to
> > > RCU semantics unique to me? 
> > 
> > Well it seems to do a !preempt_count() test too, so I wouldn't worry too
> > much about it, still using preempt_schedule() doesn't seem right, why
> > not use cond_resched()?
> 
> My guess is that Lai wants to force a call to rcu_sched_qs() even if
> !need_resched(). 

Ah, d'0h yeah, I'm not having the best of days.. monday or something :-)
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