linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Message-Id: <20180706211224.GO3593@linux.vnet.ibm.com>
Date:   Fri, 6 Jul 2018 14:12:24 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Fri, Jul 06, 2018 at 06:14:44PM +0100, David Woodhouse wrote:
> On Fri, 2018-07-06 at 10:11 -0700, Paul E. McKenney wrote:
> > > The preempt state is alread a bit complicated and shadowed in the
> > > preempt_count (on some architectures) adding additional bits to it like
> > > this is just asking for trouble.
> > 
> > How about a separate need_resched_rcu() that includes the extra cache
> > miss?  Or open-coding the rcu_urgent_qs_requested()?
> 
> Peter said "touch two cachelines". He didn't say it was a cache miss.

"... that includes the extra cache touch", then.

> Given that every single cond_resched() call touches the same cache
> line, and every single rcu_all_qs() and similar will also touch it,
> it's fairly much guaranteed *not* to be a miss...
> 
> ... which is why I didn't really understand why he cared.

Let's see what he says.  ;-)

							Thanx, Paul