linux-kernel - Re: [PATCH tip/core/timers 1/3] timers: Reduce __run

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Message-ID: <20140115162531.GA11499@redhat.com>
Date:	Wed, 15 Jan 2014 17:25:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu
Subject: Re: [PATCH tip/core/timers 1/3] timers: Reduce __run_timers()
	latency for empty list

On 01/14, Paul E. McKenney wrote:
>
> On Tue, Jan 14, 2014 at 07:48:28PM +0100, Oleg Nesterov wrote:
> > >  __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
> > >  {
> > > @@ -1146,6 +1157,10 @@ static inline void __run_timers(struct tvec_base *base)
> > >  	struct timer_list *timer;
> > >
> > >  	spin_lock_irq(&base->lock);
> >
> > Do we really need to take base->lock before catchup_timer_jiffies() ?
> > ->timer_jiffies can only be changed by us, and it seems that we do
> > not care if we race with base->active_timers++.
>
> Given that this lock should be almost always acquired by the current
> CPU, the penalty for acquiring it should be low.  After all, we were
> acquiring it prior to this patch as many times as we are after this patch,
> right?

Yes. But

	if (catchup_timer_jiffies())
		return;

looks a bit simpler and can save a couple of insn. I won't argue of course,
this is minor. And you already sent v2, I'll try add some comments...

Oleg.

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