linux-kernel - Re: [patch 2/3] timers: do not raise softirq unconditionally (spinlockless version)

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Date:   Thu, 6 Jun 2019 12:14:35 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Haris Okanovic <haris.okanovic@...com>
Subject: Re: [patch 2/3] timers: do not raise softirq unconditionally
 (spinlockless version)

On Tue, Jun 04, 2019 at 02:29:31PM +0800, Peter Xu wrote:
> On Mon, Apr 15, 2019 at 05:12:15PM -0300, Marcelo Tosatti wrote:
> > Check base->pending_map locklessly and skip raising timer softirq 
> > if empty.
> > 
> > What allows the lockless (and potentially racy against mod_timer) 
> > check is that mod_timer will raise another timer softirq after
> > modifying base->pending_map.
> > 
> > Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> > 
> > ---
> >  kernel/time/timer.c |   18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> > 
> > Index: linux-rt-devel/kernel/time/timer.c
> > ===================================================================
> > --- linux-rt-devel.orig/kernel/time/timer.c	2019-04-15 14:21:02.788704354 -0300
> > +++ linux-rt-devel/kernel/time/timer.c	2019-04-15 14:22:56.755047354 -0300
> > @@ -1776,6 +1776,24 @@
> >  		if (time_before(jiffies, base->clk))
> >  			return;
> >  	}
> > +
> > +#ifdef CONFIG_PREEMPT_RT_FULL
> > +/* On RT, irq work runs from softirq */
> > +	if (irq_work_needs_cpu())
> > +		goto raise;
> > +#endif
> > +	base = this_cpu_ptr(&timer_bases[BASE_STD]);
> > +	if (!housekeeping_cpu(base->cpu, HK_FLAG_TIMER)) {
> > +		if (!bitmap_empty(base->pending_map, WHEEL_SIZE))
> > +			goto raise;
> > +		base++;
> 
> Shall we check against CONFIG_NO_HZ_COMMON?  Otherwise the base could
> point to something else rather tha the deferred base (NR_BASES==1 if
> without nohz-common).
> 
> I see that run_local_timers() has similar pattern, actually I'm
> thinking whether we can put things like "base++" to be inside some
> "if"s of CONFIG_NO_HZ_COMMON to be clear.
> 
> Thanks,
> 
> -- 
> Peter Xu

Hi Peter,

Yes, that is a good point and needs fixing. 

I'll wait for Anna's comments before posting -v2.