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

Marcelo Tosatti
2019-06-06T15:15:27Z

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@redhat.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.