Re: [PATCH] timers: Fix up get_target_base() to use old base properly

[Peter Xu <peterx@redhat.com>]

On Mon, Jun 17, 2019 at 02:07:48PM +0200, Thomas Gleixner wrote:
> Peter,
> 
> On Mon, 17 Jun 2019, Peter Xu wrote:
> > On Mon, Jun 17, 2019 at 08:09:20AM +0200, Thomas Gleixner wrote:
> > > You might argue that in case of an explicit pinned timer, the above logic
> > > is wrong when the timer is modified as it might move to a different
> > > CPU. But from day one when the pinned logic was introduced, pinned just
> > > prevents it from being queued on a remote CPU. If you need a timer to stay
> > > on a particular CPU even if modified from a remote CPU, then the only way
> > > right now is to dequeue and requeue it with add_timer_on(). 
> > 
> > Indeed.  If add_timer_on() should always be used when with pinned
> > timers, IMHO it would be good to comment probably above TIMER_PINNED
> > about the fact so people will never misuse the interfaces (it seems to
> > be mis-used somehow but I cannot be 100% sure, please see below).
> 
> Yeah, some documentation would be good.
> 
> > > If we really want to change that, then we need to audit all usage sites of
> > > pinned timers and figure out whether this would break anything.
> > > 
> > > The proper change would be in that case:
> > > 
> > >       return pinned ? base : get_timer_this_cpu_base(tflags);
> > 
> > Purely for curiousity - why would we like to use current cpu base even
> > if it's unpinned?  My humble opinion is that if we use base directly
> > at least we can avoid potential migration of the timer.  But I can be
> > missing some real reason here...
> 
> In most cases it's desired to move the timer over. Assume you have a
> network interrupt moving from one cpu to the other and then the tcp timers
> would stay on the old cpu forever. So you'd pay the remote access price
> every time you touch it and if it fires the callback is pretty much
> guaranteed to be cache cold.

I see.

> 
> > Though, I see two outliers:
> > 
> > ======================
> > 
> > *** drivers/cpufreq/powernv-cpufreq.c:
> > powernv_cpufreq_cpu_init[867]  TIMER_PINNED | TIMER_DEFERRABLE);
> > 
> > *** net/ipv4/inet_timewait_sock.c:
> > inet_twsk_alloc[189]           timer_setup(&tw->tw_timer, tw_timer_handler, TIMER_PINNED);
> 
> That's fine. It just wants to make sure that the timer is not queued on a
> remote CPU if NOHZ is active. That gives them a serialization guarantee of
> the network softirq vs. the timer softirq so they can spare some locking
> stuff.

Thanks for the analysis.  Instead of this patch, let me post a
documentation update for pinned timers.

Thanks,

-- 
Peter Xu