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
