Re: [RFC v2] timers: Don't wake ktimersoftd on every tick

[Haris Okanovic]

On 12/23/2016 11:28 AM, Sebastian Andrzej Siewior wrote:
> On 2016-12-13 15:44:05 [-0600], Haris Okanovic wrote:
> > Changed the way timers are collected per Julia and Thomas'
>
> I can only see Julia's response to the initial thread.

I should have been more clear. Thomas commented on irc and recommended 
Julia's approach.

> > recommendation: Expired timers are now collected in interrupt context
> > and fired in ktimersoftd to avoid double-walk of `pending_map`.
> >
> > This is implemented by storing lists of expired timers in timer_base,
> > which carries a memory overhead 9*sizeof(pointer) per CPU. The timer
> > system uses hlist's which don't have end-node references, making it
> > impossible to merge 2 hlist's in constant time. I.e. Merging requires
> > walking one list. I also considered switching `vectors` to regular
> > list's which don't have this limitations, but that approach has the same
> > memory overhead. list_head is bigger than hlist_head by sizeof(pointer)
> > and is instantiated 9+ times per CPU as `vectors`. I believe the only
> > way to trim overhead is to spend more CPU cycles in interrupt context
> > either in list merging (unbounded operation) or the original double-walk
> > implementation. Any suggestions/preferences?
> >
> > As before, a 6h run of cyclictest without CPU affinity shows decrease in
> > 22-70us latency range.
> what does this mean? Your cyclictest runs on a random CPU with one thread
> only?

Yes. My point is that cyclictest only shows a significant difference 
(before and after this change) when `-S` is not used.

> > No change in max jitter.
> Does this mean your average latency went down 20-70us and your max is
> the same?

Yes. Average latency (20-70us range) goes down in a single-threaded run 
of cyclictest. Max jitter stays the same in both single and multi-thread 
runs.

> > No change when `-S` is
> > used.
>
> -S gives you one thread per core, makes sure it stays on that core and
> uses clock_nanosleep().
>
> clock_nanosleep() should be used no matter what.
>
>
> > [Before/after traces]
> >
> > ftp://ftp.ni.com/outgoing/tp02-timer-peek-traces.tgz
> > (Email me if link dies. Server periodically purges old files.)
> >
> > [Hardware/software/config]
> >
> >  NI cRIO-9033
> >   2 core Intel Atom CPU
> >
> >  Kernel 4.8.6-rt5
> >   CONFIG_HZ_PERIODIC=y
> >
> > [Outstanding concerns/issues/questions]
> >
> > I'm relatively new to the timer subsystem, so please feel free to poke
> > as many holes as possible in this change. A few things that concern me
> > at the moment are:
> >
> > Can jiffies change while one or more cpus is inside tick_sched_timer(),
> >  in interrupt context? I'm copying jiffies to a local variable in
> >  find_expired_timers() to ensure it doesn't run unbounded, but I'm not
> >  sure if that's necessary.
>
> It could change. Only the house keeping does update jiffies in
> tick_sched_do_timer().
>
> > Any special considerations for testing NO_HZ builds? (Other than letting
> > it run idle for a while)
> >
> > timers_dead_cpu() presently asserts no timer callback is actively
> > running, which suggests that timers must be canceled prior to disabling
> > CPUs; otherwise, there's a race between active timers and hotplug
> > which can crash the whole kernel. Is this a safe assumption to make and
> > are there any special considerations for CPU hotplug testing?
>
> timers_dead_cpu() and hrtimers_dead_cpu() migrate timer away. At that
> point the CPU should be down already so a timer can't run on that CPU.
>
> > Other tests/performance benchmark I should run?
> >
> > Source: https://github.com/harisokanovic/linux/tree/dev/hokanovi/timer-peek-v2
> >
> > Thanks,
> > Haris
>
> Sebastian

-- Haris