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

>> 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]
>> (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
>> [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:
>> Thanks,
>> Haris
> Sebastian

-- Haris

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