Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes

[Joel Fernandes <joel@joelfernandes.org>]

On 8/30/2022 12:22 PM, Frederic Weisbecker wrote:
> On Tue, Aug 30, 2022 at 06:03:16PM +0200, Frederic Weisbecker wrote:
>> On Tue, Aug 30, 2022 at 04:43:43AM -0700, Paul E. McKenney wrote:
>>> On Tue, Aug 30, 2022 at 12:53:24PM +0200, Frederic Weisbecker wrote:
>>>> On Mon, Aug 29, 2022 at 01:42:02PM -0700, Paul E. McKenney wrote:
>>>>> On Mon, Aug 29, 2022 at 04:36:40PM -0400, Joel Fernandes wrote:
>>>>>> On Mon, Aug 29, 2022 at 3:46 PM Frederic Weisbecker <frederic@kernel.org> wrote:
>>>>>>> On Mon, Aug 29, 2022 at 12:45:40PM -0400, Joel Fernandes wrote:
>>>>>>>> On 8/29/2022 9:40 AM, Frederic Weisbecker wrote:
>>>>>
>>>>> [ . .  . ]
>>>>>
>>>>>>>>> 2) NOCB implies performance issues.
>>>>>>>>
>>>>>>>> Which kinds of? There is slightly worse boot times, but I'm guessing that's do
>>>>>>>> with the extra scheduling overhead of the extra threads which is usually not a
>>>>>>>> problem except that RCU is used in the critical path of boot up (on ChromeOS).
>>>>>>>
>>>>>>> I never measured it myself but executing callbacks on another CPUs, with
>>>>>>> context switches and locking can only involve significant performance issues if callbacks
>>>>>>> are frequent. So it's a tradeoff between power and performance.
>>>>>>
>>>>>> In my testing of benchmarks on real systems with 8-16 CPUs, the
>>>>>> performance hit is down in the noise. It is possible though that maybe
>>>>>> one can write a non-realistic synthetic test to force the performance
>>>>>> issues, but I've not seen it in the real world. Maybe on
>>>>>> networking-heavy servers with lots of cores, you'll see it but their
>>>>>> batteries if any would be pretty big :-).
>>>>>
>>>>> To Frederic's point, if you have enough servers, even a 1% decrease in
>>>>> power consumption is a very big deal.  ;-)
>>>>
>>>> The world has enough servers, for that matters ;-)
>>>
>>> True enough!  Now you need only demonstrate that call_rcu_lazy() for
>>> !rcu_nocbs servers would actually deliver that 1%.  ;-)
>>
>> Well, !rcu_nocbs is not only used by server but also by pretty much
>> everything else, except android IIUC. I can't really measure the whole
>> world but I don't see how the idleness of a server/router/desktop/embedded/rt/hpc
>> device differs from the idleness of an android device.
>>
>> But ok I'll try to measure that.
> 
> Although who knows, may be some periodic file operation while idle are specific
> to Android. I'll try to trace lazy callbacks while idle and the number of grace
> periods associated.

One potential usecase could be logging if the logger is opening and closing log
files during logging updates. Uladzislau reported this on Android, that a system
logger was doing file open/close and triggering RCU that way (file table,
dentry, inode code all queue RCU callbacks).

Thanks,

 - Joel