Re: [PATCH v6 1/3] perf/core: use rb trees for pinned/flexible groups

[Alexey Budankov]

On 07.08.2017 18:55, Peter Zijlstra wrote:
> On Mon, Aug 07, 2017 at 06:32:16PM +0300, Alexey Budankov wrote:
>> On 07.08.2017 12:13, Peter Zijlstra wrote:
>>> On Mon, Aug 07, 2017 at 10:39:13AM +0200, Peter Zijlstra wrote:
>>>> On Mon, Aug 07, 2017 at 10:17:46AM +0300, Alexey Budankov wrote:
>>>>> Makes sense. The implementation becomes a bit simpler. The drawbacks 
>>>>> may be several rotations of potentially big tree on the critical path, 
>>>>> instead of updating four pointers in case of the tree of lists.
>>>>
>>>> Yes, but like said, it allows implementing a better scheduler than RR,
>>>> allowing us to fix rotation artifacts where task runtimes are near the
>>>> rotation window.
>>
>> Could you elaborate more on the artifacts or my be share some link to the theory?
> 
> In the extreme, if you construct your program such that you'll never get
> hit by the tick (this used to be a popular measure to hide yourself from
> time accounting)

Well, some weird thing for me. Never run longer than one tick? 
I could imaging some I/O bound code that would fast serve some short 
messages, all the other time waiting for incoming requests.
Not sure if CPU events monitoring is helpful in this case.

> , you'll never rotate the counters, even though you can
> rack up quite a lot of runtime.> 
> By doing a runtime based scheduler, instead of a tick based RR, we'll
> still get rotation, and the tick will only function as a forced
> reprogram point.
> 
>>>> A slightly more complicated, but also interested scheduling problem is
>>>> the per-cpu flexible vs the per-task flexible. Ideally we'd rotate them
>>>> at the same priority based on service, without strictly prioritizing the
>>>> per-cpu events.
>>>>
>>>> Again, that is something that should be possible once we have a more
>>>> capable event scheduler.
>>>>
>>>>
>>>> So yes, cons and pros.. :-)
>>>
>>> Also, I think for AVL tree you could do the erase and (re)insert
>>> combined and then rebalance in one go, not sure RB allows the same
>>> thing, but it might be fun looking into.
>>
>> Not sure if AVL is more practical here. You get better balancing what gives 
>> you faster average search for the price of longer modifications 
>> so yes, need to measure and compare ... :-)
> 
> Oh, I wasn't suggesting using AVL (the last thing we need is another
> balanced tree in the kernel), I was merely wondering if you could do
> compound/bulk updates on RB as you can with AVL.

Aww, I see.

>