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

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Date:   Wed, 23 Aug 2017 17:18:54 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] perf/core: use rb trees for pinned/flexible groups

On 23.08.2017 16:39, Alexander Shishkin wrote:
> Alexey Budankov <alexey.budankov@...ux.intel.com> writes:
> 
>>>>>> bool event_less(left, right)
>>>>>> {
>>>>>>   if (left->cpu < right->cpu)
>>>>>>     return true;
>>>>>>
>>>>>>   if (left->cpu > right_cpu)
>>>>>>     return false;
>>>>>>
>>>>>>   if (left->vtime < right->vtime)
>>>>>>     return true;
>>>>>>
>>>>>>   return false;
>>>>>> }
>>>>>>
>>>>>> insert_group(group, event, tail)
>>>>>> {
>>>>>>   if (tail)
>>>>>>     event->vtime = ++group->vtime;
>>>>>>
>>>>>>   tree_insert(&group->root, event);
>>>>>> }
> 
> [ ... ]
> 
>> 2. implementing special _less() function and rotation by re-inserting
>>    group with incremented index;
>>
> 
> [ ... ]
> 
>> Now I figured that not all indexed events are always located under 
>> the root with the same cpu, and it depends on the order of insertion
>> e.g. with insertion order 01,02,03,14,15,16 we get this:
>>
>>      02
>>     /  \
>>    01  14
>>       /  \
>>      03  15
>>            \
>>            16
> 
> How did you arrive at this? Seeing the actual code would help, because
> this is not the ordering we're looking for. With Peter's earlier example
> (quoted above) it shouldn't look like this.

I implemented the solution Peter suggested. Then I was testing and noticed
considerable difference in amount of collected samples when multiplexing 
event, in comparison to the version with tree of lists. 

I then looked for a fast way to emulate the idea with virtual index as 
a secondary key and found this RB tree emulator:

https://www.cs.usfca.edu/~galles/visualization/RedBlack.html

and it showed me the picture I mentioned above:

      02
     /  \
    01  14
       /  \
      03  15
            \
            16

I understand it is not 100% proof that index idea doesn't work 
however it means that in order to apply the idea to this patch 
some more changes are required additionally to what Peter 
shared earlier.

> 
> Regards,
> --
> Alex
> 
> 
>