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SubjectRe: [PATCH v6 1/3] perf/core: use rb trees for pinned/flexible groups
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On 15.08.2017 20:28, Alexey Budankov wrote:
> Hi Peter,
>
> On 07.08.2017 10:17, Alexey Budankov wrote:
>> On 04.08.2017 17:36, Peter Zijlstra wrote:
>>> On Thu, Aug 03, 2017 at 11:30:09PM +0300, Alexey Budankov wrote:
>>>> On 03.08.2017 16:00, Peter Zijlstra wrote:
>>>>> On Wed, Aug 02, 2017 at 11:13:54AM +0300, Alexey Budankov wrote:
>>>
>>>>>> +/*
>>>>>> + * Find group list by a cpu key and rotate it.
>>>>>> + */
>>>>>> +static void
>>>>>> +perf_event_groups_rotate(struct rb_root *groups, int cpu)
>>>>>> +{
>>>>>> + struct rb_node *node;
>>>>>> + struct perf_event *node_event;
>>>>>> +
>>>>>> + node = groups->rb_node;
>>>>>> +
>>>>>> + while (node) {
>>>>>> + node_event = container_of(node,
>>>>>> + struct perf_event, group_node);
>>>>>> +
>>>>>> + if (cpu < node_event->cpu) {
>>>>>> + node = node->rb_left;
>>>>>> + } else if (cpu > node_event->cpu) {
>>>>>> + node = node->rb_right;
>>>>>> + } else {
>>>>>> + list_rotate_left(&node_event->group_list);
>>>>>> + break;
>>>>>> + }
>>>>>> + }
>>>>>> +}
>>>>>
>>>>> Ah, you worry about how to rotate inside a tree?
>>>>
>>>> Exactly.
>>>>
>>>>>
>>>>> You can do that by adding (run)time based ordering, and you'll end up
>>>>> with a runtime based scheduler.
>>>>
>>>> Do you mean replacing a CPU indexed rb_tree of lists with
>>>> an CPU indexed rb_tree of counter indexed rb_trees?
>>>
>>> No, single tree, just more complicated ordering rules.
>>>
>>>>> A trivial variant keeps a simple counter per tree that is incremented
>>>>> for each rotation. That should end up with the events ordered exactly
>>>>> like the list. And if you have that comparator like above, expressing
>>>>> that additional ordering becomes simple ;-)
>>>>>
>>>>> Something like:
>>>>>
>>>>> struct group {
>>>>> u64 vtime;
>>>>> rb_tree tree;
>>>>> };
>>>>>
>>>>> 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);
>>>>> }
>>>>>
>>>>> Then every time you use insert_group(.tail=1) it goes to the end of that
>>>>> CPU's 'list'.
>>>>>
>>>>
>>>> Could you elaborate more on how to implement rotation?
>>>
>>> Its almost all there, but let me write a complete replacement for your
>>> perf_event_group_rotate() above.
>>>
>>> /* find the leftmost event matching @cpu */
>>> /* XXX not sure how to best parametrise a subtree search, */
>>> /* again, C sucks... */
>>> struct perf_event *__group_find_cpu(group, cpu)
>>> {
>>> struct rb_node *node = group->tree.rb_node;
>>> struct perf_event *event, *match = NULL;
>>>
>>> while (node) {
>>> event = container_of(node, struct perf_event, group_node);
>>>
>>> if (cpu > event->cpu) {
>>> node = node->rb_right;
>>> } else if (cpu < event->cpu) {
>>> node = node->rb_left;
>>> } else {
>>> /*
>>> * subtree match, try left subtree for a
>>> * 'smaller' match.
>>> */
>>> match = event;
>>> node = node->rb_left;
>>> }
>>> }
>>>
>>> return match;
>>> }
>>>
>>> void perf_event_group_rotate(group, int cpu)
>>> {
>>> struct perf_event *event = __group_find_cpu(cpu);
>>>
>>> if (!event)
>>> return;
>>>
>>> tree_delete(&group->tree, event);
>>> insert_group(group, event, 1);
>>> }
>>>
>>> So we have a tree ordered by {cpu,vtime} and what we do is find the
>>> leftmost {cpu} entry, that is the smallest vtime entry for that cpu. We
>>> then take it out and re-insert it with a vtime number larger than any
>>> other, which places it as the rightmost entry for that cpu.
>>>
>>>
>>> So given:
>>>
>>> {1,1}
>>> / \
>>> {0,5} {1,2}
>>> / \ \
>>> {0,1} {0,6} {1,4}
>>>
>>>
>>> __group_find_cpu(.cpu=1) will return {1,1} as being the leftmost entry
>>> with cpu=1. We'll then remove it, update its vtime to 7 and re-insert.
>>> resulting in something like:
>>>
>>> {1,2}
>>> / \
>>> {0,5} {1,4}
>>> / \ \
>>> {0,1} {0,6} {1,7}
>>>
>>
>> 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.
>
> I implemented the approach you had suggested (as I understood it),
> tested it and got results that are drastically different from what
> I am getting for the tree of lists. Specifically I did:
>
> 1. keeping all groups in the same single tree by employing a 64-bit index
> additionally to CPU key;
>
> 2. implementing special _less() function and rotation by re-inserting
> group with incremented index;
>
> 3. replacing API with a callback in the signature by a macro
> perf_event_groups_for_each();
>
> Employing all that shrunk the total patch size, however I am still
> struggling with the correctness issues.
>
> 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
>
> and it is unclear how to iterate cpu==0 part of tree in this case.
>
> Iterating cpu specific subtree like this:
>
> #define for_each_group_event(event, group, cpu, pmu, field) \
> for (event = rb_entry_safe(group_first(group, cpu, pmu), \
> typeof(*event), field); \
> event && event->cpu == cpu && event->pmu == pmu; \
> event = rb_entry_safe(rb_next(&event->field), \
> typeof(*event), field))
>
> misses event==03 for the case above and I guess this is where I loose
> samples in my testing.

I eventually managed to overcome difficulties with implementation
of rb_tree indexed by {cpu,index} for event groups so please
see patches v9.

>
> Please advise how to proceed.
>
> Thanks,
> Alexey
>
>>
>>>
>>>
>>>
>>>
>>
>>
>
>

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