linux-kernel - Re: [PATCH]: perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

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Date:   Thu, 15 Jun 2017 20:44:32 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:     Andi Kleen <ak@...ux.intel.com>, Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        Stephane Eranian <eranian@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH]: perf/core: addressing 4x slowdown during per-process
 profiling of STREAM benchmark on Intel Xeon Phi

On 14.06.2017 13:07, Alexey Budankov wrote:
> On 30.05.2017 11:29, Alexander Shishkin wrote:
>> Alexey Budankov <alexey.budankov@...ux.intel.com> writes:
>>
>>> On 29.05.2017 15:03, Alexander Shishkin wrote:
>>>> Alexey Budankov <alexey.budankov@...ux.intel.com> writes:
>>>>
>>>> Here (above the function) you could include a comment describing what
>>>> happens when this is called, locking considerations, etc.
>>>
>>> I can put the short description from the initial thread message here.
>>> Would it be sufficient?
>>
>> Sure, this is where API descriptions fit better than in commit messages.

addressed in v3.

>>
>>>
>>>>
>>>>> +static int
>>>>> +perf_cpu_tree_insert(struct rb_root *tree, struct perf_event *event)
>>>>> +{
>>>>> +    struct rb_node **node;
>>>>> +    struct rb_node *parent;
>>>>> +
>>>>> +    if (!tree || !event)
>>>>> +        return 0;
>>>>
>>>> I don't think this should be happening, should it? And either way you
>>>> probably don't want to return 0 here, unless you're using !0 for
>>>> success.
>>>
>>> As you might notice already, currently return codes of the tree API are
>>> not checked all other the implementation. I suggest replacing that int
>>> error code by void and simplify the stuff.
>>
>> Your call. But I'd still either drop the redundant checks or wrap them
>> in WARN_ON_ONCE().
> 
> Ok. WARN_ON_ONCE() then.

addressed in v3.

> 
>>
>>>
>>>>
>>>>> +
>>>>> +    node = &tree->rb_node;
>>>>> +    parent = *node;
>>>>> +
>>>>> +    while (*node) {
>>>>> +        struct perf_event *node_event =    container_of(*node,
>>>>> +                struct perf_event, group_node);
>>>>> +
>>>>> +        parent = *node;
>>>>> +
>>>>> +        if (event->cpu < node_event->cpu) {
>>>>> +            node = &((*node)->rb_left);
>>>>
>>>> this would be the same as node = &parent->rb_left, right?
> 
> Yes, that is right.

addressed in v3.

> 
>>>
>>> Please ask more.
>>
>> Side note: between commit message, comments and the actual code, in an
>> ideal situation one doesn't have to 'ask' anything, because everything
>> is already clear. Not the case here.
>>
>>> node is the leaf node and parent is the parent of the
>>> node at the end of cycle. We need the both to insert a new node into a
>>> tree.
>>
>> Not sure I understand. You'd still have both.
>>
>>>
>>>>
>>>>> +        } else if (event->cpu > node_event->cpu) {
>>>>> +            node = &((*node)->rb_right);
>>>>> +        } else {
>>>>> +            list_add_tail(&event->group_list_entry,
>>>>> +                    &node_event->group_list);
>>>>
>>>> So why is this better than simply having per-cpu event lists plus one
>>>> for per-thread events?
>>>
>>> Good question. Choice of data structure and layout depends on the
>>> operations applied to the data so keeping groups as a tree simplifies
>>> and improves the implementation in terms of scalability and performance.
>>> Please ask more if any.
>>
>> Please be more specific on how scalability and performance are
>> improved. In general, try to avoid vagues statements like "this is
>> better for performance".
> 
> Accepted. Peter already provided more specifics on this. Thanks.
> 
>>
>> Thanks,
>> -- 
>> Alex
>>
>