linux-kernel - Re: [RESEND PATCH v3 1/n] perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

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Date:   Tue, 20 Jun 2017 18:12:08 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        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>,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND PATCH v3 1/n] perf/core: addressing 4x slowdown during
 per-process profiling of STREAM benchmark on Intel Xeon Phi

On 20.06.2017 16:44, Mark Rutland wrote:
> On Fri, Jun 16, 2017 at 02:03:58AM +0300, Alexey Budankov wrote:
>> perf/core: use rb trees for pinned/flexible groups
>>
>> By default, the userspace perf tool opens per-cpu task-bound events
>> when sampling, so for N logical events requested by the user, the tool
>> will open N * NR_CPUS events.
>>
>> In the kernel, we mux events with a hrtimer, periodically rotating the
>> flexible group list and trying to schedule each group in turn. We
>> skip groups whose cpu filter doesn't match. So when we get unlucky,
>> we can walk N * (NR_CPUS - 1) groups pointlessly for each hrtimer
>> invocation.
>>
>> This has been observed to result in significant overhead when running
>> the STREAM benchmark on 272 core Xeon Phi systems.
>>
>> One way to avoid this is to place our events into an rb tree sorted by
>> CPU filter, so that our hrtimer can skip to the current CPU's
>> list and ignore everything else.
>>
>> As a step towards that, this patch replaces event group lists with rb
>> trees.
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com>
>> ---
>>   include/linux/perf_event.h |  18 ++-
>>   kernel/events/core.c       | 393
>> +++++++++++++++++++++++++++++++++------------
>>   2 files changed, 307 insertions(+), 104 deletions(-)
>>
>> Addressed Mark Rutland's comments from the previous patch version.
> 
> ... then this should be v4, no?
> 
> Which comments? Could you pelase write a changelog in future?

Changes are:

1. changed type of pinned_groups/flexible_groups to rb_tree;
2. removed group_list_entry and reused group_entry for that purposes;
3. added add_to_groups()/del_from_groups() helper functions;

> 
> In future, please send patches as a series, with a known upper-bound
> rather than N. It's really painful to find them when they're sent
> separately, without a known upper bound.

Accepted.

> 
> [...]
> 
>> +/*
>> + * Delete a group from a tree. If the group is directly attached to
>> the tree
>> + * it also detaches all groups on the group's group_list list.
>> + */
>> +static void
>> +perf_cpu_tree_delete(struct rb_root *tree, struct perf_event *event)
>> +{
>> +	WARN_ON_ONCE(!tree || !event);
>> +
>> +	if (RB_EMPTY_NODE(&event->group_node)) {
>> +		list_del_init(&event->group_entry);
>> +	} else {
>> +		struct perf_event *list_event, *list_next;
>> +
>> +		rb_erase(&event->group_node, tree);
>> +		list_for_each_entry_safe(list_event, list_next,
>> +				&event->group_list, group_entry)
>> +			list_del_init(&list_event->group_entry);
>> +	}
>> +}
> 
> As I commented on the last version, this means that all groups which
> were (incidentally) hanging off of this one are removed, and can
> no longer be reached via the tree.
> 
> Surely one of the remaining groups should be added to the tree?

Aww, I see. That needs to implemented. Thanks.

> 
> I don't beleive that is correct.
> 
> I beleive it would be simpler to reason about a threaded rb-tree here,
> since that special case wouldn't exist.
> 
> Thanks,
> Mark.
>