Re: [RFC 0/6] optimize ctx switch with rb-tree

[David Carrillo-Cisneros]

Hi Kan,

It's still on my list, but I won't have time to work on it for at
least another month.

What issues did you encounter?

Thanks,
David

On Tue, Apr 25, 2017 at 10:27 AM, Liang, Kan <kan.liang@xxxxxxxxx> wrote:
> Hi David,
>
> Is there any update about the patch series?
>
> We recently encountered another performance issue on KNL. I think the RB-tree solution also has benefits for it.
>
> Thanks,
> Kan
>
>> Subject: [RFC 0/6] optimize ctx switch with rb-tree
>>
>> Following the discussion in:
>> https://patchwork.kernel.org/patch/9420035/
>>
>> This is is an early version of a series of perf context switches optimizations.
>>
>> The main idea is to create and maintain a list of inactive events sorted by
>> timestamp, and a rb-tree index to index it. The rb-tree's key are
>> {cpu,flexible,stamp} for task contexts and {cgroup,flexible,stamp} for CPU
>> contexts.
>>
>> The rb-tree provides functions to find intervals in the inactive event list so
>> that ctx_sched_in only has to visit the events that can be potentially be
>> scheduled (i.e. avoid iterations over events bound to CPUs or cgroups that
>> are not current).
>>
>> Since the inactive list is sort by timestamp, rotation can be done by simply
>> scheduling out and in the events. This implies that each timer interrupt, the
>> events will rotate by q events (where q is the number of hardware counters).
>> This changes the current behavior of rotation.
>> Feedback welcome!
>>
>> I haven't profiled the new approach. I am only assuming it will be superior
>> when the number of per-cpu or distict cgroup events is large.
>>
>> The last patch shows how perf_iterate_ctx can use the new rb-tree index to
>> reduce the number of visited events. I haven't looked carefully if locking and
>> other things are correct.
>>
>> If this changes are in the right direction. A next version could remove some
>> existing code, specifically the lists ctx->pinned_groups and
>> ctx->flexible_groups could be removed. Also, event_filter_match could be
>> simplified when called on events groups filtered using the rb-tree, since both
>> perform similar checks.
>>
>> David Carrillo-Cisneros (6):
>>   perf/core: create active and inactive event groups
>>   perf/core: add a rb-tree index to inactive_groups
>>   perf/core: use rb-tree to sched in event groups
>>   perf/core: avoid rb-tree traversal when no inactive events
>>   perf/core: rotation no longer neccesary. Behavior has changed. Beware
>>   perf/core: use rb-tree index to optimize filtered  perf_iterate_ctx
>>
>>  include/linux/perf_event.h |  13 ++
>>  kernel/events/core.c       | 466
>> +++++++++++++++++++++++++++++++++++++++------
>>  2 files changed, 426 insertions(+), 53 deletions(-)
>>
>> --
>> 2.11.0.390.gc69c2f50cf-goog
>