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 17:38:50 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.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 Tue, Jun 20, 2017 at 06:12:08PM +0300, Alexey Budankov wrote:
> 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;

Thanks; the new changelog is much appreciated.

Mark.