linux-kernel - Re: [RFC][PATCH] perf: Rewrite core context handling

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Date:   Mon, 15 Oct 2018 10:26:06 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        songliubraving@...com, eranian@...gle.com, tglx@...utronix.de,
        mark.rutland@....com, megha.dey@...el.com, frederic@...nel.org
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling

Hi,

On 10.10.2018 13:45, Peter Zijlstra wrote:
> Hi all,
> 
> There have been various issues and limitations with the way perf uses
> (task) contexts to track events. Most notable is the single hardware PMU
> task context, which has resulted in a number of yucky things (both
> proposed and merged).
> 
> Notably:
> 
>  - HW breakpoint PMU
>  - ARM big.little PMU
>  - Intel Branch Monitoring PMU
> 
> Since we now track the events in RB trees, we can 'simply' add a pmu
> order to them and have them grouped that way, reducing to a single
> context. Of course, reality never quite works out that simple, and below
> ends up adding an intermediate data structure to bridge the context ->
> pmu mapping.
> 
> Something a little like:
> 
>               ,------------------------[1:n]---------------------.
>               V                                                  V
>     perf_event_context <-[1:n]-> perf_event_pmu_context <--- perf_event
>               ^                      ^     |                     |
>               `--------[1:n]---------'     `-[n:1]-> pmu <-[1:n]-'
> 
> This patch builds (provided you disable CGROUP_PERF), boots and survives
> perf-top without the machine catching fire.
> 
> There's still a fair bit of loose ends (look for XXX), but I think this
> is the direction we should be going.
> 
> Comments?
> 
> Not-Quite-Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/powerpc/perf/core-book3s.c |    4 
>  arch/x86/events/core.c          |    4 
>  arch/x86/events/intel/core.c    |    6 
>  arch/x86/events/intel/ds.c      |    6 
>  arch/x86/events/intel/lbr.c     |   16 
>  arch/x86/events/perf_event.h    |    6 
>  include/linux/perf_event.h      |   80 +-
>  include/linux/sched.h           |    2 
>  kernel/events/core.c            | 1412 ++++++++++++++++++++--------------------
>  9 files changed, 815 insertions(+), 721 deletions(-)

Rewrite is impressive however it doesn't result in code base reduction as it is.
Nonetheless there is a clear demand for per pmu events groups tracking and rotation 
in single cpu context (HW breakpoints, ARM big.little, Intel LBRs) and there is 
a supply thru groups ordering on RB-tree.

This might be driven into the kernel by some new Perf features that would base on 
that RB-tree groups ordering or by refactoring of existing code but in the way it 
would result in overall code base reduction thus lowering support cost.

Thanks,
Alexey