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    SubjectRe: [PATCH v2] perf c2c: Add report option to show false sharing in adjacent cachelines
    Hi Arnaldo,

    On Tue, Feb 14, 2023 at 04:30:56PM -0300, Arnaldo Carvalho de Melo wrote:
    > Em Tue, Feb 14, 2023 at 03:58:23PM +0800, Feng Tang escreveu:
    > > Many platforms have feature of adjacent cachelines prefetch, when it
    > > is enabled, for data in RAM of 2 cachelines (2N and 2N+1) granularity,
    > > if one is fetched to cache, the other one could likely be fetched too,
    > > which sort of extends the cacheline size to double, thus the false
    > > sharing could happens in adjacent cachelines.
    > >
    > > 0Day has captured performance changed related with this [1], and some
    > > commercial software explicitly makes its hot global variables 128 bytes
    > > aligned (2 cache lines) to avoid this kind of extended false sharing.
    > >
    > > So add an option "-a" or "--double-cl" for c2c report to show false
    > > sharing in double cache line granularity, which acts just like the
    > > cacheline size is doubled. There is no change to c2c record. The
    > > hardware events of shared cacheline are still per cacheline, and
    > > this option just changes the granularity of how events are grouped
    > > and displayed.
    > >
    > > In the c2c report below (will-it-scale's 'pagefault2' case on old kernel):
    > >
    > > ----------------------------------------------------------------------
    > > 26 31 2 0 0 0 0xffff888103ec6000
    > > ----------------------------------------------------------------------
    > > 35.48% 50.00% 0.00% 0.00% 0.00% 0x10 0 1 0xffffffff8133148b 1153 66 971 3748 74 [k] get_mem_cgroup_from_mm
    > > 6.45% 0.00% 0.00% 0.00% 0.00% 0x10 0 1 0xffffffff813396e4 570 0 1531 879 75 [k] mem_cgroup_charge
    > > 25.81% 50.00% 0.00% 0.00% 0.00% 0x54 0 1 0xffffffff81331472 949 70 593 3359 74 [k] get_mem_cgroup_from_mm
    > > 19.35% 0.00% 0.00% 0.00% 0.00% 0x54 0 1 0xffffffff81339686 1352 0 1073 1022 74 [k] mem_cgroup_charge
    > > 9.68% 0.00% 0.00% 0.00% 0.00% 0x54 0 1 0xffffffff813396d6 1401 0 863 768 74 [k] mem_cgroup_charge
    > > 3.23% 0.00% 0.00% 0.00% 0.00% 0x54 0 1 0xffffffff81333106 618 0 804 11 9 [k] uncharge_batch
    > >
    > > The offset 0x10 and 0x54 used to displayed in 2 groups, and now they
    > > are listed together to give users a hint of extended false sharing.
    > >
    > > [1]. https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/
    > >
    > > Signed-off-by: Feng Tang <feng.tang@intel.com>
    > > Reviewed-by: Andi Kleen <ak@linux.intel.com>
    > > Reviewed-by: Leo Yan <leo.yan@linaro.org>
    > > Tested-by: Leo Yan <leo.yan@linaro.org>
    > > ---
    > > Changelog:
    > >
    > > v1 -> v2:
    > > * Refine comments and fix typos (Leo Yan)
    > > * Add reviewd-by and tested-by(for Arm64) tag from Leo Yan
    > > * Refine cmd description and commit log to avoid using
    > > architecture specific name (Joe Mario)
    > >
    > > tools/perf/Documentation/perf-c2c.txt | 7 +++++++
    > > tools/perf/builtin-c2c.c | 22 +++++++++++++---------
    > > tools/perf/util/cacheline.h | 25 ++++++++++++++++++++-----
    > > tools/perf/util/sort.c | 13 ++++++++++---
    > > tools/perf/util/sort.h | 1 +
    > > 5 files changed, 51 insertions(+), 17 deletions(-)
    > >
    > > diff --git a/tools/perf/Documentation/perf-c2c.txt b/tools/perf/Documentation/perf-c2c.txt
    > > index 4e8c263e1721..cc18d61ec5d5 100644
    > > --- a/tools/perf/Documentation/perf-c2c.txt
    > > +++ b/tools/perf/Documentation/perf-c2c.txt
    > > @@ -130,6 +130,13 @@ REPORT OPTIONS
    > > The known limitations include exception handing such as
    > > setjmp/longjmp will have calls/returns not match.
    > >
    > > +-a::
    > > +--double-cl::
    > > + Group the detection of shared cacheline events into double cacheline
    > > + granularity. Some architectures have an Adjacent Cacheline Prefetch
    > > + feature, which causes cacheline sharing to behave like the cacheline
    > > + size is doubled.
    > > +
    >
    > Humm, this is something not that usual, so I think we should have it
    > just as --double-cl, ok?
    >
    > I can do the adjustment here if you agree.

    Sure. Many thanks for the review and suggestion!

    - Feng

    > Thanks,
    >
    > - Arnaldo


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