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 "--double-cl" for 'perf 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 'perf c2c report' output 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/
Committer notes:
Link: https://lore.kernel.org/r/Y+wvVNWqXb70l4uy@feng-clx
Removed -a, leaving just as --double-cl, as this probably is not used so
frequently and perhaps will be even auto-detected if we manage to record
the MSR where this is configured.
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Tested-by: Leo Yan <leo.yan@linaro.org>
Acked-by: Joe Mario <jmario@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20230214075823.246414-1-feng.tang@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
1470a108a6