Re: [PATCH] perf c2c: Add report option to show false sharing in adjacent cachelines

[Leo Yan]

On Mon, Feb 13, 2023 at 11:17:33AM +0800, Feng Tang wrote:
> 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 HITM events are still per cacheline. The 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.
> 
> [1]. https://lore.kernel.org/lkml/20201102091543.GM31092@shao2-debian/
> 
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>

LGTM and I verified it on my Arm64 platform with peer flag:

Reviewed-by: Leo Yan <leo.yan@xxxxxxxxxx>
Tested-by: Leo Yan <leo.yan@xxxxxxxxxx>