Re: [PATCH v2] perf c2c: Add report option to show false sharing in adjacent cachelines
From: Arnaldo Carvalho de Melo
Date: Tue Feb 14 2023 - 14:31:05 EST
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@xxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Reviewed-by: Leo Yan <leo.yan@xxxxxxxxxx>
> Tested-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
> 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.
Thanks,
- Arnaldo
> C2C RECORD
> ----------
> The perf c2c record command setup options related to HITM cacheline analysis
> diff --git a/tools/perf/builtin-c2c.c b/tools/perf/builtin-c2c.c
> index 52d94c7dd836..1a98c6e97a4b 100644
> --- a/tools/perf/builtin-c2c.c
> +++ b/tools/perf/builtin-c2c.c
> @@ -524,7 +524,7 @@ static int dcacheline_entry(struct perf_hpp_fmt *fmt, struct perf_hpp *hpp,
> char buf[20];
>
> if (he->mem_info)
> - addr = cl_address(he->mem_info->daddr.addr);
> + addr = cl_address(he->mem_info->daddr.addr, chk_double_cl);
>
> return scnprintf(hpp->buf, hpp->size, "%*s", width, HEX_STR(buf, addr));
> }
> @@ -562,7 +562,7 @@ static int offset_entry(struct perf_hpp_fmt *fmt, struct perf_hpp *hpp,
> char buf[20];
>
> if (he->mem_info)
> - addr = cl_offset(he->mem_info->daddr.al_addr);
> + addr = cl_offset(he->mem_info->daddr.al_addr, chk_double_cl);
>
> return scnprintf(hpp->buf, hpp->size, "%*s", width, HEX_STR(buf, addr));
> }
> @@ -574,9 +574,10 @@ offset_cmp(struct perf_hpp_fmt *fmt __maybe_unused,
> uint64_t l = 0, r = 0;
>
> if (left->mem_info)
> - l = cl_offset(left->mem_info->daddr.addr);
> + l = cl_offset(left->mem_info->daddr.addr, chk_double_cl);
> +
> if (right->mem_info)
> - r = cl_offset(right->mem_info->daddr.addr);
> + r = cl_offset(right->mem_info->daddr.addr, chk_double_cl);
>
> return (int64_t)(r - l);
> }
> @@ -2590,7 +2591,7 @@ perf_c2c_cacheline_browser__title(struct hist_browser *browser,
> he = cl_browser->he;
>
> if (he->mem_info)
> - addr = cl_address(he->mem_info->daddr.addr);
> + addr = cl_address(he->mem_info->daddr.addr, chk_double_cl);
>
> scnprintf(bf, size, "Cacheline 0x%lx", addr);
> return 0;
> @@ -2788,15 +2789,16 @@ static int ui_quirks(void)
> if (!c2c.use_stdio) {
> dim_offset.width = 5;
> dim_offset.header = header_offset_tui;
> - nodestr = "CL";
> + nodestr = chk_double_cl ? "Double-CL" : "CL";
> }
>
> dim_percent_costly_snoop.header = percent_costly_snoop_header[c2c.display];
>
> /* Fix the zero line for dcacheline column. */
> - buf = fill_line("Cacheline", dim_dcacheline.width +
> - dim_dcacheline_node.width +
> - dim_dcacheline_count.width + 4);
> + buf = fill_line(chk_double_cl ? "Double-Cacheline" : "Cacheline",
> + dim_dcacheline.width +
> + dim_dcacheline_node.width +
> + dim_dcacheline_count.width + 4);
> if (!buf)
> return -ENOMEM;
>
> @@ -3037,6 +3039,8 @@ static int perf_c2c__report(int argc, const char **argv)
> OPT_BOOLEAN('f', "force", &symbol_conf.force, "don't complain, do it"),
> OPT_BOOLEAN(0, "stitch-lbr", &c2c.stitch_lbr,
> "Enable LBR callgraph stitching approach"),
> + OPT_BOOLEAN('a', "double-cl", &chk_double_cl,
> + "Detect adjacent cacheline false sharing"),
> OPT_PARENT(c2c_options),
> OPT_END()
> };
> diff --git a/tools/perf/util/cacheline.h b/tools/perf/util/cacheline.h
> index dec8c0fb1f4a..fe6d5b60a031 100644
> --- a/tools/perf/util/cacheline.h
> +++ b/tools/perf/util/cacheline.h
> @@ -6,16 +6,31 @@
>
> int __pure cacheline_size(void);
>
> -static inline u64 cl_address(u64 address)
> +
> +/*
> + * Some architectures have 'Adjacent Cacheline Prefetch' feature,
> + * which performs like the cacheline size being doubled.
> + */
> +static inline u64 cl_address(u64 address, bool double_cl)
> {
> + u64 size = cacheline_size();
> +
> + if (double_cl)
> + size *= 2;
> +
> /* return the cacheline of the address */
> - return (address & ~(cacheline_size() - 1));
> + return (address & ~(size - 1));
> }
>
> -static inline u64 cl_offset(u64 address)
> +static inline u64 cl_offset(u64 address, bool double_cl)
> {
> - /* return the cacheline of the address */
> - return (address & (cacheline_size() - 1));
> + u64 size = cacheline_size();
> +
> + if (double_cl)
> + size *= 2;
> +
> + /* return the offset inside cacheline */
> + return (address & (size - 1));
> }
>
> #endif // PERF_CACHELINE_H
> diff --git a/tools/perf/util/sort.c b/tools/perf/util/sort.c
> index 4a648231fe72..093a0c8b2e3d 100644
> --- a/tools/perf/util/sort.c
> +++ b/tools/perf/util/sort.c
> @@ -53,6 +53,13 @@ enum sort_mode sort__mode = SORT_MODE__NORMAL;
> static const char *const dynamic_headers[] = {"local_ins_lat", "ins_lat", "local_p_stage_cyc", "p_stage_cyc"};
> static const char *const arch_specific_sort_keys[] = {"local_p_stage_cyc", "p_stage_cyc"};
>
> +/*
> + * Some architectures have Adjacent Cacheline Prefetch feature, which
> + * behaves like the cacheline size is doubled. Enable this flag to
> + * check things in double cacheline granularity.
> + */
> +bool chk_double_cl;
> +
> /*
> * Replaces all occurrences of a char used with the:
> *
> @@ -1500,8 +1507,8 @@ sort__dcacheline_cmp(struct hist_entry *left, struct hist_entry *right)
>
> addr:
> /* al_addr does all the right addr - start + offset calculations */
> - l = cl_address(left->mem_info->daddr.al_addr);
> - r = cl_address(right->mem_info->daddr.al_addr);
> + l = cl_address(left->mem_info->daddr.al_addr, chk_double_cl);
> + r = cl_address(right->mem_info->daddr.al_addr, chk_double_cl);
>
> if (l > r) return -1;
> if (l < r) return 1;
> @@ -1520,7 +1527,7 @@ static int hist_entry__dcacheline_snprintf(struct hist_entry *he, char *bf,
> if (he->mem_info) {
> struct map *map = he->mem_info->daddr.ms.map;
>
> - addr = cl_address(he->mem_info->daddr.al_addr);
> + addr = cl_address(he->mem_info->daddr.al_addr, chk_double_cl);
> ms = &he->mem_info->daddr.ms;
>
> /* print [s] for shared data mmaps */
> diff --git a/tools/perf/util/sort.h b/tools/perf/util/sort.h
> index 9a91d0df2833..d79a100e5999 100644
> --- a/tools/perf/util/sort.h
> +++ b/tools/perf/util/sort.h
> @@ -35,6 +35,7 @@ extern struct sort_entry sort_sym_from;
> extern struct sort_entry sort_sym_to;
> extern struct sort_entry sort_srcline;
> extern const char default_mem_sort_order[];
> +extern bool chk_double_cl;
>
> struct res_sample {
> u64 time;
> --
> 2.34.1
>
--
- Arnaldo