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Date: Wed, 2 Sep 2020 11:07:38 -0700
From: Ian Rogers <irogers@...gle.com>
To: Rob Herring <robh@...nel.org>
Cc: Will Deacon <will@...nel.org>,
Catalin Marinas <catalin.marinas@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Jiri Olsa <jolsa@...hat.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Namhyung Kim <namhyung@...nel.org>,
Raphael Gault <raphael.gault@....com>,
Mark Rutland <mark.rutland@....com>,
Jonathan Cameron <Jonathan.Cameron@...wei.com>,
honnappa.nagarahalli@....com
Subject: Re: [PATCH v2 5/9] libperf: Add support for user space counter access
On Fri, Aug 28, 2020 at 1:56 PM Rob Herring <robh@...nel.org> wrote:
>
> x86 and arm64 can both support direct access of event counters in
> userspace. The access sequence is less than trivial and currently exists
> in perf test code (tools/perf/arch/x86/tests/rdpmc.c) with copies in
> projects such as PAPI and libpfm4.
>
> In order to support usersapce access, an event must be mmapped. While
> there's already mmap support for evlist, the usecase is a bit different
> than the self monitoring with userspace access. So let's add a new
> perf_evsel__mmap() function to mmap an evsel. This allows implementing
> userspace access as a fastpath for perf_evsel__read().
>
> The mmapped address is returned by perf_evsel__mmap() primarily for
> users/tests to check if userspace access is enabled.
>
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
> tools/lib/perf/Documentation/libperf.txt | 1 +
> tools/lib/perf/evsel.c | 33 +++++++++
> tools/lib/perf/include/internal/evsel.h | 2 +
> tools/lib/perf/include/internal/mmap.h | 3 +
> tools/lib/perf/include/perf/evsel.h | 1 +
> tools/lib/perf/libperf.map | 1 +
> tools/lib/perf/mmap.c | 90 ++++++++++++++++++++++++
> tools/lib/perf/tests/test-evsel.c | 64 +++++++++++++++++
> 8 files changed, 195 insertions(+)
>
> diff --git a/tools/lib/perf/Documentation/libperf.txt b/tools/lib/perf/Documentation/libperf.txt
> index 0c74c30ed23a..ca7478acc97c 100644
> --- a/tools/lib/perf/Documentation/libperf.txt
> +++ b/tools/lib/perf/Documentation/libperf.txt
> @@ -136,6 +136,7 @@ SYNOPSIS
> struct perf_thread_map *threads);
> void perf_evsel__close(struct perf_evsel *evsel);
> void perf_evsel__close_cpu(struct perf_evsel *evsel, int cpu);
> + void *perf_evsel__mmap(struct perf_evsel *evsel);
> int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
> struct perf_counts_values *count);
> int perf_evsel__enable(struct perf_evsel *evsel);
> diff --git a/tools/lib/perf/evsel.c b/tools/lib/perf/evsel.c
> index 4dc06289f4c7..b0c94ef4d9b6 100644
> --- a/tools/lib/perf/evsel.c
> +++ b/tools/lib/perf/evsel.c
> @@ -11,10 +11,12 @@
> #include <stdlib.h>
> #include <internal/xyarray.h>
> #include <internal/cpumap.h>
> +#include <internal/mmap.h>
> #include <internal/threadmap.h>
> #include <internal/lib.h>
> #include <linux/string.h>
> #include <sys/ioctl.h>
> +#include <sys/mman.h>
>
> void perf_evsel__init(struct perf_evsel *evsel, struct perf_event_attr *attr)
> {
> @@ -156,6 +158,34 @@ void perf_evsel__close_cpu(struct perf_evsel *evsel, int cpu)
> perf_evsel__close_fd_cpu(evsel, cpu);
> }
>
> +void *perf_evsel__mmap(struct perf_evsel *evsel)
> +{
> + int ret;
> + struct perf_mmap *map;
> + struct perf_mmap_param mp = {
> + .mask = -1,
> + .prot = PROT_READ | PROT_WRITE,
> + };
> +
> + if (FD(evsel, 0, 0) < 0)
> + return NULL;
> +
> + map = zalloc(sizeof(*map));
> + if (!map)
> + return NULL;
> +
> + perf_mmap__init(map, NULL, false, NULL);
> +
> + ret = perf_mmap__mmap(map, &mp, FD(evsel, 0, 0), 0);
> + if (ret) {
> + free(map);
> + return NULL;
> + }
> +
> + evsel->mmap = map;
> + return map->base;
> +}
> +
> int perf_evsel__read_size(struct perf_evsel *evsel)
> {
> u64 read_format = evsel->attr.read_format;
> @@ -191,6 +221,9 @@ int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
> if (FD(evsel, cpu, thread) < 0)
> return -EINVAL;
>
> + if (evsel->mmap && !perf_mmap__read_self(evsel->mmap, count))
> + return 0;
> +
> if (readn(FD(evsel, cpu, thread), count->values, size) <= 0)
> return -errno;
>
> diff --git a/tools/lib/perf/include/internal/evsel.h b/tools/lib/perf/include/internal/evsel.h
> index 1ffd083b235e..a7985dbb68ff 100644
> --- a/tools/lib/perf/include/internal/evsel.h
> +++ b/tools/lib/perf/include/internal/evsel.h
> @@ -9,6 +9,7 @@
>
> struct perf_cpu_map;
> struct perf_thread_map;
> +struct perf_mmap;
> struct xyarray;
>
> /*
> @@ -40,6 +41,7 @@ struct perf_evsel {
> struct perf_cpu_map *cpus;
> struct perf_cpu_map *own_cpus;
> struct perf_thread_map *threads;
> + struct perf_mmap *mmap;
> struct xyarray *fd;
> struct xyarray *sample_id;
> u64 *id;
> diff --git a/tools/lib/perf/include/internal/mmap.h b/tools/lib/perf/include/internal/mmap.h
> index be7556e0a2b2..5e3422f40ed5 100644
> --- a/tools/lib/perf/include/internal/mmap.h
> +++ b/tools/lib/perf/include/internal/mmap.h
> @@ -11,6 +11,7 @@
> #define PERF_SAMPLE_MAX_SIZE (1 << 16)
>
> struct perf_mmap;
> +struct perf_counts_values;
>
> typedef void (*libperf_unmap_cb_t)(struct perf_mmap *map);
>
> @@ -52,4 +53,6 @@ void perf_mmap__put(struct perf_mmap *map);
>
> u64 perf_mmap__read_head(struct perf_mmap *map);
>
> +int perf_mmap__read_self(struct perf_mmap *map, struct perf_counts_values *count);
> +
> #endif /* __LIBPERF_INTERNAL_MMAP_H */
> diff --git a/tools/lib/perf/include/perf/evsel.h b/tools/lib/perf/include/perf/evsel.h
> index c82ec39a4ad0..6d0da962870c 100644
> --- a/tools/lib/perf/include/perf/evsel.h
> +++ b/tools/lib/perf/include/perf/evsel.h
> @@ -27,6 +27,7 @@ LIBPERF_API int perf_evsel__open(struct perf_evsel *evsel, struct perf_cpu_map *
> struct perf_thread_map *threads);
> LIBPERF_API void perf_evsel__close(struct perf_evsel *evsel);
> LIBPERF_API void perf_evsel__close_cpu(struct perf_evsel *evsel, int cpu);
> +LIBPERF_API void *perf_evsel__mmap(struct perf_evsel *evsel);
> LIBPERF_API int perf_evsel__read(struct perf_evsel *evsel, int cpu, int thread,
> struct perf_counts_values *count);
> LIBPERF_API int perf_evsel__enable(struct perf_evsel *evsel);
> diff --git a/tools/lib/perf/libperf.map b/tools/lib/perf/libperf.map
> index 7be1af8a546c..733a0647be8b 100644
> --- a/tools/lib/perf/libperf.map
> +++ b/tools/lib/perf/libperf.map
> @@ -23,6 +23,7 @@ LIBPERF_0.0.1 {
> perf_evsel__disable;
> perf_evsel__open;
> perf_evsel__close;
> + perf_evsel__mmap;
> perf_evsel__read;
> perf_evsel__cpus;
> perf_evsel__threads;
> diff --git a/tools/lib/perf/mmap.c b/tools/lib/perf/mmap.c
> index 79d5ed6c38cc..cb07969cfdbf 100644
> --- a/tools/lib/perf/mmap.c
> +++ b/tools/lib/perf/mmap.c
> @@ -8,9 +8,11 @@
> #include <linux/perf_event.h>
> #include <perf/mmap.h>
> #include <perf/event.h>
> +#include <perf/evsel.h>
> #include <internal/mmap.h>
> #include <internal/lib.h>
> #include <linux/kernel.h>
> +#include <linux/math64.h>
> #include "internal.h"
>
> void perf_mmap__init(struct perf_mmap *map, struct perf_mmap *prev,
> @@ -273,3 +275,91 @@ union perf_event *perf_mmap__read_event(struct perf_mmap *map)
>
> return event;
> }
> +
> +#if defined(__i386__) || defined(__x86_64__)
> +static u64 read_perf_counter(unsigned int counter)
> +{
> + unsigned int low, high;
> +
> + asm volatile("rdpmc" : "=a" (low), "=d" (high) : "c" (counter));
> +
> + return low | ((u64)high) << 32;
> +}
> +
> +static u64 read_timestamp(void)
> +{
> + unsigned int low, high;
> +
> + asm volatile("rdtsc" : "=a" (low), "=d" (high));
> +
> + return low | ((u64)high) << 32;
> +}
> +#else
> +static u64 read_perf_counter(unsigned int counter) { return 0; }
> +static u64 read_timestamp(void) { return 0; }
> +#endif
> +
> +int perf_mmap__read_self(struct perf_mmap *map, struct perf_counts_values *count)
> +{
> + struct perf_event_mmap_page *pc = map->base;
> + u32 seq, idx, time_mult = 0, time_shift = 0;
> + u64 cnt, cyc = 0, time_offset = 0, time_cycles = 0, time_mask = ~0ULL;
> +
> + BUG_ON(!pc);
> +
> + if (!pc->cap_user_rdpmc)
> + return -1;
> +
> + do {
> + seq = READ_ONCE(pc->lock);
> + barrier();
> +
> + count->ena = READ_ONCE(pc->time_enabled);
> + count->run = READ_ONCE(pc->time_running);
> +
> + if (pc->cap_user_time && count->ena != count->run) {
> + cyc = read_timestamp();
> + time_mult = READ_ONCE(pc->time_mult);
> + time_shift = READ_ONCE(pc->time_shift);
> + time_offset = READ_ONCE(pc->time_offset);
> +
> + if (pc->cap_user_time_short) {
> + time_cycles = READ_ONCE(pc->time_cycles);
> + time_mask = READ_ONCE(pc->time_mask);
> + }
> + }
> +
> + idx = READ_ONCE(pc->index);
> + cnt = READ_ONCE(pc->offset);
> + if (pc->cap_user_rdpmc && idx) {
> + u64 evcnt = read_perf_counter(idx - 1);
> + u16 width = READ_ONCE(pc->pmc_width);
> +
> + evcnt <<= 64 - width;
> + evcnt >>= 64 - width;
> + cnt += evcnt;
> + } else
> + return -1;
> +
> + barrier();
> + } while (READ_ONCE(pc->lock) != seq);
> +
> + if (count->ena != count->run) {
There's an existing bug here that I tried to resolve in this patch:
https://lore.kernel.org/lkml/CAP-5=fVRdqvswtyQMg5cB+ntTGda+SAYskjTQednEH-AeZo13g@mail.gmail.com/
Due to multiplexing, enabled may be > 0 but run == 0 and the divide
below can end up with divide by zero.
I like the idea of this code being in a library, there's an intent
that the perf_event.h and test code be copy-paste-able, but there is
some pre-existing divergence. It would be nice if this code could be
closer to the sample code in both the test and perf_event.h.
As per the change above, I think running and enabled times need to be
out arguments.
Thanks,
Ian
> + u64 delta;
> +
> + /* Adjust for cap_usr_time_short, a nop if not */
> + cyc = time_cycles + ((cyc - time_cycles) & time_mask);
> +
> + delta = time_offset + mul_u64_u32_shr(cyc, time_mult, time_shift);
> +
> + count->ena += delta;
> + if (idx)
> + count->run += delta;
> +
> + cnt = mul_u64_u64_div64(cnt, count->ena, count->run);
> + }
> +
> + count->val = cnt;
> +
> + return 0;
> +}
> diff --git a/tools/lib/perf/tests/test-evsel.c b/tools/lib/perf/tests/test-evsel.c
> index 135722ac965b..fd637d23216b 100644
> --- a/tools/lib/perf/tests/test-evsel.c
> +++ b/tools/lib/perf/tests/test-evsel.c
> @@ -120,6 +120,68 @@ static int test_stat_thread_enable(void)
> return 0;
> }
>
> +static int test_stat_user_read(int event)
> +{
> + struct perf_counts_values counts = { .val = 0 };
> + struct perf_thread_map *threads;
> + struct perf_evsel *evsel;
> + struct perf_event_mmap_page *pc;
> + struct perf_event_attr attr = {
> + .type = PERF_TYPE_HARDWARE,
> + .config = event,
> + };
> + int err, i;
> +
> + threads = perf_thread_map__new_dummy();
> + __T("failed to create threads", threads);
> +
> + perf_thread_map__set_pid(threads, 0, 0);
> +
> + evsel = perf_evsel__new(&attr);
> + __T("failed to create evsel", evsel);
> +
> + err = perf_evsel__open(evsel, NULL, threads);
> + __T("failed to open evsel", err == 0);
> +
> + pc = perf_evsel__mmap(evsel);
> + __T("failed to mmap evsel", pc);
> +
> +#if defined(__i386__) || defined(__x86_64__) || defined(__aarch64__)
> + __T("userspace counter access not supported", pc->cap_user_rdpmc);
> + __T("userspace counter access not enabled", pc->index);
> + __T("userspace counter width not set", pc->pmc_width >= 32);
> +#endif
> +
> + perf_evsel__read(evsel, 0, 0, &counts);
> + __T("failed to read value for evsel", counts.val != 0);
> +
> + fputs("\n", stderr);
> + for (i = 0; i < 5; i++) {
> + volatile int count = 0x10000 << i;
> + __u64 start, end, last = 0;
> +
> + fprintf(stderr, "\tloop = %u, ", count);
> +
> + perf_evsel__read(evsel, 0, 0, &counts);
> + start = counts.val;
> +
> + while (count--) ;
> +
> + perf_evsel__read(evsel, 0, 0, &counts);
> + end = counts.val;
> +
> + __T("invalid counter data", (end - start) > last);
> + last = end - start;
> + fprintf(stderr, "count = %llu\n", end - start);
> + }
> +
> + perf_evsel__close(evsel);
> + perf_evsel__delete(evsel);
> +
> + perf_thread_map__put(threads);
> + return 0;
> +}
> +
> int main(int argc, char **argv)
> {
> __T_START;
> @@ -129,6 +191,8 @@ int main(int argc, char **argv)
> test_stat_cpu();
> test_stat_thread();
> test_stat_thread_enable();
> + test_stat_user_read(PERF_COUNT_HW_INSTRUCTIONS);
> + test_stat_user_read(PERF_COUNT_HW_CPU_CYCLES);
>
> __T_END;
> return 0;
> --
> 2.25.1
>
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