Re: [PATCH v3] perf test x86: address multiplexing in rdpmc test

From: Ian Rogers
Date: Tue Jun 02 2020 - 17:53:18 EST

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On Wed, Apr 29, 2020 at 12:08 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> Counters may be being used for pinned or other events which inhibit the
> instruction counter in the test from being scheduled - time_enabled > 0
> but time_running == 0. This causes the test to fail with division by 0.
> Make time_running and enabled out parameters of mmap_read_self and add a
> sleep loop to ensure that the counter is running before computing the
> delta.

Ping. Peter does this address your feedback?

Thanks,
Ian

> v3. Cleans up the code by adding a single scale_count boolean. It aligns
> the code with the comments in perf_event.h as suggested by Peter
> Zijlstra.
> v2. Address review feedback from Peter Zijlstra.
>
> Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> ---
> include/uapi/linux/perf_event.h | 34 +++++++-----
> tools/include/uapi/linux/perf_event.h | 34 +++++++-----
> tools/perf/arch/x86/tests/rdpmc.c | 80 +++++++++++++++++++--------
> 3 files changed, 98 insertions(+), 50 deletions(-)
>
> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
> index 7b2d6fc9e6ed..4936cce0a6d0 100644
> --- a/include/uapi/linux/perf_event.h
> +++ b/include/uapi/linux/perf_event.h
> @@ -488,19 +488,22 @@ struct perf_event_mmap_page {
> /*
> * Bits needed to read the hw events in user-space.
> *
> - * u32 seq, time_mult, time_shift, index, width;
> - * u64 count, enabled, running;
> - * u64 cyc, time_offset;
> - * s64 pmc = 0;
> + * u32 seq, time_mult, time_shift, index;
> + * u64 cyc, time_offset, enabled, running = -1;
> + * s64 count = 0;
> + * bool scale_count = false;
> *
> * do {
> * seq = pc->lock;
> - * barrier()
> + * barrier();
> *
> - * enabled = pc->time_enabled;
> - * running = pc->time_running;
> + * if (pc->cap_user_time) {
> + * enabled = pc->time_enabled;
> + * running = pc->time_running;
> + * scale_count = enabled != running && running != 0;
> + * }
> *
> - * if (pc->cap_usr_time && enabled != running) {
> + * if (scale_count) {
> * cyc = rdtsc();
> * time_offset = pc->time_offset;
> * time_mult = pc->time_mult;
> @@ -509,9 +512,13 @@ struct perf_event_mmap_page {
> *
> * index = pc->index;
> * count = pc->offset;
> - * if (pc->cap_user_rdpmc && index) {
> - * width = pc->pmc_width;
> - * pmc = rdpmc(index - 1);
> + * if (pc->cap_user_rdpmc && index && running != 0) {
> + * u16 width = pc->pmc_width;
> + * s64 pmc = rdpmc(index - 1);
> + *
> + * pmc <<= 64 - width;
> + * pmc >>= 64 - width; // signed shift right
> + * count += pmc;
> * }
> *
> * barrier();
> @@ -562,8 +569,9 @@ struct perf_event_mmap_page {
> * ((rem * time_mult) >> time_shift);
> *
> * Where time_offset,time_mult,time_shift and cyc are read in the
> - * seqcount loop described above. This delta can then be added to
> - * enabled and possible running (if index), improving the scaling:
> + * seqcount loop described above when scale_count is true. This delta
> + * can then be added to enabled and possible running (if index),
> + * improving the scaling:
> *
> * enabled += delta;
> * if (index)
> diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
> index 7b2d6fc9e6ed..4936cce0a6d0 100644
> --- a/tools/include/uapi/linux/perf_event.h
> +++ b/tools/include/uapi/linux/perf_event.h
> @@ -488,19 +488,22 @@ struct perf_event_mmap_page {
> /*
> * Bits needed to read the hw events in user-space.
> *
> - * u32 seq, time_mult, time_shift, index, width;
> - * u64 count, enabled, running;
> - * u64 cyc, time_offset;
> - * s64 pmc = 0;
> + * u32 seq, time_mult, time_shift, index;
> + * u64 cyc, time_offset, enabled, running = -1;
> + * s64 count = 0;
> + * bool scale_count = false;
> *
> * do {
> * seq = pc->lock;
> - * barrier()
> + * barrier();
> *
> - * enabled = pc->time_enabled;
> - * running = pc->time_running;
> + * if (pc->cap_user_time) {
> + * enabled = pc->time_enabled;
> + * running = pc->time_running;
> + * scale_count = enabled != running && running != 0;
> + * }
> *
> - * if (pc->cap_usr_time && enabled != running) {
> + * if (scale_count) {
> * cyc = rdtsc();
> * time_offset = pc->time_offset;
> * time_mult = pc->time_mult;
> @@ -509,9 +512,13 @@ struct perf_event_mmap_page {
> *
> * index = pc->index;
> * count = pc->offset;
> - * if (pc->cap_user_rdpmc && index) {
> - * width = pc->pmc_width;
> - * pmc = rdpmc(index - 1);
> + * if (pc->cap_user_rdpmc && index && running != 0) {
> + * u16 width = pc->pmc_width;
> + * s64 pmc = rdpmc(index - 1);
> + *
> + * pmc <<= 64 - width;
> + * pmc >>= 64 - width; // signed shift right
> + * count += pmc;
> * }
> *
> * barrier();
> @@ -562,8 +569,9 @@ struct perf_event_mmap_page {
> * ((rem * time_mult) >> time_shift);
> *
> * Where time_offset,time_mult,time_shift and cyc are read in the
> - * seqcount loop described above. This delta can then be added to
> - * enabled and possible running (if index), improving the scaling:
> + * seqcount loop described above when scale_count is true. This delta
> + * can then be added to enabled and possible running (if index),
> + * improving the scaling:
> *
> * enabled += delta;
> * if (index)
> diff --git a/tools/perf/arch/x86/tests/rdpmc.c b/tools/perf/arch/x86/tests/rdpmc.c
> index 1ea916656a2d..93e1770d03d3 100644
> --- a/tools/perf/arch/x86/tests/rdpmc.c
> +++ b/tools/perf/arch/x86/tests/rdpmc.c
> @@ -16,13 +16,13 @@
> #include <internal/lib.h> // page_size
> #include "arch-tests.h"
>
> -static u64 rdpmc(unsigned int counter)
> +static s64 rdpmc(unsigned int counter)
> {
> unsigned int low, high;
>
> asm volatile("rdpmc" : "=a" (low), "=d" (high) : "c" (counter));
>
> - return low | ((u64)high) << 32;
> + return low | ((s64)high) << 32;
> }
>
> static u64 rdtsc(void)
> @@ -34,49 +34,67 @@ static u64 rdtsc(void)
> return low | ((u64)high) << 32;
> }
>
> -static u64 mmap_read_self(void *addr)
> +/*
> + * Return a user rdpmc result as well as the time enabled and running. In the
> + * situation that the counter was ran and the kernel is multiplexing events, the
> + * counter is scaled.
> + */
> +static s64 mmap_read_self(void *addr, u64 *enabled, u64 *running)
> {
> struct perf_event_mmap_page *pc = addr;
> - u32 seq, idx, time_mult = 0, time_shift = 0;
> - u64 count, cyc = 0, time_offset = 0, enabled, running, delta;
> + u32 seq, time_mult = 0, time_shift = 0, index;
> + u64 cyc = 0, time_offset = 0;
> + s64 count = 0;
> + bool scale_count = false;
>
> + *enabled = -1;
> + *running = -1;
> do {
> seq = pc->lock;
> barrier();
>
> - enabled = pc->time_enabled;
> - running = pc->time_running;
> + if (pc->cap_user_time) {
> + *enabled = pc->time_enabled;
> + *running = pc->time_running;
> + scale_count = *enabled != *running && *running != 0;
> + }
>
> - if (enabled != running) {
> + if (scale_count) {
> cyc = rdtsc();
> time_mult = pc->time_mult;
> time_shift = pc->time_shift;
> time_offset = pc->time_offset;
> }
>
> - idx = pc->index;
> + index = pc->index;
> count = pc->offset;
> - if (idx)
> - count += rdpmc(idx - 1);
> + if (pc->cap_user_rdpmc && index && *running != 0) {
> + u16 width = pc->pmc_width;
> + s64 pmc = rdpmc(index - 1);
> +
> + pmc <<= 64 - width;
> + pmc >>= 64 - width; // signed shift right
> + count += pmc;
> + }
>
> barrier();
> } while (pc->lock != seq);
>
> - if (enabled != running) {
> - u64 quot, rem;
> + if (scale_count) {
> + u64 quot, rem, delta;
>
> quot = (cyc >> time_shift);
> rem = cyc & (((u64)1 << time_shift) - 1);
> delta = time_offset + quot * time_mult +
> ((rem * time_mult) >> time_shift);
>
> - enabled += delta;
> - if (idx)
> - running += delta;
> + *enabled += delta;
> + if (index)
> + *running += delta;
>
> - quot = count / running;
> - rem = count % running;
> - count = quot * enabled + (rem * enabled) / running;
> + quot = count / *running;
> + rem = count % *running;
> + count = quot * *enabled + (rem * *enabled) / *running;
> }
>
> return count;
> @@ -104,7 +122,7 @@ static int __test__rdpmc(void)
> .config = PERF_COUNT_HW_INSTRUCTIONS,
> .exclude_kernel = 1,
> };
> - u64 delta_sum = 0;
> + u64 delta_sum = 0, sleep_count = 0;
> struct sigaction sa;
> char sbuf[STRERR_BUFSIZE];
>
> @@ -130,14 +148,23 @@ static int __test__rdpmc(void)
> }
>
> for (n = 0; n < 6; n++) {
> - u64 stamp, now, delta;
> -
> - stamp = mmap_read_self(addr);
> + u64 stamp, now, delta, enabled, running;
> +
> + for (;;) {
> + stamp = mmap_read_self(addr, &enabled, &running);
> + if (enabled && running)
> + break;
> + /* Try to wait for event to be running. */
> + sleep_count++;
> + if (sleep_count > 20)
> + goto out_never_run;
> + sleep(1);
> + }
>
> for (i = 0; i < loops; i++)
> tmp++;
>
> - now = mmap_read_self(addr);
> + now = mmap_read_self(addr, &enabled, &running);
> loops *= 10;
>
> delta = now - stamp;
> @@ -155,6 +182,11 @@ static int __test__rdpmc(void)
> return -1;
>
> return 0;
> +
> +out_never_run:
> + close(fd);
> + pr_err("Event counter failed to multiplexed in. Are higher priority counters being sampled by a different process?\n");
> + return -1;
> }
>
> int test__rdpmc(struct test *test __maybe_unused, int subtest __maybe_unused)
> --
> 2.26.2.303.gf8c07b1a785-goog
>

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