Re: [PATCH lttng-ust v2 2/2] Add perf context support for ARMv7

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jun 23, 2016, at 6:52 PM, Julien Desfossez jdesfossez@efficios.com wrote:

> Allow to add perf context to UST traces. ARMv7 does not have a reliable
> way to read perf PMU counters entirely from user-space like we do on
> x86, so this approach requires a system call every time a counter needs
> to be read which has a significant performance impact.
> 
> ARMv7 does not have way to read PMU from userspace because it requires
> write access to the debug coprocessor to select which PMU counter to
> read which defeats user-space/kernel protection. For that reason, the
> bits required to allow user-space access to those registers are not
> enabled in the kernel and Perf does not expose any information in the
> shared mmap page, so we do not know what is the counter index. Also, for
> ARMv7 we cannot the exclude_kernel flag, so the counter stays active

we cannot .. set ?

> even when the process is executing in kernel context (system calls
> mainly).
> 
> This generic approach might work on other architecture, but it has not
> yet been tested so it is not enabled in the code.
> 
> Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
> ---
> configure.ac                               |  1 +
> liblttng-ust-ctl/ustctl.c                  |  2 +-
> liblttng-ust/lttng-context-perf-counters.c | 84 +++++++++++++++++++++---------
> 3 files changed, 62 insertions(+), 25 deletions(-)
> 
> diff --git a/configure.ac b/configure.ac
> index de462ff..5475640 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -219,6 +219,7 @@ AS_CASE([$host_cpu],
> 	[i[[3456]]86], [UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS=yes],
> 	[x86_64], [UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS=yes],
> 	[amd64], [UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS=yes],
> +	[armv7l], [UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS=yes],
> 	[UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS=no])
> AC_MSG_RESULT([$UST_SUPPORT_FOR_ARCH_PERF_EVENT_COUNTERS])
> 
> diff --git a/liblttng-ust-ctl/ustctl.c b/liblttng-ust-ctl/ustctl.c
> index 0165786..97445d5 100644
> --- a/liblttng-ust-ctl/ustctl.c
> +++ b/liblttng-ust-ctl/ustctl.c
> @@ -1742,7 +1742,7 @@ int ustctl_get_instance_id(struct ustctl_consumer_stream
> *stream,
> 	return client_cb->instance_id(buf, handle, id);
> }
> 
> -#if defined(__x86_64__) || defined(__i386__)
> +#if defined(__x86_64__) || defined(__i386__) || defined(__ARM_ARCH_7A__)
> 
> int ustctl_has_perf_counters(void)
> {
> diff --git a/liblttng-ust/lttng-context-perf-counters.c
> b/liblttng-ust/lttng-context-perf-counters.c
> index 725b1b4..a981aa9 100644
> --- a/liblttng-ust/lttng-context-perf-counters.c
> +++ b/liblttng-ust/lttng-context-perf-counters.c
> @@ -91,17 +91,13 @@ uint64_t rdpmc(unsigned int counter)
> 	return low | ((uint64_t) high) << 32;
> }
> 
> -#else /* defined(__x86_64__) || defined(__i386__) */
> -
> -#error "Perf event counters are only supported on x86 so far."
> -
> -#endif /* #else defined(__x86_64__) || defined(__i386__) */
> -
> static
> -uint64_t read_perf_counter(struct perf_event_mmap_page *pc)
> +uint64_t read_perf_counter(
> +		struct lttng_perf_counter_thread_field *thread_field)
> {
> 	uint32_t seq, idx;
> 	uint64_t count;
> +	struct perf_event_mmap_page *pc = thread_field->pc;
> 
> 	if (caa_unlikely(!pc))
> 		return 0;
> @@ -122,6 +118,29 @@ uint64_t read_perf_counter(struct perf_event_mmap_page *pc)
> 	return count;
> }
> 
> +#elif defined (__ARM_ARCH_7A__)
> +
> +static
> +uint64_t read_perf_counter(
> +		struct lttng_perf_counter_thread_field *thread_field)
> +{
> +	uint64_t count;
> +
> +	if (caa_unlikely(thread_field->fd < 0))
> +		return 0;
> +
> +	if (read(thread_field->fd, &count, sizeof(count)) < sizeof(count))

^ This could also be a caa_unlikely().

Thanks,

Mathieu

> +		return 0;
> +
> +	return count;
> +}
> +
> +#else /* defined(__x86_64__) || defined(__i386__) || defined(__ARM_ARCH_7A__)
> */
> +
> +#error "Perf event counters are only supported on x86 and ARMv7 so far."
> +
> +#endif /* #else defined(__x86_64__) || defined(__i386__) ||
> defined(__ARM_ARCH_7A__) */
> +
> static
> int sys_perf_event_open(struct perf_event_attr *attr,
> 		pid_t pid, int cpu, int group_fd,
> @@ -144,6 +163,20 @@ int open_perf_fd(struct perf_event_attr *attr)
> }
> 
> static
> +void close_perf_fd(int fd)
> +{
> +	int ret;
> +
> +	if (fd < 0)
> +		return;
> +
> +	ret = close(fd);
> +	if (ret) {
> +		perror("Error closing LTTng-UST perf memory mapping FD");
> +	}
> +}
> +
> +static
> struct perf_event_mmap_page *setup_perf(
> 		struct lttng_perf_counter_thread_field *thread_field)
> {
> @@ -160,25 +193,10 @@ struct perf_event_mmap_page *setup_perf(
> 	thread_field->fd = -1;
> #endif
> 
> -end:
> 	return perf_addr;
> }
> 
> static
> -void close_perf_fd(int fd)
> -{
> -	int ret;
> -
> -	if (fd < 0)
> -		return;
> -
> -	ret = close(fd);
> -	if (ret) {
> -		perror("Error closing LTTng-UST perf memory mapping FD");
> -	}
> -}
> -
> -static
> void unmap_perf_page(struct perf_event_mmap_page *pc)
> {
> 	int ret;
> @@ -292,7 +310,7 @@ uint64_t wrapper_perf_counter_read(struct lttng_ctx_field
> *field)
> 
> 	perf_field = field->u.perf_counter;
> 	perf_thread_field = get_thread_field(perf_field);
> -	return read_perf_counter(perf_thread_field->pc);
> +	return read_perf_counter(perf_thread_field);
> }
> 
> static
> @@ -363,6 +381,24 @@ void lttng_destroy_perf_counter_field(struct
> lttng_ctx_field *field)
> 	free(perf_field);
> }
> 
> +#ifdef __ARM_ARCH_7A__
> +
> +static
> +int perf_get_exclude_kernel(void)
> +{
> +	return 0;
> +}
> +
> +#else /* __ARM_ARCH_7A__ */
> +
> +static
> +int perf_get_exclude_kernel(void)
> +{
> +	return 1;
> +}
> +
> +#endif /* __ARM_ARCH_7A__ */
> +
> /* Called with UST lock held */
> int lttng_add_perf_counter_to_ctx(uint32_t type,
> 				uint64_t config,
> @@ -413,7 +449,7 @@ int lttng_add_perf_counter_to_ctx(uint32_t type,
> 
> 	perf_field->attr.type = type;
> 	perf_field->attr.config = config;
> -	perf_field->attr.exclude_kernel = 1;
> +	perf_field->attr.exclude_kernel = perf_get_exclude_kernel();
> 	CDS_INIT_LIST_HEAD(&perf_field->thread_field_list);
> 	field->u.perf_counter = perf_field;
> 
> --
> 1.9.1

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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