From: Pawel Moll <pawel.m...@arm.com> Until now, perf framework never defined the meaning of the timestamps captured as PERF_SAMPLE_TIME sample type. The values were obtaining from local (sched) clock, which is unavailable in userspace. This made it impossible to correlate perf data with any other events. Other tracing solutions have the source configurable (ftrace) or just share a common time domain between kernel and userspace (LTTng).
Follow the trend by using monotonic clock, which is readily available as POSIX CLOCK_MONOTONIC. Also add a sysctl "perf_sample_time_clk_id" attribute (usually available as "/proc/sys/kernel/perf_sample_time_clk_id") which can be used by the user to obtain the clk_id to be used with POSIX clock API (eg. clock_gettime()) to obtain a time value comparable with perf samples. Old behaviour can be restored by using "perf_use_local_clock" kernel parameter. Signed-off-by: Pawel Moll <pawel.m...@arm.com> --- Documentation/kernel-parameters.txt | 9 ++++++++ kernel/events/core.c | 43 ++++++++++++++++++++++++++++++++++++- 2 files changed, 51 insertions(+), 1 deletion(-) diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt index bfcb1a6..e80da0b 100644 --- a/Documentation/kernel-parameters.txt +++ b/Documentation/kernel-parameters.txt @@ -91,6 +91,7 @@ the beginning of each description states the restrictions within which a NUMA NUMA support is enabled. NFS Appropriate NFS support is enabled. OSS OSS sound support is enabled. + PERF Performance events and counters support is enabled. PV_OPS A paravirtualized kernel is enabled. PARIDE The ParIDE (parallel port IDE) subsystem is enabled. PARISC The PA-RISC architecture is enabled. @@ -2813,6 +2814,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted. allocator. This parameter is primarily for debugging and performance comparison. + perf_use_local_clock + [PERF] + Use local_clock() as a source for perf timestamps + generation. This was be the default behaviour and + this parameter can be used to maintain backward + compatibility or on older hardware with expensive + monotonic clock source. + pf. [PARIDE] See Documentation/blockdev/paride.txt. diff --git a/kernel/events/core.c b/kernel/events/core.c index b01dfb6..f9d7a32 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -42,6 +42,8 @@ #include <linux/module.h> #include <linux/mman.h> #include <linux/compat.h> +#include <linux/sysctl.h> +#include <linux/jump_label.h> #include "internal.h" @@ -322,9 +324,43 @@ extern __weak const char *perf_pmu_name(void) return "pmu"; } +static struct static_key perf_use_local_clock_key = STATIC_KEY_INIT_FALSE; +static bool perf_use_local_clock_param __initdata; +static int __init perf_use_local_clock_setup(char *__unused) +{ + perf_use_local_clock_param = true; + return 1; +} +__setup("perf_use_local_clock", perf_use_local_clock_setup); + +static int sysctl_perf_sample_time_clk_id = CLOCK_MONOTONIC; + +static struct ctl_table perf_sample_time_kern_table[] = { + { + .procname = "perf_sample_time_clk_id", + .data = &sysctl_perf_sample_time_clk_id, + .maxlen = sizeof(int), + .mode = 0444, + .proc_handler = proc_dointvec, + }, + {} +}; + +static struct ctl_table perf_sample_time_root_table[] = { + { + .procname = "kernel", + .mode = 0555, + .child = perf_sample_time_kern_table, + }, + {} +}; + static inline u64 perf_clock(void) { - return local_clock(); + if (static_key_false(&perf_use_local_clock_key)) + return local_clock(); + else + return ktime_get_mono_fast_ns(); } static inline struct perf_cpu_context * @@ -8524,6 +8560,11 @@ void __init perf_event_init(void) */ BUILD_BUG_ON((offsetof(struct perf_event_mmap_page, data_head)) != 1024); + + if (perf_use_local_clock_param) + static_key_slow_inc(&perf_use_local_clock_key); + else + register_sysctl_table(perf_sample_time_root_table); } ssize_t perf_event_sysfs_show(struct device *dev, struct device_attribute *attr, -- 1.9.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/