Re: [PATCH v4 3/3] perf: Sample additional clock value
From: Peter Zijlstra <hidden>
Date: 2015-01-05 13:45:32
Also in:
lkml
On Thu, Nov 06, 2014 at 04:51:58PM +0000, Pawel Moll wrote:
Currently three clocks are implemented: CLOCK_REALITME = 0, CLOCK_MONOTONIC = 1 and CLOCK_MONOTONIC_RAW = 2. The clock field is 5 bits wide to allow for future extension to custom, non-POSIX clock sources(MAX_CLOCK for those is 16, see include/uapi/linux/time.h) like ARM CoreSight (hardware trace) timestamp generator.
quoted hunk ↗ jump to hunk
@@ -304,7 +305,16 @@ struct perf_event_attr { mmap2 : 1, /* include mmap with inode data */ comm_exec : 1, /* flag comm events that are due to an exec */ uevents : 1, /* allow uevents into the buffer */ - __reserved_1 : 38; + + /* + * clock: one of the POSIX clock IDs: + * + * 0 - CLOCK_REALTIME + * 1 - CLOCK_MONOTONIC + * 4 - CLOCK_MONOTONIC_RAW + */ + clock : 5, /* clock type */ + __reserved_1 : 33; union { __u32 wakeup_events; /* wakeup every n events */
This would put a constraint on actually changing MAX_CLOCKS, are the time people OK with that? Thomas, John? I'm also not quite sure of using the >MAX_CLOCKS space for 'special' clocks, preferably those would register themselves with the POSIX clock interface.
quoted hunk ↗ jump to hunk
@@ -4631,6 +4637,24 @@ static void __perf_event_header__init_id(struct perf_event_header *header, data->cpu_entry.cpu = raw_smp_processor_id(); data->cpu_entry.reserved = 0; } + + if (sample_type & PERF_SAMPLE_CLOCK) { + switch (event->attr.clock) { + case CLOCK_REALTIME: + data->clock = ktime_get_real_ns(); + break; + case CLOCK_MONOTONIC: + data->clock = ktime_get_mono_fast_ns(); + break; + case CLOCK_MONOTONIC_RAW: + data->clock = ktime_get_raw_ns(); + break; + default: + data->clock = 0; + break; + } + } + }
This is broken. There is nothing stopping this from being called from
NMI context and only the MONO one is NMI safe.
Also, one would expect something like:
default: {
struct k_clock *kc = clockid_to_kclock(event->attr.clock);
struct timespec ts;
if (kc) {
kc->clock_get(event->attr.clock, &ts);
data->clock = ktime_to_ns(timespec_to_ktime(ts));
} else {
data->clock = 0;
}
}
Albeit preferably slightly less horrible -- of course, one would first
need to deal with the NMI issue.