Messages in this thread | | | Date | Tue, 14 Feb 2023 15:38:35 -0500 | Subject | Re: [RFC PATCH V2 2/9] perf: Extend ABI to support post-processing monotonic raw conversion | From | "Liang, Kan" <> |
| |
On 2023-02-14 3:11 p.m., John Stultz wrote: > On Tue, Feb 14, 2023 at 9:00 AM Liang, Kan <kan.liang@linux.intel.com> wrote: >> On 2023-02-14 9:51 a.m., Liang, Kan wrote: >>> If I understand correctly, the idea is to let the user space tool run >>> the above interpoloation algorithm several times to 'guess' the atomic >>> mapping. Using the mapping information to covert the TSC from the PEBS >>> record. Is my understanding correct? >>> >>> If so, to be honest, I doubt we can get the accuracy we want. >>> >> >> I implemented a simple test to evaluate the error. > > Very cool! > >> I collected TSC -> CLOCK_MONOTONIC_RAW mapping using the above algorithm >> at the start and end of perf cmd. >> MONO_RAW TSC >> start 89553516545645 223619715214239 >> end 89562251233830 223641517000376 >> >> Here is what I get via mult/shift conversion from this patch. >> MONO_RAW TSC >> PEBS 89555942691466 223625770878571 >> >> Then I use the time information from start and end to create a linear >> function and 'guess' the MONO_RAW of PEBS from the TSC. I get >> 89555942692721. >> There is a 1255 ns difference. >> I tried several different PEBS records. The error is ~1000ns. >> I think it should be an observable error. > > Interesting. That's a good bit higher than I'd expect as I'd expect a > clock_gettime() call to take ~ double digit nanoseconds range on > average, so the error should be within that. > > Can you share your logic? >
I run the algorithm right before and after the perf command as below. (The source code of time is attached.)
$./time $perf record -e cycles:upp --clockid monotonic_raw $some_workaround $./time
The time will dump both MONO_RAW and TSC. That's where "start" and "end" from. The perf command print out both TSC and converted MONO_RAW (using the mul/shift from this patch series). That's where "PEBS" value from.
Than I use the below formula to calculate the guessed MONO_RAW of PEBS TSC. Guessed_MONO_RAW = (PEBS_TSC - start_TSC) / (end_TSC - start_TSC) * (end_MONO_RAW - start_MONO_RAW) + start_MONO_RAW.
The guessed_MONO_RAW is 89555942692721. The PEBS_MONO_RAW is 89555942691466. The difference is 1255.
Is the calculation correct?
Thanks, Kan#include <sys/time.h> #include <time.h> #include <stdio.h> #include <errno.h>
static inline unsigned long rdtsc () { unsigned long var; unsigned int hi, lo;
asm volatile ("rdtsc" : "=a" (lo), "=d" (hi)); var = ((unsigned long long int) hi << 32) | lo;
return var; }
typedef unsigned long long u64;
int main() { struct timespec ts; u64 start, end, delta, mid; do { start= rdtsc(); clock_gettime(CLOCK_MONOTONIC_RAW, &ts); end = rdtsc(); delta = end-start; } while (delta > 20000); // make sure the reads were not preempted mid = start + (delta +(delta/2))/2; //round-closest printf("%llu %llu %llu\n", start, end, delta); printf("MONO_RAW: %llu TSC: %llu\n", (u64)ts.tv_sec * 1000000000 + ts.tv_nsec, mid); }
| |