Messages in this thread | | | From | Jason Vas Dias <> | Date | Tue, 13 Mar 2018 23:45:45 +0000 | Subject | Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW |
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On 12/03/2018, Peter Zijlstra <peterz@infradead.org> wrote: > On Mon, Mar 12, 2018 at 07:01:20AM +0000, Jason Vas Dias wrote: >> Sometimes, particularly when correlating elapsed time to performance >> counter values, > > So what actual problem are you tring to solve here? Perf can already > give you sample time in various clocks, including MONOTONIC_RAW. > >
Yes, I am sampling perf counters, including CPU_CYCLES , INSTRUCTIONS, CPU_CLOCK, TASK_CLOCK, etc, in a Group FD I open with perf_event_open() , for the current thread on the current CPU - I am doing this for 4 threads , on Intel & ARM cpus.
Reading performance counters does involve 2 ioctls and a read() , which takes time that already far exceeds the time required to read the TSC or CNTPCT in the VDSO .
The CPU_CLOCK software counter should give the converted TSC cycles seen between the ioctl( grp_fd, PERF_EVENT_IOC_ENABLE , ...) and the ioctl( grp_fd, PERF_EVENT_IOC_DISABLE ), and the difference between the event->time_running and time_enabled should also measure elapsed time .
This gives the "inner" elapsed time, from the perpective of the kernel, while the measured code section had the counters enabled.
But unless the user-space program also has a way of measuring elapsed time from the CPU's perspective , ie. without being subject to operator or NTP / PTP adjustment, it has no way of correlating this inner elapsed time with any "outer" elapsed time measurement it may have made - I also measure the time taken by I/O operations between threads, for instance.
So that is my primary motivation - for each thread's main run loop, I enable performance counters and count several PMU counters and the CPU_CLOCK & TASK_CLOCK . I want to determine with maximal accuracy how much elapsed time was used actually executing the task's instructions on the CPU , and how long they took to execute. I want to try to exclude the time spent gathering and making and analysing the performance measurements from the time spent running the threads' main loop .
To do this accurately, it is best to exclude variations in time that occur because of operator or NTP / PTP adjustments .
The CLOCK_MONOTONIC_RAW clock is the ONLY clock that is MEANT to be immune from any adjustment.
It is meant to be high - resolution clock with 1ns resolution that should be subject to no adjustment, and hence one would expect it it have the lowest latency.
But the way Linux has up to now implemented it , CLOCK_MONOTONIC_RAW has a resolution (minimum time that can be measured) that varies from 300 - 1000ns .
I can read the TSC and store a 16-byte timespec value in @ 8ns on the same CPU .
I understand that linux must conform to the POSIX interface which means it cannot provide sub-nanosecond resolution timers, but it could allow user-space programs to easily discover the timer calibration so that user-space programs can read the timers themselves.
Currently, users must parse the log file or use gdb / objdump to inspect /proc/kcore to get the TSC calibration and exact mult+shift values for the TSC value conversion.
Intel does not publish, nor does the CPU come with in ROM or firmware, the actual precise TSC frequency - this must be calibrated against the other clocks , according to a complicated procedure in section 18.2 of the SDM . My TSC has a "rated" / nominal TSC frequency , which one can compute from CPUID leaves, of 2.3ghz, but the "Refined TSC frequency" is 2.8333ghz .
Hence I think Linux should export this calibrated frequency somehow ; its "calibration" is expressed as the raw clocksource 'mult' and 'shift' values, and is exported to the VDSO .
I think the VDSO should read the TSC and use the calibration to render the raw, unadjusted time from the CPU's perspective.
Hence, the patch I am preparing , which is again attached.
I will submit it properly via email once I figure out how to obtain the 'git-send-mail' tool, and how to use it to send multiple patches, which seems to be the only way to submit acceptable patches.
Also the attached timer program measures a latency of @ 20ns with my patch 4.15.9 kernel, when it measured a latency of 300-1000ns without it.
Thanks & Regards,
Jason [unhandled content-type:application/octet-stream]/* * Program to measure high-res timer latency. * */ #include <stdint.h> #include <stdbool.h> #include <sys/types.h> #include <unistd.h> #include <time.h> #include <errno.h> #include <string.h> #include <stdio.h>
#ifndef N_SAMPLES #define N_SAMPLES 100 #endif #define _STR(_S_) #_S_ #define STR(_S_) _STR(_S_)
int main(int argc, char *const* argv, char *const* envp) { clockid_t clk = CLOCK_MONOTONIC_RAW; bool do_dump = false; int argn=1; for(; argn < argc; argn+=1) if( argv[argn] != NULL ) if( *(argv[argn]) == '-') switch( *(argv[argn]+1) ) { case 'm': case 'M': clk = CLOCK_MONOTONIC; break; case 'd': case 'D': do_dump = true; break; case '?': case 'h': case 'u': case 'U': case 'H': fprintf(stderr,"Usage: timer_latency [ -m : use CLOCK_MONOTONIC clock (not CLOCK_MONOTONIC_RAW) ; -d : dump timespec contents. N_SAMPLES: " STR(N_SAMPLES) "\n\t" "Calculates average timer latency (minimum time that can be measured) over N_SAMPLES.\n" ); return 0; } struct timespec sample[N_SAMPLES+1]; unsigned int cnt=N_SAMPLES, s=0 ; do { if( 0 != clock_gettime(clk, &sample[s++]) ) { fprintf(stderr,"oops, clock_gettime() failed: %d: '%s'.\n", errno, strerror(errno)); return 1; } }while( --cnt ); clock_gettime(clk, &sample[s]); #define TS2NS(_TS_) ((((unsigned long long)(_TS_).tv_sec)*1000000000ULL) + (((unsigned long long)((_TS_).tv_nsec)))) unsigned long long deltas [ N_SAMPLES ] , t1, t2, sum=0, zd=0 , t_start = TS2NS(sample[0]); for(s=1; s < (N_SAMPLES+1); s+=1) { t1 = TS2NS(sample[s-1]); t2 = TS2NS(sample[s]); if ( t1 > t2 ) { fprintf(stderr,"Inconsistency: %llu %llu %lu.%lu %lu.%lu\n", t1 , t2 , sample[s-1].tv_sec, sample[s-1].tv_nsec , sample[s].tv_sec, sample[s].tv_nsec ); continue; } unsigned long long d =t2-t1; if ( d == 0 ) { if( zd == 0 ) fprintf(stderr, "0 delta!\n"); zd += 1; } deltas[s-1] = d; } if( zd > 0 ) fprintf(stderr,"%u 0 deltas\n"); for(s = 0; s < N_SAMPLES; s+=1) sum += deltas[s]; fprintf(stderr,"sum: %llu\n",sum); unsigned long long avg_ns = sum / N_SAMPLES; t1=(t2 - t_start); printf("Total time: %1.1llu.%9.9lluS - Average Latency: %1.1llu.%9.9lluS\n", t1 / 1000000000, t1 % 1000000000, avg_ns / 1000000000, avg_ns % 1000000000 ); return 0; }
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