Messages in this thread | | | From | Jason Vas Dias <> | Date | Sat, 17 Mar 2018 22:52:20 +0000 | Subject | re: [PATCH v4.16-rc5 (2)] x86/vdso: VDSO should handle clock_gettime(CLOCK_MONOTONIC_RAW) without syscall |
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Hi -
I submitted a new stripped-down to bare essentials version of the patch, (see LKML emails with $subject) which passes all checkpatch.pl tests and addresses all concerns raised by reviewers, which uses only rdtsc_ordered(), and which only only updates in vsyscall_gtod_data the new fields: u32 raw_mult, raw_shift ; ... gtod_long_t monotonic_time_raw_sec /* == tk->raw_sec */ , monotonic_time_raw_nsec /* == tk->tkr_raw.nsec */; (this is NOT the formatting used in vgtod.h - sorry about previous formatting issues . ) .
I don't see how one could present the raw timespec in user-space properly without tk->tkr_raw.xtime_nsec and and tk->raw_sec ; monotonic has gtod->monotonic_time_sec and gtod->monotonic_time_snsec, and I am only trying to follow exactly the existing algorithm in timekeeping.c's getrawmonotonic64() .
When I submitted the initial version of this stripped down patch, I got an email back from robot<lkp@intel.com> reporting a compilation error saying :
> > arch/x86/entry/vdso/vclock_gettime.o: In function `__vdso_clock_gettime': > vclock_gettime.c:(.text+0xf7): undefined reference to >`__x86_indirect_thunk_rax' > /usr/bin/ld: arch/x86/entry/vdso/vclock_gettime.o: relocation R_X86_64_PC32 >against undefined symbol `__x86_indirect_thunk_rax' can not be used when making >a shared object; recompile with -fPIC > /usr/bin/ld: final link failed: Bad value >>> collect2: error: ld returned 1 exit status >-- >>> arch/x86/entry/vdso/vdso32.so.dbg: undefined symbols found >-- >>> objcopy: 'arch/x86/entry/vdso/vdso64.so.dbg': No such file >---
I had fixed this problem with the patch to the RHEL kernel attached to bug #198161 (attachment #274751: https://bugzilla.kernel.org/attachment.cgi?id=274751) , by simply reducing the number of clauses in __vdso_clock_gettime's switch(clock) from 6 to 5 , but at the cost of an extra test of clock & second switch(clock).
I reported this as GCC bug : https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84908 because I don't think GCC should fail to do anything for a switch with 6 clauses and not for one with 5, but the response I got from H.J. Liu was:
H.J. Lu wrote @ 2018-03-16 22:13:27 UTC: > > vDSO isn't compiled with $(KBUILD_CFLAGS). Why does your kernel do it? > > Please try my kernel patch at comment 4.. >
So that patch to the arch/x86/vdso/Makefile only prevents it enabling the RETPOLINE_CFLAGS for building the vDSO .
I defer to H.J.'s expertise on GCC + binutils & advisability of enabling RETPOLINE_CFLAGS in the VDSO - GCC definitely behaves strangely for the vDSO when RETPOLINE _CFLAGS are enabled.
Please provide something like the patch in a future version of Linux , and I suggest not compiling the vDSO with RETPOLINE_CFLAGS as does H.J. .
The inconsistency_check program in tools/testing/selftests/timers produces no errors for long runs and the timer_latency.c program (attached) also produces no errors and latencies of @ 20ns for CLOCK_MONOTONIC_RAW and latencies of @ 40ns for CLOCK_MONOTONIC - this is however with the additional rdtscp patches , and under 4.15.9, for use on my system ; the 4.16-rc5 version submitted still uses barrier() + rdtsc , and that has a latency of @ 30ns for CLOCK_MONOTONIC_RAW and @40ns for CLOCK_MONOTONIC ; but both are much, much better that 200-1000ns for CLOCK_MONOTONIC_RAW that the unpatched kernels have (all times refer to 'Average Latency' output produced by 'timer_latency.c').
I do apologize for whitespace errors, unread emails and resends and confusion of previous emails - I now understand the process and standards much better and will attempt to adhere to them more closely in future.
Thanks & Best Regards, Jason Vas Dias /* * 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 <alloca.h> #include <string.h> #include <stdio.h> #include <stdlib.h>
#ifndef N_SAMPLES #define N_SAMPLES 100 #endif #define _STR(_S_) #_S_ #define STR(_S_) _STR(_S_)
#define TS2NS(_TS_) ((((unsigned long long)(_TS_).tv_sec)*1000000000ULL) + (((unsigned long long)((_TS_).tv_nsec))))
int main(int argc, char *const* argv, char *const* envp) { struct timespec sample[N_SAMPLES+1]; unsigned int cnt=N_SAMPLES, s=0 , avg_n=0; unsigned long long deltas [ N_SAMPLES ] , t1, t2, sum=0, zd=0, ic=0, d , t_start, avg_ns, *avgs=0; clockid_t clk = CLOCK_MONOTONIC_RAW; bool do_dump = false; int argn=1, repeat=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 'r': case 'R': if( (argn < argc) && (argv[argn+1] != NULL)) repeat = atoi(argv[argn+=1]); break; case '?': case 'h': case 'u': case 'U': case 'H': fprintf(stderr,"Usage: timer_latency [\n\t-m : use CLOCK_MONOTONIC clock (not CLOCK_MONOTONIC_RAW)\n\t-d : dump timespec contents. N_SAMPLES: " STR(N_SAMPLES) "\n\t" "-r <repeat count>\n]\t" "Calculates average timer latency (minimum time that can be measured) over N_SAMPLES.\n" ); return 0; } if( repeat > 1 ) { avgs=alloca(sizeof(unsigned long long) * (N_SAMPLES + 1)); if( ((unsigned long) avgs) & 7 ) avgs = ((unsigned long long*)(((unsigned char*)avgs)+(8-((unsigned long) avgs) & 7))); } do { 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]); for(s=1; s < (N_SAMPLES+1); s+=1) { t1 = TS2NS(sample[s-1]); t2 = TS2NS(sample[s]); if ( (t1 > t2) ||(sample[s-1].tv_sec > sample[s].tv_sec) ||((sample[s-1].tv_sec == sample[s].tv_sec) &&(sample[s-1].tv_nsec > sample[s].tv_nsec) ) ) { 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 ); ic+=1; continue; } d = t2 - t1; if ( d == 0 ) { zd += 1; } deltas[s-1] = d; if(do_dump) fprintf(stderr, "%lu %lu %llu\n", sample[s].tv_sec, sample[s].tv_nsec, d ); } for(s = 0, sum=0; s < N_SAMPLES; s+=1) sum += deltas[s]; fprintf(stderr,"sum: %llu\n",sum); avg_ns = sum / N_SAMPLES; t_start = TS2NS(sample[0]); t1=(t2 - t_start); printf("Total time: %1.1llu.%9.9lluS - Average Latency: %1.1llu.%9.9lluS N zero deltas: %u N inconsistent deltas: %u\n", t1 / 1000000000, t1 % 1000000000, avg_ns / 1000000000, avg_ns % 1000000000 , zd, ic ); if (avgs != ((void*)0UL)) avgs[avg_n++] = avg_ns; } while (--repeat); if (avgs != ((void*)0UL)) { for( s=0, sum=0; s < avg_n; s+=1) sum += avgs[s]; printf("Average of %u average latencies of " STR(N_SAMPLES) " samples : %1.1llu.%9.9lluS\n" , avg_n, (sum / N_SAMPLES) / 1000000000, (sum / N_SAMPLES) % 1000000000 ); } return 0; }
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