While doing this for small amounts may be okay, the unconditional use of CPU0's value here has been found to be a problem when the boot time TSC of the BSP was behind that of all APs by more than a second. In particular because of get_s_time_fixed() producing insane output when the calculated delta is negative, we can't allow this to happen.
On the first iteration have all other CPUs sort out the highest TSC value any one of them has read. On the second iteration, if that maximum is higher than CPU0's, update its recorded value from that taken in the first iteration. Use the resulting value on the last iteration to write everyone's TSCs. To account for the possible discontinuity, have time_calibration_rendezvous_tail() record the newly written value, but extrapolate local stime using the value read. Reported-by: Claudemir Todo Bom <claude...@todobom.com> Signed-off-by: Jan Beulich <jbeul...@suse.com> Reviewed-by: Roger Pau Monné <roger....@citrix.com> --- v3: Simplify cmpxchg() loop. v2: Don't update r->master_stime by calculation. Re-base over new earlier patch. Make time_calibration_rendezvous_tail() take two TSC values. --- Since CPU0 reads its TSC last on the first iteration, if TSCs were perfectly sync-ed there shouldn't ever be a need to update. However, even on the TSC-reliable system I first tested this on (using "tsc=skewed" to get this rendezvous function into use in the first place) updates by up to several thousand clocks did happen. I wonder whether this points at some problem with the approach that I'm not (yet) seeing. Considering the sufficiently modern CPU it's using, I suspect the reporter's system wouldn't even need to turn off TSC_RELIABLE, if only there wasn't the boot time skew. Hence another approach might be to fix this boot time skew. Of course to recognize whether the TSCs then still aren't in sync we'd need to run tsc_check_reliability() sufficiently long after that adjustment. Which is besides the need to have this "fixing" be precise enough for the TSCs to not look skewed anymore afterwards. As per the comment ahead of it, the original purpose of the function was to deal with TSCs halted in deep C states. While this probably explains why only forward moves were ever expected, I don't see how this could have been reliable in case CPU0 was deep-sleeping for a sufficiently long time. My only guess here is a hidden assumption of CPU0 never being idle for long enough. Furthermore that comment looks to be contradicting the actual use of the function: It gets installed when !RELIABLE_TSC, while the comment would suggest !NONSTOP_TSC. I suppose the comment is simply misleading, because RELIABLE_TSC implies NONSTOP_TSC according to all the places where either of the two feature bits gets played with. Plus in the !NONSTOP_TSC case we write the TSC explicitly anyway when coming back out of a (deep; see below) C-state. As an implication from the above mwait_idle_cpu_init() then looks to pointlessly clear "reliable" when "nonstop" is clear. It further looks odd that mwait_idle() (unlike acpi_processor_idle()) calls cstate_restore_tsc() independent of what C-state was active. --- a/xen/arch/x86/time.c +++ b/xen/arch/x86/time.c @@ -1658,17 +1658,17 @@ struct calibration_rendezvous { cpumask_t cpu_calibration_map; atomic_t semaphore; s_time_t master_stime; - u64 master_tsc_stamp; + uint64_t master_tsc_stamp, max_tsc_stamp; }; static void time_calibration_rendezvous_tail(const struct calibration_rendezvous *r, - uint64_t tsc) + uint64_t old_tsc, uint64_t new_tsc) { struct cpu_time_stamp *c = &this_cpu(cpu_calibration); - c->local_tsc = tsc; - c->local_stime = get_s_time_fixed(c->local_tsc); + c->local_tsc = new_tsc; + c->local_stime = get_s_time_fixed(old_tsc ?: new_tsc); c->master_stime = r->master_stime; raise_softirq(TIME_CALIBRATE_SOFTIRQ); @@ -1683,6 +1683,7 @@ static void time_calibration_tsc_rendezv int i; struct calibration_rendezvous *r = _r; unsigned int total_cpus = cpumask_weight(&r->cpu_calibration_map); + uint64_t tsc = 0; /* Loop to get rid of cache effects on TSC skew. */ for ( i = 4; i >= 0; i-- ) @@ -1692,8 +1693,15 @@ static void time_calibration_tsc_rendezv while ( atomic_read(&r->semaphore) != (total_cpus - 1) ) cpu_relax(); - if ( r->master_tsc_stamp == 0 ) - r->master_tsc_stamp = rdtsc_ordered(); + if ( tsc == 0 ) + r->master_tsc_stamp = tsc = rdtsc_ordered(); + else if ( r->master_tsc_stamp < r->max_tsc_stamp ) + /* + * We want to avoid moving the TSC backwards for any CPU. + * Use the largest value observed anywhere on the first + * iteration. + */ + r->master_tsc_stamp = r->max_tsc_stamp; else if ( i == 0 ) r->master_stime = read_platform_stime(NULL); @@ -1712,6 +1720,15 @@ static void time_calibration_tsc_rendezv while ( atomic_read(&r->semaphore) < total_cpus ) cpu_relax(); + if ( tsc == 0 ) + { + uint64_t cur = ACCESS_ONCE(r->max_tsc_stamp); + + tsc = rdtsc_ordered(); + while ( tsc > cur ) + cur = cmpxchg(&r->max_tsc_stamp, cur, tsc); + } + if ( i == 0 ) write_tsc(r->master_tsc_stamp); @@ -1719,9 +1736,12 @@ static void time_calibration_tsc_rendezv while ( atomic_read(&r->semaphore) > total_cpus ) cpu_relax(); } + + /* Just in case a read above ended up reading zero. */ + tsc += !tsc; } - time_calibration_rendezvous_tail(r, r->master_tsc_stamp); + time_calibration_rendezvous_tail(r, tsc, r->master_tsc_stamp); } /* Ordinary rendezvous function which does not modify TSC values. */ @@ -1746,7 +1766,7 @@ static void time_calibration_std_rendezv smp_rmb(); /* receive signal /then/ read r->master_stime */ } - time_calibration_rendezvous_tail(r, rdtsc_ordered()); + time_calibration_rendezvous_tail(r, 0, rdtsc_ordered()); } /*