Messages in this thread | | | From | Andi Kleen <> | Subject | Re: [patch] x86: unify/rewrite SMP TSC sync code | Date | Fri, 24 Nov 2006 22:06:36 +0100 |
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On Friday 24 November 2006 21:46, Ingo Molnar wrote: > > * Andi Kleen <ak@suse.de> wrote: > > > > > > yeah - the main new bit for x86-64 is the unconditional check for time > > > warps. We shouldnt (and cannot) really trust the CPU and the board/BIOS > > > getting it right. There were always some motherboards using Intel CPUs > > > that had the TSCs wrong. > > > > In the 64bit capable generation I don't know of any run in spec > > (except for multinode systems and there was one overclocked system > > where the cores got unsync, but overclocking is an operator error) > > i have one (Intel based), 64-bit, fully in spec, which is off by > ~3000-4000 cycles. So it happens.
More details?
> I was in fact surprised when i noticed that you removed the > unconditional TSC check that i put there years ago
I removed it because you pointed out that it usually caused trouble on Intel systems: we would always detect errors due to measurement errors and then make things worse by trying to fix it.
But you're right it might have been better to keep a check with a threshold to catch totally broken cases.
> but which apps are using RDTSC natively? Trapping isnt too good i agree
The only sure way would be to trap+printk -- but from previous user complaints it's a substantial number.
> - if then we should remove it from the CPU features and hence apps wont > (or shouldnt) use it.
I doubt the majority checks any cpu features first ...
> > > > nor can the TSC really be synced up properly in the hotplug CPU > > > case, after the fact - what if the app already read out an older TSC > > > value and a new CPU is added. If the TSC isnt sync on SMP then it > > > quickly gets pretty messy, and we should rather take a look at /why/ > > > these apps are using RDTSC. > > > > Because gettimeofday is too slow. > > as i indicated it in another discussion, i can fix that. Next patch will > be that.
Well I hope it's not making it HZ resolution. As noted earlier we tried that already and it didn't work (it violates the "forward monotonity" that is commonly expected)
Ok I could imagine it making sense as a new CLOCK_FASTBUTLOUSYRESOLUTION timer in clock_gettime() [together with the new vdso fastpath], but not as default.
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