From: Joao Martins <email@example.com> To: Jan Beulich <JBeulich@suse.com> Cc: Andrew Cooper <firstname.lastname@example.org>, Dario Faggioli <email@example.com>, Wei Liu <firstname.lastname@example.org>, xen-devel <email@example.com> Subject: Re: [PATCH] x86/time: use correct (local) time stamp in constant-TSC calibration fast path Date: Thu, 9 Jun 2016 19:19:04 +0100 [thread overview] Message-ID: <5759B318.firstname.lastname@example.org> (raw) In-Reply-To: <5759ACD802000078000F3A78@prv-mh.provo.novell.com> [changing Dario address to citrix.com as it was bouncing for me ] On 06/09/2016 04:52 PM, Jan Beulich wrote: >>>> On 09.06.16 at 17:00, <email@example.com> wrote: >> On 06/09/2016 01:57 PM, Jan Beulich wrote: >>>>>> On 09.06.16 at 14:11, <firstname.lastname@example.org> wrote: >>> So in effect for the fast path the patch >>> changes the situation from c->stime_local_stamp being effectively >>> unused to c->stime_master_stamp being so. In the former case, if >>> that really hadn't been a typo, deleting the write of that field from >>> time_calibration_std_rendezvous() would have made sense, as >>> get_s_time() certainly is more overhead than the simply memory >>> read and write needed for keeping c->stime_master_stamp up to >>> date (despite not being used). >> I agree, but what I meant previously was more of a concern meaning: CPU 0 is >> doing an >> expensive read_platform_time (e.g. HPET supposedly microseconds order, plus >> a >> non-contented lock) to set stime_master_stamp that doesn't get used at all - >> effectively not using the clocksource set initially at boot. > > Yeah, there's likely some cleanup potential here, but of course we > need to be pretty careful about not doing something that may be > needed by some code paths but not others. But if you think you > can help the situation without harming maintainability, feel free to > go ahead. > OK, Makes sense. I'll likely do already so of it on my related series. >> What if verify_tsc_reliability clears out X86_FEATURE_TSC_RELIABLE when >> failing >> the warp test? The calibration function is set early on right after >> interrupts are >> enabled and the time warp check later on when all CPUs are up. So on >> problematic >> platforms it's possible that std_rendezvous is used with a constant TSC but >> still >> deemed unreliable. We still keep incrementing deltas at roughly about the >> same time, >> but in effect with this change the stime_local_stamp would be TSC-only based >> thus >> leading to warps with an unreliable TSC? And there's also the CPU >> hotplug/onlining >> case that we once discussed. > > I agree that we're likely in trouble in such a case. But for the > moment I'd be glad if we could get the "normal" case work right. > OK. Apologies for the noise, I was just pointing out things that I tried and some also discussed here in the PVCLOCK_TSC_STABLE_BIT series, although didn't cross me that Xen own idea of time could be a little broken. IMO adding another clocksource for TSC would be more correct if we are only using TSC (and having its associated limitations made aware/explicit to the user) rather then being on the back of another clocksource in use. But it wouldn't cover the normal case :( unless set manually NB: Guests on the other hand aren't affected since they take care of keeping the latest stamp when different vCPUS slightly diverge. Joao _______________________________________________ Xen-devel mailing list Xenemail@example.com http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-06-09 18:18 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-06-09 12:01 Jan Beulich 2016-06-09 12:10 ` Andrew Cooper 2016-06-09 12:11 ` Joao Martins 2016-06-09 12:57 ` Jan Beulich 2016-06-09 15:00 ` Joao Martins 2016-06-09 15:52 ` Jan Beulich 2016-06-09 18:19 ` Joao Martins [this message] 2016-06-10 6:59 ` Jan Beulich 2016-06-10 9:29 ` Jan Beulich 2016-06-10 17:07 ` Joao Martins 2016-06-09 12:12 ` Wei Liu
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