From: Joao Martins <email@example.com> To: Jan Beulich <JBeulich@suse.com> Cc: Andrew Cooper <firstname.lastname@example.org>, Keir Fraser <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH 2/5] x86/time: implement tsc as clocksource Date: Wed, 23 Mar 2016 12:05:12 +0000 [thread overview] Message-ID: <56F28678.email@example.com> (raw) In-Reply-To: <56F2539D02000078000DF78F@prv-mh.provo.novell.com> On 03/23/2016 07:28 AM, Jan Beulich wrote: >>>> On 22.03.16 at 21:40, <firstname.lastname@example.org> wrote: >> On 03/22/2016 04:02 PM, Jan Beulich wrote: >>>>>> On 22.03.16 at 16:51, <email@example.com> wrote: >>>> On 03/22/2016 12:46 PM, Jan Beulich wrote: >>>>>>>> On 22.03.16 at 13:41, <firstname.lastname@example.org> wrote: >>>>>>> >>>>>> I found out one issue in the tsc clocksource initialization path with respect to >>>>>> the reliability check. This check is running when initializing platform timer, >>>>>> but not all CPUS are up at that point (init_xen_time()) which means that the >>>>>> check will always succeed. So for clocksource=tsc I need to defer initialization >>>>>> to a later point (on verify_tsc_reliability()) and if successful switch to that >>>>>> clocksource. Unless you disagree, v2 would have this and just requires one >>>>>> additional preparatory change prior to this patch. >>>>> >>>>> Hmm, that's suspicious when thinking about CPU hotplug: What >>>>> do you intend to do when a CPU comes online later, failing the >>>>> check? >>>> Good point, but I am not sure whether that would happen. The initcall >>>> verify_tsc_reliability seems to be called only for the boot processor. Or are >>>> you saying that it's case that initcalls are called too when hotplugging cpus >>>> later on? If that's the case then my suggestion wouldn't work as you point out - >>>> or rather without having runtime switching support as you point out below. >>> >>> Looks like I didn't express myself clearly enough: "failing the check" >>> wasn't meant to imply the failure would actually occur, but rather >>> that failure would occur if the check was run on that CPU. IOW the >>> case of a CPU getting hotplugged which doesn't satisfy the needs >>> for selecting TSC as the clock source. >> Ah, I see. I believe this wouldn't be an issue with the current rendezvous >> mechanism (std_rendezvous), as the delta is computed between local_tsc_stamp >> taken in that (hotplugged) CPU in the calibration and the rdtsc() in the >> guest >> same CPU, even though having CPU0 TSC (master) as system_time. >> >> However it can be a problem with PVCLOCK_TSC_STABLE_BIT as the hotplugged CPU >> could have its TSC drifted, and since setting this flag relies on >> synchronization of TSCs we would need to clear the flag enterily. > > Except that we can't, after guests already got started, validly clear > that flag afaics. Correct. > The only option I see here would be to never set > this flag if CPU hotplug is possible - by looking at the hot pluggable > CPU count and, if non-zero, perhaps allowing a command line > override to indicate no hotplug is intended (it may well be that such > is already implicitly available). OK, will add this then to allow the flag only if the conditions above are met. Thanks for the pointer! >>>>> So far we don't do runtime switching of the clock source, >>>>> and I'm not sure that's a good idea to do when there are running >>>>> guests. >>>> Totally agree, but I would be proposing to be at initialization phase where >>>> there wouldn't be guests running. We would start with HPET, then only switch >>>> to >>>> TSC if that check has passed (and would happen once). >>>> >>>> Simpler alternative would be to call init_xen_time after all CPUs are >>>> brought up >>>> (and would also keep this patch as is), but I am not sure about the >>>> repercussions of that. >>> >>> I don't see how either would help with the hotplug case. >> This was in response to what I thought you meant with your earlier question >> (which I misunderstood). But my question is still valid I believe. The >> reason >> for choosing between my suggested approaches is that tsc_check_reliability() >> requires all CPUs up so that the check is correctly performed. > > Sure - it seems quite obvious that all boot time available CPUs > should be checked. Cool, so I will go with moving init_xen_time right after all CPUs are up but before initcalls are invoked. Joao _______________________________________________ Xen-devel mailing list Xenemail@example.com http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-03-23 12:05 UTC|newest] Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-03-17 16:12 [PATCH 0/5] x86/time: PVCLOCK_TSC_STABLE_BIT support Joao Martins 2016-03-17 16:12 ` [PATCH 1/5] public/xen.h: add flags field to vcpu_time_info Joao Martins 2016-03-18 20:12 ` Andrew Cooper 2016-03-21 11:42 ` Joao Martins 2016-03-21 11:43 ` Andrew Cooper 2016-03-21 11:51 ` Joao Martins 2016-03-21 15:10 ` Jan Beulich 2016-03-21 15:27 ` Andrew Cooper 2016-03-21 15:40 ` Joao Martins 2016-03-17 16:12 ` [PATCH 2/5] x86/time: implement tsc as clocksource Joao Martins 2016-03-18 20:21 ` Andrew Cooper 2016-03-21 11:43 ` Joao Martins 2016-03-22 12:41 ` Joao Martins 2016-03-22 12:46 ` Jan Beulich 2016-03-22 15:51 ` Joao Martins 2016-03-22 16:02 ` Jan Beulich 2016-03-22 20:40 ` Joao Martins 2016-03-23 7:28 ` Jan Beulich 2016-03-23 12:05 ` Joao Martins [this message] 2016-03-23 14:05 ` Jan Beulich 2016-03-17 16:12 ` [PATCH 3/5] x86/time: streamline platform time init on plt_init() Joao Martins 2016-03-18 20:32 ` Andrew Cooper 2016-03-21 11:45 ` Joao Martins 2016-03-17 16:12 ` [PATCH 4/5] x86/time: refactor read_platform_stime() Joao Martins 2016-03-18 20:34 ` Andrew Cooper 2016-03-21 11:45 ` Joao Martins 2016-03-21 13:08 ` Andrew Cooper 2016-03-17 16:12 ` [PATCH 5/5] x86/time: implement PVCLOCK_TSC_STABLE_BIT Joao Martins 2016-03-18 20:58 ` Andrew Cooper 2016-03-21 11:50 ` Joao Martins
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