From: "Jan Beulich" <JBeulich@suse.com> To: Joao Martins <firstname.lastname@example.org> Cc: Andrew Cooper <email@example.com>, Keir Fraser <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH 2/5] x86/time: implement tsc as clocksource Date: Wed, 23 Mar 2016 01:28:13 -0600 [thread overview] Message-ID: <56F2539D02000078000DF78F@prv-mh.provo.novell.com> (raw) In-Reply-To: <56F1ADD6.firstname.lastname@example.org> >>> On 22.03.16 at 21:40, <email@example.com> wrote: > On 03/22/2016 04:02 PM, Jan Beulich wrote: >>>>> On 22.03.16 at 16:51, <firstname.lastname@example.org> wrote: >>> On 03/22/2016 12:46 PM, Jan Beulich wrote: >>>>>>> On 22.03.16 at 13:41, <email@example.com> 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. 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). >>>> 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. Jan _______________________________________________ Xen-devel mailing list Xenfirstname.lastname@example.org http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-03-23 7:28 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 [this message] 2016-03-23 12:05 ` Joao Martins 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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