From: "Jan Beulich" <JBeulich@suse.com> To: Joao Martins <email@example.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: Tue, 22 Mar 2016 06:46:10 -0600 [thread overview] Message-ID: <56F14CA202000078000DF28B@prv-mh.provo.novell.com> (raw) In-Reply-To: <56F13D92.email@example.com> >>> On 22.03.16 at 13:41, <firstname.lastname@example.org> wrote: > > On 03/18/2016 08:21 PM, Andrew Cooper wrote: >> On 17/03/16 16:12, Joao Martins wrote: >>> Introduce support for using TSC as platform time which is the highest >>> resolution time and most performant to get (~20 nsecs). Though there >>> are also several problems associated with its usage, and there isn't a >>> complete (and architecturally defined) guarantee that all machines >>> will provide reliable and monotonic TSC across all CPUs, on different >>> sockets and different P/C states. I believe Intel to be the only that >>> can guarantee that. For this reason it's set with less priority when >>> compared to HPET unless adminstrator changes "clocksource" boot option >>> to "tsc". Upon initializing it, we also check for time warps and >>> appropriate CPU features i.e. TSC_RELIABLE, CONSTANT_TSC and >>> NONSTOP_TSC. And in case none of these conditions are met, we fail in >>> order to fallback to the next available clocksource. >>> >>> It is also worth noting that with clocksource=tsc there isn't any >>> need to synchronise with another clocksource, and I could verify that >>> great portion the time skew was eliminated and seeing much less time >>> warps happening. With HPET I observed ~500 warps in the period >>> of 1h of around 27 us, and with TSC down to 50 warps in the same >>> period having each warp < 100 ns. The warps still exist though but >>> are only related to cross CPU calibration (being how much it takes to >>> rendezvous with master), in which a later patch in this series aims to >>> solve. >>> >>> Signed-off-by: Joao Martins <email@example.com> >>> --- >>> Cc: Keir Fraser <firstname.lastname@example.org> >>> Cc: Jan Beulich <email@example.com> >>> Cc: Andrew Cooper <firstname.lastname@example.org> >> >> Some style corrections, but no functional problems. >> >> Reviewed-by Andrew Cooper <email@example.com> >> > 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? 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. Jan _______________________________________________ Xen-devel mailing list Xenfirstname.lastname@example.org http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-03-22 12:46 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 [this message] 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 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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