Re: [PATCH v5 3/5] x86/time: implement tsc as clocksource

["Jan Beulich" <JBeulich@suse.com>]

>>> On 23.09.16 at 12:42, <joao.m.martins@oracle.com> wrote:
> Recent x86/time changes improved a lot of the monotonicity in xen
> timekeeping, making it much harder to observe time going backwards.
> Although platform timer can't be expected to be perfectly in sync with
> TSC and so get_s_time won't be guaranteed to always return
> monotonically increasing values across cpus. This is the case in some
> of the boxes I am testing with, observing sometimes ~100 warps (of
> very few nanoseconds each) after a few hours.
> 
> This patch introduces support for using TSC as platform time source
> which is the highest resolution time and most performant to get.
> 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 in all cases (I
> believe Intel to be the only that can guarantee that?). For this reason
> it's not used unless administrator changes "clocksource" boot option
> to "tsc". Initializing TSC clocksource requires all CPUs up to have
> the tsc reliability checks performed. init_xen_time is called before
> all CPUs are up, so for example we would start with HPET (or ACPI,
> PIT) at boot time, and switch later to TSC. The switch then happens on
> verify_tsc_reliability initcall that is invoked when all CPUs are up.
> When attempting to initialize TSC we also check for time warps and if
> it has invariant TSC. Note that while we deem reliable a CONSTANT_TSC
> with no deep C-states, it might not always be the case, so we're
> conservative and allow TSC to be used as platform timer only with
> invariant TSC. Additionally we check if CPU Hotplug isn't meant to be
> performed on the host which will either be when max vcpus and
> num_present_cpu are the same. This is because a newly hotplugged CPU
> may not satisfy the condition of having all TSCs synchronized - so
> when having tsc clocksource being used we allow offlining CPUs but not
> onlining any ones back. Finally we prevent TSC from being used as
> clocksource on multiple sockets because it isn't guaranteed to be
> invariant. Further relaxing of this last requirement is added in a
> separate patch, such that we allow vendors with such guarantee to use
> TSC as clocksource. In case any of these conditions is not met, we
> keep the clocksource that was previously initialized on init_xen_time.
> 
> Since b64438c7c ("x86/time: use correct (local) time stamp in
> constant-TSC calibration fast path") updates to cpu time use local
> stamps, which means platform timer is only used to seed the initial
> cpu time. We further introduce a new rendezvous function
> (nop_rendezvous) which doesn't require synchronization between master
> and slave CPUS and just reads calibration_rendezvous struct and writes
> it down the stime and stamp to the cpu_calibration struct to be used
> later on. With clocksource=tsc there is no need to be in sync with
> another clocksource, so we reseed the local/master stamps to be values
> of TSC and update the platform time stamps accordingly. Time
> calibration is set to 1sec after we switch to TSC, thus these stamps
> are reseeded to also ensure monotonic returning values right after the
> point we switch to TSC. This is to remove the possibility of having
> inconsistent readings in this short period (i.e. until calibration
> fires).
> 
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>

with one further adjustment (no need to re-send):

> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -264,9 +264,13 @@ minimum of 32M, subject to a suitably aligned and sized contiguous
>  region of memory being available.
>  
>  ### clocksource
> -> `= pit | hpet | acpi`
> +> `= pit | hpet | acpi | tsc`
>  
>  If set, override Xen's default choice for the platform timer.
> +Having TSC as platform timer requires being explicitly set. This is because
> +TSC can only be safely used if CPU hotplug isn't performed on the system. In
> +some platforms, "maxcpus" parameter may require further adjustment to the
> +number of online cpus.

I think I'll re-write this last sentence to "On some platforms,
the "maxcpus" option may need to be used to further adjust the
number of allowed CPUs."

Jan

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