From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Joao Martins <joao.m.martins@oracle.com>, xen-devel@lists.xen.org
Cc: Keir Fraser <keir@xen.org>, Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH 2/5] x86/time: implement tsc as clocksource
Date: Fri, 18 Mar 2016 20:21:09 +0000 [thread overview]
Message-ID: <56EC6335.7010502@citrix.com> (raw)
In-Reply-To: <1458231136-13457-3-git-send-email-joao.m.martins@oracle.com>
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 <joao.m.martins@oracle.com>
> ---
> Cc: Keir Fraser <keir@xen.org>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Some style corrections, but no functional problems.
Reviewed-by Andrew Cooper <andrew.cooper3@citrix.com>
>
> Changes since RFC:
> - Spelling fixes in the commit message.
> - Remove unused clocksource_is_tsc variable and introduce it instead
> on the patch that uses it.
> - Move plt_tsc from second to last in the available clocksources.
> ---
> xen/arch/x86/time.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 60 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/time.c b/xen/arch/x86/time.c
> index 687e39b..1311c58 100644
> --- a/xen/arch/x86/time.c
> +++ b/xen/arch/x86/time.c
> @@ -432,6 +432,63 @@ uint64_t ns_to_acpi_pm_tick(uint64_t ns)
> }
>
> /************************************************************
> + * PLATFORM TIMER 4: TSC
> + */
> +static u64 tsc_freq;
> +static unsigned long tsc_max_warp;
> +static void tsc_check_reliability(void);
> +
> +static int __init init_tsctimer(struct platform_timesource *pts)
> +{
> + bool_t tsc_reliable = 0;
> +
> + tsc_check_reliability();
> +
> + if ( tsc_max_warp > 0 )
> + {
> + tsc_reliable = 0;
> + printk("TSC: didn't passed warp test\n");
printk(XENLOG_INFO "TSC ...
> + }
> + else if ( boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
> + ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
> + boot_cpu_has(X86_FEATURE_NONSTOP_TSC) ) )
You don't need extra spaces on inner brackets, so
boot_cpu_has(X86_FEATURE_TSC_RELIABLE) ||
(boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
boot_cpu_has(X86_FEATURE_NONSTOP_TSC)) )
is fine.
> + {
> + tsc_reliable = 1;
> + }
> + else if ( boot_cpu_has(X86_FEATURE_CONSTANT_TSC) )
> + {
> + tsc_reliable = (max_cstate <= 2);
> +
> + if (tsc_reliable)
Please add some extra ones here though.
> + printk("TSC: no deep Cstates, deemed reliable\n");
> + else
> + printk("TSC: deep Cstates possible, so not reliable\n");
XENLOG_INFO as well please.
~Andrew
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next prev parent reply other threads:[~2016-03-18 20:21 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 [this message]
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
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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