From: Joao Martins <joao.m.martins@oracle.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
Keir Fraser <keir@xen.org>,
xen-devel@lists.xen.org
Subject: Re: [PATCH v2 3/6] x86/time: implement tsc as clocksource
Date: Tue, 5 Apr 2016 18:07:35 +0100 [thread overview]
Message-ID: <5703F0D7.4080708@oracle.com> (raw)
In-Reply-To: <5703F20C02000078000E3414@prv-mh.provo.novell.com>
On 04/05/2016 04:12 PM, Jan Beulich wrote:
>>>> On 05.04.16 at 16:56, <joao.m.martins@oracle.com> wrote:
>> On 04/05/2016 11:43 AM, Jan Beulich wrote:
>>>>>> On 29.03.16 at 15:44, <joao.m.martins@oracle.com> wrote:
>>>> @@ -541,6 +613,10 @@ static int __init try_platform_timer(struct platform_timesource *pts)
>>>> if ( rc <= 0 )
>>>> return rc;
>>>>
>>>> + /* We have a platform timesource already so reset it */
>>>> + if ( plt_src.counter_bits != 0 )
>>>> + reset_platform_timer();
>>>
>>> What if any of the time functions get used between here and the
>>> setting of the new clock source? For example, what will happen to
>>> log messages when "console_timestamps=..." is in effect?
>> time functions will use NOW() (i.e. get_s_time) which uses cpu_time structs
>> being updated on local_time_calibration() or cpu frequency changes.
>> reset_platform_timer() will zero out some of the variables used by the
>> plt_overflow and read_platform_stime(). The overflow and calibration isn't an
>> issue as the timers are previously killed so any further calls will use what's
>> on cpu_time while plt_src is being changed.
>>
>> The case I could see this being different is if cpu_frequency_change is called
>> between reset_platform_time() and the next update of stime_platform_stamp. In
>> this situation the calculation would end up a bit different meaning subsequent
>> calls of read_platform_stime() would return the equivalent to
>> scale_delta(plt_src->read_counter(), plt_scale), as opposed to computing with a
>> previous taken stime_platform_stamp and incrementing a difference with a counter
>> read. But can this situation actually happen?
>
> No matter which CPU freq model is being used (right now, may
> change when the Intel P-state driver finally arrives), Dom0 is
> required to be executing already, so no issue here.
Cool.
> Since you didn't go into any detail on the specific example of log
> time stamps - am I to imply that they're completely unaffected by
> this (i.e. there's also no risk of them going backwards for a brief
> period of time)?
log timestamps uses wallclock_time() function which uses NOW() and follows the
first paragraph I explained. But I need to correct that statement as get_s_time
uses a previously seeded value from stime_local_stamp (which is set through
read_platform_time()). Time going backwards could happen only if TSC is behind
the currently-in-use clocksource. During that brief period it uses the values
taken from cpu_time but after I set the new clocksource it would go backwards as
I zeroed the previous stime/stamp snapshots in reset_platform_time. In this case
with I propose I don't have any other timestamps (platform_timer_stamp =
(stime_platform_stamp = 0)) to calculate the difference with, so it would go
backwards if the TSC falls behind. I could prevent this situation from happening
by having an offset which would be used until the new clocksource catches up to
the old one? I am also searching in the manual, if that can situation can happen.
Joao
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next prev parent reply other threads:[~2016-04-05 17:07 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-29 13:44 [PATCH v2 0/6] x86/time: PVCLOCK_TSC_STABLE_BIT support Joao Martins
2016-03-29 13:44 ` [PATCH v2 1/6] public/xen.h: add flags field to vcpu_time_info Joao Martins
2016-03-30 15:49 ` Ian Jackson
2016-03-30 16:33 ` Joao Martins
2016-03-31 7:09 ` Jan Beulich
2016-03-31 7:13 ` Jan Beulich
2016-03-31 11:04 ` Joao Martins
2016-04-05 10:16 ` Jan Beulich
2016-04-05 10:59 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 2/6] x86/time: refactor init_platform_time() Joao Martins
2016-04-01 16:10 ` Konrad Rzeszutek Wilk
2016-04-01 18:26 ` Joao Martins
2016-04-05 10:09 ` Jan Beulich
2016-04-05 10:55 ` Joao Martins
2016-04-05 11:16 ` Jan Beulich
2016-03-29 13:44 ` [PATCH v2 3/6] x86/time: implement tsc as clocksource Joao Martins
2016-03-29 17:39 ` Konrad Rzeszutek Wilk
2016-03-29 17:52 ` Joao Martins
2016-04-01 16:43 ` Konrad Rzeszutek Wilk
2016-04-01 18:38 ` Joao Martins
2016-04-01 18:45 ` Konrad Rzeszutek Wilk
2016-04-03 18:47 ` Joao Martins
2016-04-05 10:43 ` Jan Beulich
2016-04-05 14:56 ` Joao Martins
2016-04-05 15:12 ` Jan Beulich
2016-04-05 17:07 ` Joao Martins [this message]
2016-03-29 13:44 ` [PATCH v2 4/6] x86/time: streamline platform time init on plt_init() Joao Martins
2016-04-05 11:46 ` Jan Beulich
2016-04-05 15:12 ` Joao Martins
2016-04-05 15:22 ` Jan Beulich
2016-04-05 17:17 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 5/6] x86/time: refactor read_platform_stime() Joao Martins
2016-04-01 18:32 ` Konrad Rzeszutek Wilk
2016-04-05 11:52 ` Jan Beulich
2016-04-05 15:22 ` Joao Martins
2016-04-05 15:26 ` Jan Beulich
2016-04-05 17:08 ` Joao Martins
2016-03-29 13:44 ` [PATCH v2 6/6] x86/time: implement PVCLOCK_TSC_STABLE_BIT Joao Martins
2016-04-05 12:22 ` Jan Beulich
2016-04-05 21:34 ` Joao Martins
2016-04-07 15:58 ` Jan Beulich
2016-04-07 21:17 ` Joao Martins
2016-04-07 21:32 ` Jan Beulich
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