From: "Jan Beulich" <JBeulich@suse.com>
To: Joao Martins <joao.m.martins@oracle.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, 05 Apr 2016 09:12:44 -0600 [thread overview]
Message-ID: <5703F20C02000078000E3414@prv-mh.provo.novell.com> (raw)
In-Reply-To: <5703D20F.6080407@oracle.com>
>>> 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.
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)?
Jan
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next prev parent reply other threads:[~2016-04-05 15:12 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 [this message]
2016-04-05 17:07 ` Joao Martins
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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