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 15:56:15 +0100 [thread overview]
Message-ID: <5703D20F.6080407@oracle.com> (raw)
In-Reply-To: <5703B30002000078000E320D@prv-mh.provo.novell.com>
On 04/05/2016 11:43 AM, Jan Beulich wrote:
>>>> On 29.03.16 at 15:44, <joao.m.martins@oracle.com> 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 only set when adminstrator
>> changes "clocksource" boot option to "tsc". Initializing TSC
>> clocksource requires all CPUs to have the tsc reliability checks
>> performed. init_xen_time is called before all CPUs are up, and so we
>> start with HPET at boot time, and switch later to TSC.
>
> Please don't make possibly wrong statements: There's no guarantee
> we'd start with HPET - might as well be PMTMR or PIT.
My apologies - while the commit message doesn't express this, I meant it to be
"for example we would start with HPET (...)". I gave that example as it's the
one preferable in plt_timers.
>
>> The switch then
>> happens on verify_tsc_reliability initcall that is invoked when all
>> CPUs are up. When attempting to initializing TSC 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 keep the clocksource that was previously initialized on
>> init_xen_time.
>
> DYM "And in case any of these conditions is not met, ..."?
Yes. My use of "none" may be wrong here so just to be sure what I mean is: (...)
If all of the conditions in the previous sentence are not met (...)
> Or,
> looking at the code, something more complex?
>
>> --- 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;
>
> Redundant assignment.
>
Yes, Konrad had point that out too. Fixed that.
>> +static void resume_tsctimer(struct platform_timesource *pts)
>> +{
>> +}
>
> No need for an empty function - other clock sources don't have
> such empty stubs either (i.e. the caller checks for NULL).
>
OK.
>> @@ -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?
>
>> @@ -566,7 +642,9 @@ static void __init init_platform_timer(void)
>> struct platform_timesource *pts = NULL;
>> int i, rc = -1;
>>
>> - if ( opt_clocksource[0] != '\0' )
>> + /* clocksource=tsc is initialized later when all CPUS are up */
>> + if ( (opt_clocksource[0] != '\0') &&
>> + (strcmp(opt_clocksource, "tsc") != 0) )
>
> In line with the inverse check done below (using ! instead of == 0)
> please omit the != 0 here.
>
OK.
>> @@ -1437,6 +1515,20 @@ static int __init verify_tsc_reliability(void)
>> }
>> }
>>
>> + if ( !strcmp(opt_clocksource, "tsc") )
>> + {
>> + if ( try_platform_timer(&plt_tsc) > 0 )
>> + {
>> + printk(XENLOG_INFO "Switched to Platform timer %s TSC\n",
>> + freq_string(plt_src.frequency));
>> +
>> + init_percpu_time();
>
> So you re-do this for the BP, but not for any of the APs?
I had overlooked the invocation made in start_secondary(). I also need to redo
this on the APs.
>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
>
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next prev parent reply other threads:[~2016-04-05 14:56 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 [this message]
2016-04-05 15:12 ` Jan Beulich
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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