From: Joao Martins <joao.m.martins@oracle.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Jan Beulich <jbeulich@suse.com>,
xen-devel@lists.xen.org
Subject: Re: [PATCH 2/5] x86/time: implement tsc as clocksource
Date: Mon, 21 Mar 2016 11:43:01 +0000 [thread overview]
Message-ID: <56EFDE45.3090308@oracle.com> (raw)
In-Reply-To: <56EC6335.7010502@citrix.com>
On 03/18/2016 08:21 PM, Andrew Cooper wrote:
> 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>
>
I've fixed all your comments for v2, Thanks!
>>
>> 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
>
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-03-21 11:43 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
2016-03-21 11:43 ` Joao Martins [this message]
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=56EFDE45.3090308@oracle.com \
--to=joao.m.martins@oracle.com \
--cc=andrew.cooper3@citrix.com \
--cc=jbeulich@suse.com \
--cc=keir@xen.org \
--cc=xen-devel@lists.xen.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).