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From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: xen-devel <xen-devel@lists.xenproject.org>,
	Dario Faggioli <dario.faggioli@citrix.com>,
	Joao Martins <joao.m.martins@oracle.com>
Subject: Re: [PATCH 3/8] x86/time: introduce and use rdtsc_ordered()
Date: Mon, 20 Jun 2016 14:07:45 +0100	[thread overview]
Message-ID: <38e20462-a6e9-8e84-b129-64eb1e8ef773@citrix.com> (raw)
In-Reply-To: <5768067302000078000F6CBD@prv-mh.provo.novell.com>

On 20/06/16 14:06, Jan Beulich wrote:
>>>> On 20.06.16 at 14:59, <andrew.cooper3@citrix.com> wrote:
>> On 15/06/16 11:27, Jan Beulich wrote:
>>> Matching Linux commit 03b9730b76 ("x86/asm/tsc: Add rdtsc_ordered() and
>>> use it in trivial call sites") and earlier ones it builds upon, let's
>>> make sure timing loops don't have their rdtsc()-s re-ordered, as that
>>> would harm precision of the result (values were observed to be several
>>> hundred clocks off without this adjustment).
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
> I have these two additional hunks for v2:
>
> @@ -1124,16 +1124,13 @@ static void local_time_calibration(void)
>   */
>  static void check_tsc_warp(unsigned long tsc_khz, unsigned long *max_warp)
>  {
> -#define rdtsc_barrier() mb()
>      static DEFINE_SPINLOCK(sync_lock);
>      static cycles_t last_tsc;
>  
>      cycles_t start, now, prev, end;
>      int i;
>  
> -    rdtsc_barrier();
> -    start = get_cycles();
> -    rdtsc_barrier();
> +    start = rdtsc_ordered();
>  
>      /* The measurement runs for 20 msecs: */
>      end = start + tsc_khz * 20ULL;
> @@ -1148,9 +1145,7 @@ static void check_tsc_warp(unsigned long
>           */
>          spin_lock(&sync_lock);
>          prev = last_tsc;
> -        rdtsc_barrier();
> -        now = get_cycles();
> -        rdtsc_barrier();
> +        now = rdtsc_ordered();
>          last_tsc = now;
>          spin_unlock(&sync_lock);
>  
>
> May I consider those covered as well?

Yes.

I need to dust off my series removing most of the misuse of mb() in the
Xen codebase now that 4.8 is open.

~Andrew

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  reply	other threads:[~2016-06-20 13:07 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-06-15  9:50 [PATCH 0/8] x86/time: improve cross-CPU clock monotonicity (and more) Jan Beulich
2016-06-15 10:26 ` [PATCH 1/8] " Jan Beulich
2016-06-15 10:32   ` Jan Beulich
2016-06-15 22:51   ` Joao Martins
2016-06-16  8:27     ` Jan Beulich
2016-06-16 20:27       ` Joao Martins
2016-06-17  7:32         ` Jan Beulich
2016-06-21 12:05           ` Joao Martins
2016-06-21 12:28             ` Jan Beulich
2016-06-21 13:57               ` Joao Martins
2016-08-02 19:30   ` Andrew Cooper
2016-06-15 10:26 ` [PATCH 2/8] x86: also generate assembler usable equates for synthesized features Jan Beulich
2016-06-20 12:50   ` Andrew Cooper
2016-06-15 10:27 ` [PATCH 3/8] x86/time: introduce and use rdtsc_ordered() Jan Beulich
2016-06-20 12:59   ` Andrew Cooper
2016-06-20 13:06     ` Jan Beulich
2016-06-20 13:07       ` Andrew Cooper [this message]
2016-07-11 11:39     ` Dario Faggioli
2016-06-15 10:28 ` [PATCH 4/8] x86/time: calibrate TSC against platform timer Jan Beulich
2016-06-20 14:20   ` Andrew Cooper
2016-06-20 15:19     ` Jan Beulich
2016-08-02 19:25       ` Andrew Cooper
2016-08-03  9:32         ` Jan Beulich
2016-06-15 10:28 ` [PATCH 5/8] x86/time: correctly honor late clearing of TSC related feature flags Jan Beulich
2016-06-20 14:32   ` Andrew Cooper
2016-06-20 15:20     ` Jan Beulich
2016-07-04 15:39       ` Andrew Cooper
2016-07-04 15:53         ` Jan Beulich
2016-08-02 19:08           ` Andrew Cooper
2016-08-03  9:43             ` Jan Beulich
2016-08-31 13:42               ` Andrew Cooper
2016-08-31 14:03                 ` Jan Beulich
2016-06-15 10:29 ` [PATCH 6/8] x86/time: support 32-bit wide ACPI PM timer Jan Beulich
2016-07-04 15:40   ` Andrew Cooper
2016-06-15 10:30 ` [PATCH 7/8] x86/time: fold recurring code Jan Beulich
2016-07-04 15:43   ` Andrew Cooper
2016-06-15 10:30 ` [PATCH 8/8] x86/time: group time stamps into a structure Jan Beulich
2016-07-04 15:57   ` Andrew Cooper
2016-07-01  7:44 ` Ping: [PATCH 0/8] x86/time: improve cross-CPU clock monotonicity (and more) Jan Beulich

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