linux-kernel - Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts

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Date:   Mon, 09 Jan 2017 14:05:43 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     "devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alex Ng <alexng@...rosoft.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH v2 4/4] hv_util: improve time adjustment accuracy by disabling interrupts

John Stultz <john.stultz@...aro.org> writes:

> On Wed, Jan 4, 2017 at 9:24 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>> If we happen to receive interrupts during hv_set_host_time() execution
>> our adjustments may get inaccurate. Make the whole function atomic.
>> Unfortunately, we can's call do_settimeofday64() with interrupts
>> disabled as some cross-CPU work is being done but this call happens
>> very rarely.
>>
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>> ---
>>  drivers/hv/hv_util.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/hv/hv_util.c b/drivers/hv/hv_util.c
>> index 7e97231..4e50a42 100644
>> --- a/drivers/hv/hv_util.c
>> +++ b/drivers/hv/hv_util.c
>> @@ -187,6 +187,9 @@ static void hv_set_host_time(struct work_struct *work)
>>         struct timespec64 host_ts, our_ts;
>>         struct timex txc = {0};
>>         int ret;
>> +       unsigned long flags;
>> +
>> +       local_irq_save(flags);
>>
>>         wrk = container_of(work, struct adj_time_work, work);
>>
>> @@ -218,6 +221,7 @@ static void hv_set_host_time(struct work_struct *work)
>>          * ordered to sync our time by the host.
>>          */
>>         if (abs(delta) > MAXPHASE || wrk->flags & ICTIMESYNCFLAG_SYNC) {
>> +               local_irq_restore(flags);
>>                 do_settimeofday64(&host_ts);
>>                 return;
>>         }
>> @@ -232,6 +236,8 @@ static void hv_set_host_time(struct work_struct *work)
>>         ret = do_adjtimex(&txc);
>>         if (ret)
>>                 pr_debug("Failed to adjust system time: %d\n", ret);
>> +
>> +       local_irq_restore(flags);
>
> This seems like a long time to disable irqs for what your trying to
> do. I'd guess you really only want to disable irqs while you aquire
> the host and guest timestamps (so they are as close together as
> possible).  Since the delta is then calculated, I'm not sure what
> disabling irqs for calling adjtimex gets you.
>

True,

interrupts disabling would only be beneficial for the
do_settimeofday64() case but we can't actually do it there. I'll shorten
the interrupts disabling to the delta calculation only.

-- 
  Vitaly