linux-kernel - Re: [PATCH v2 5/7] KVM-GST: KVM Steal time accounting

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Date:	Mon, 20 Jun 2011 09:07:51 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Glauber Costa <glommer@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Anthony Liguori <aliguori@...ibm.com>,
	Eric B Munson <emunson@...bm.net>
Subject: Re: [PATCH v2 5/7] KVM-GST: KVM Steal time accounting

On 06/20/2011 05:38 AM, Glauber Costa wrote:
> On 06/19/2011 07:04 AM, Avi Kivity wrote:
>> On 06/17/2011 01:20 AM, Glauber Costa wrote:
>>> This patch accounts steal time time in kernel/sched.
>>> I kept it from last proposal, because I still see advantages
>>> in it: Doing it here will give us easier access from scheduler
>>> variables such as the cpu rq. The next patch shows an example of
>>> usage for it.
>>>
>>> Since functions like account_idle_time() can be called from
>>> multiple places, not only account_process_tick(), steal time
>>> grabbing is repeated in each account function separatedely.
>>>
>>> /*
>>> + * We have to at flush steal time information every time something 
>>> else
>>> + * is accounted. Since the accounting functions are all visible to
>>> the rest
>>> + * of the kernel, it gets tricky to do them in one place. This helper
>>> function
>>> + * helps us.
>>> + *
>>> + * When the system is idle, the concept of steal time does not apply.
>>> We just
>>> + * tell the underlying hypervisor that we grabbed the data, but skip
>>> steal time
>>> + * accounting
>>> + */
>>> +static inline bool touch_steal_time(int is_idle)
>>> +{
>>> + u64 steal, st = 0;
>>> +
>>> + if (static_branch(&paravirt_steal_enabled)) {
>>> +
>>> + steal = paravirt_steal_clock(smp_processor_id());
>>> +
>>> + steal -= this_rq()->prev_steal_time;
>>> + if (is_idle) {
>>> + this_rq()->prev_steal_time += steal;
>>> + return false;
>>> + }
>>> +
>>> + while (steal>= TICK_NSEC) {
>>> + /*
>>> + * Inline assembly required to prevent the compiler
>>> + * optimising this loop into a divmod call.
>>> + * See __iter_div_u64_rem() for another example of this.
>>> + */
>>
>> Why not use said function?
>
> because here we want to do work during each loop. The said function
> would have to be adapted for that, possibly using a macro, to run 
> arbitrary code during each loop iteration, in a way that I don't think
> it is worthy given the current number of callers (2 counting this new 
> one)

You mean adding to prev_steal_time?  That can be done outside the loop.

>
>>> + asm("" : "+rm" (steal));
>>> +
>>> + steal -= TICK_NSEC;
>>> + this_rq()->prev_steal_time += TICK_NSEC;
>>> + st++;
>>
>> Suppose a live migration or SIGSTOP causes lots of steal time. How long
>> will we spend here?
> Silly me. I actually used this same argument with Peter to cap it with 
> "delta" in the next patch in this series. So I think you are 100 % 
> right. Here, however, we do want to account all that time, I believe.
>
> How about we do a slow division if we're > 10 sec (unlikely), and 
> account everything as steal time in this scenario ?

Okay.  Division would be faster for a lot less than 10s though.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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