Re: [Xen-devel] [PATCH 0/2] make hypercall preemption checks consistent

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 04/03/14 12:00, Jan Beulich wrote:
>>>> On 04.03.14 at 12:52, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 04/03/14 11:21, Jan Beulich wrote:
>>> - never preempt on the first iteration (ensure forward progress)
>>> - never preempt on the last iteration (pointless/wasteful)
>>> - do cheap checks first
>>>
>>> 1: common: make hypercall preemption checks consistent
>>> 2: x86: make hypercall preemption checks consistent
>>>
>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>> All in all, this is a good improvement over what is currently present.
>>
>> However, given the overhead of creating continuations (particularly for
>> 32bit HVM guests, which have been seen to unconditionally fail the
>> preemption check by the time the compat layer has run), some of these
>> operations would probably be better having more than a single guaranteed
>> operation.
> 
> I agree, but I wanted to do one step at a time. Judging how much
> work we want to permit done between preemption points will be
> either heavy guess work, or require quite a bit of performance
> measurement...

Perhaps something time-based?  Record the time at start and make
hypercall_preempt_check() return true if more than T time has elapsed?

David

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