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Re: [qemu-s390x] [PATCH v1 for-2-12 06/15] s390x/flic: factor out inject

From: David Hildenbrand
Subject: Re: [qemu-s390x] [PATCH v1 for-2-12 06/15] s390x/flic: factor out injection of floating interrupts
Date: Wed, 13 Dec 2017 10:31:58 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 13.12.2017 10:16, Christian Borntraeger wrote:
> On 12/12/2017 04:28 PM, Cornelia Huck wrote:
>> On Tue, 12 Dec 2017 16:17:17 +0100
>> David Hildenbrand <address@hidden> wrote:
>>> On 12.12.2017 15:29, Cornelia Huck wrote:
>>>> On Tue, 12 Dec 2017 15:13:46 +0100
>>>> Christian Borntraeger <address@hidden> wrote:
>>>>> On 12/12/2017 02:49 PM, Cornelia Huck wrote:  
>>>>>> One thing I noticed: You removed the caching of the flic (in the old
>>>>>> kvm inject routine), and you generally do more qom invocations (first,
>>>>>> to find the common flic; then, to translate to the qemu or kvm flic).
>>>>>> Not sure if this might be a problem (probably not).    
>>>>> Is any of these calls on a potential fast path (e.g. guest without adapter
>>>>> interrupts)? If yes, then QOM is a no-go since it is really slow.  
>>>> At least the new airq interface was using QOM without caching before.
>>>> It's basically about any interrupt; but otoh we are (for kvm) in
>>>> userspace already. Caching the flic and just keeping the casting to the
>>>> specialized flic might be ok (I'd guess that the lookup is the slowest
>>>> path.)
>>> Please note that the lookup is already cached in s390_get_flic(); That
>>> should be sufficient, as it does the expensive lookup. One cache should
>>> be enough, no?
>> Ah, missed that. So the old code actually did double caching...
>>> The other conversions should be cheap (and already were in place in a
>>> couple of places before).
>> Yes, object_resolve_path() is probably the most expensive one.
>> Did anyone ever check if the (existing) conversions are actually
>> measurable?
> Did some quick measurement.
> the initial object resolve takes about 20000ns, the cached ones then is 2-5ns.
> (measuring s390_get_flic function).
> The other conversions (S390FLICStateClass *fsc = 
> takes about 15-25ns (up to 100 cycles). 
> For irqfd users this does not matter, but things like plan9fs might benefit
> if we speedup this lookup as well?

Did you also measure the state conversion like QEMU_S390_FLIC() or
KVM_S390_FLIC() ?

As we call e.g. QEMU_S390_FLIC() on a regular basis in TCG, it might
then also make sense to speed that up.

a) cache the (converted) state and class in every function. Somewhat
uglifies the code.

b) introduce new functions that return the cached value

Not sure what's better. I prefer to do this as a separate addon patch
and can prepare something.

> PS: We can improve the initial object_resolve_path by doing the resolve not 
> for
> but 
> "/machine/" TYPE_KVM_S390_FLIC
> (2500ns instead of 20000)
> but its still way too slow.

Specifying the absolute path would be even faster I guess.

/machine/s390-flic-qemu/ ...



David / dhildenb

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