Re: [PATCH 5/5] qom: Make container_get() strict to always walk or return container

[Peter Xu <peterx@redhat.com>]

On Tue, Nov 19, 2024 at 09:30:09PM +0100, Paolo Bonzini wrote:
> >
> > Do we have known places that we care a lot on object[_class]_dynamic_cast()
> > performance?
> 
> The easiest way to check is probably to print the type of every successful
> object_dynamic_cast and object_class_dynamic_cast. I suspect the result
> will be virtio-blk-device and/or scsi-hd, but maybe those already do an
> unsafe cast (pointer type cast) instead of object_dynamic_cast.

Yes, it sounds more reasonable to me to optimize specific call sites so far
rather than provides something generic..  Though it could still be a
generic API so that devices can opt-in.  Maybe still not as fast as an
unsafe cast, though.. I think I'll leave that to block experts when it may
needs some good measurements.

> 
> I can give it some measurement if there is, otherwise I'm
> > guessing whatever changes could fall into the noise.
> 
> 
> Yes, probably. At most you can identify if there any heavy places out of
> the 34000 calls, and see if they can use an unsafe cast.

I can still trivially do this.

I traced qemu using bpf and interestingly in my case close to half (over
10000+) of the calls are about ahci_irq_lower() from different higher level
stack (yeah I used IDE in my setup.. with a split irqchi..), where it has:

    PCIDevice *pci_dev = (PCIDevice *) object_dynamic_cast(OBJECT(dev_state),
                                                           TYPE_PCI_DEVICE);

So IIUC that can be open to a unsafe cast too, but considering IDE is ODD
FIXES stage, I'm not sure if I should send a patch at all.  However I
copied John regardless.

Thanks,

-- 
Peter Xu