Re: [dm-devel] DM-CRYPT: Scale to multiple CPUs v3

[Milan Broz <mbroz redhat com>]

On 10/10/2010 09:16 PM, Andi Kleen wrote:
>> Not if in_interrupt is set though?
>> +       if (per_cpu(io_wq_cpu, cpu) == current && !in_interrupt()) {
>>
>> What I am missing here?
> 
> The interrupt doesn't block on the task.
> 
> Actually most likely that check isn't needed anyways because
> that should not happen, was just pure paranoia from my side.

I don't think so. If you run crypto in async mode, you get asynchronous
callback (kcryptd_asynnc_done() here).

AFAIK this callback is called in interrupt context. This callback
decreases pending counter and if it reach zero it calls
kcryptd_crypt_write_io_submit() -> kcryptd_queue_io().

You cannot call direct encryption if it is called from async callback,
so the IO must be always queued to IO workqueue for later.

So the in_interrupt() is IMHO equivalent of async flag and it is
properly placed there.

But previously, there were threads per device, so if one IO thread blocks,
others stacked mappings can continue
Now I see possibility for deadlock there because we have one io thread now
(assuming that 1 CPU situation Alasdair mentioned).

Or is there a mistake in my analysis?

> 
>>
>> (And assume there is only 1 CPU too for worst case behaviour, presumably.)
> 
> One per process, previously it was always one per CPU.

Nope, one singlethread per crypt device (resp. two: io + crypt).

Milan