[LSF/MM TOPIC][LSF/MM ATTEND] NAPI polling for block drivers

[Sagi Grimberg]

> Just for the record, all tests you've run are with the upper irq_poll_budget of
> 256 [1]?

Yes, but that's the point, I never ever reach this budget because
I'm only processing 1-2 completions per interrupt.

> We (Hannes and me) recently stumbed accross this when trying to poll for more
> than 256 queue entries in the drivers we've been testing.

What do you mean by stumbed? irq-poll should be agnostic to the fact
that drivers can poll more than their given budget?

> Did your system load reduce with irq polling? In theory it should but I have
> seen increases with AHCI at least according to fio. IIRC Hannes saw decreases
> with his SAS HBA tests, as expected.

I didn't see any reduction. When I tested on a single cpu core (to
simplify for a single queue) the cpu was at 100% cpu but got less iops
(which makes sense, a single cpu-core is not enough to max out the nvme
device, at least not the core I'm using). Before irqpoll I got
~230 KIOPs on a single cpu-core and after irqpoll I got ~205 KIOPs
which is consistent with the ~10% iops decrease I've reported in the
original submission.