On Wed, 4 Apr 2018, Thomas Gleixner wrote: > I'm aware how that hw-queue stuff works. But that only works if the > spreading algorithm makes the interrupts affine to offline/not-present CPUs > when the block device is initialized. > > In the example above: > > > > > irq 39, cpu list 0,4 > > > > irq 40, cpu list 1,6 > > > > irq 41, cpu list 2,5 > > > > irq 42, cpu list 3,7 > > and assumed that at driver init time only CPU 0-3 are online then the > hotplug of CPU 4-7 will not result in any interrupt delivered to CPU 4-7. > > So the extra assignment to CPU 4-7 in the affinity mask has no effect > whatsoever and even if the spreading result is 'perfect' it just looks > perfect as it is not making any difference versus the original result: > > > > > irq 39, cpu list 0 > > > > irq 40, cpu list 1 > > > > irq 41, cpu list 2 > > > > irq 42, cpu list 3 And looking deeper into the changes, I think that the first spreading step has to use cpu_present_mask and not cpu_online_mask. Assume the following scenario: Machine with 8 present CPUs is booted, the 4 last CPUs are unplugged. Device with 4 queues is initialized. The resulting spread is going to be exactly your example: irq 39, cpu list 0,4 irq 40, cpu list 1,6 irq 41, cpu list 2,5 irq 42, cpu list 3,7 Now the 4 offline CPUs are plugged in again. These CPUs won't ever get an interrupt as all interrupts stay on CPU 0-3 unless one of these CPUs is unplugged. Using cpu_present_mask the spread would be: irq 39, cpu list 0,1 irq 40, cpu list 2,3 irq 41, cpu list 4,5 irq 42, cpu list 6,7 while on a machine where CPU 4-7 are NOT present, but advertised as possible the spread would be: irq 39, cpu list 0,4 irq 40, cpu list 1,6 irq 41, cpu list 2,5 irq 42, cpu list 3,7 Hmm? Thanks, tglx
