Re: [PATCH v4 0/2] irqchip/gic-v3-its: Balance LPI affinity across CPUs

[John Garry]

On 15/05/2020 17:57, Marc Zyngier wrote:
> When mapping a LPI, the ITS driver picks the first possible
> affinity, which is in most cases CPU0, assuming that if
> that's not suitable, someone will come and set the affinity
> to something more interesting.
>
> It apparently isn't the case, and people complain of poor
> performance when many interrupts are glued to the same CPU.
> So let's place the interrupts by finding the "least loaded"
> CPU (that is, the one that has the fewer LPIs mapped to it).
> So called 'managed' interrupts are an interesting case where
> the affinity is actually dictated by the kernel itself, and
> we should honor this.

Cheers Marc, I ran these again and the figures look good:
NVMe with nvme.use_threaded_interrupts=1/0
Before: 1000K/1100K IOPS
After: 1100K/1400K IOPS

For some reason v5.7-rc5 looks more stable than rc4 (which I tested 
previously).

Tested-by: John Garry <john.garry@xxxxxxxxxx>

> * From v3:
>    - Always pre-decrement/post-increment affinity to avoid useless
>      changes of affinity (John)
>    - Don't use the node mask as a superset of the proposed affinity
>      as the ACPI tables can't really describe this (John)
>    - Rebased on v5.7-rc5
>
> * From v2:
>    - Split accounting from CPU selection
>    - Track managed and unmanaged interrupts separately
>
> Marc Zyngier (2):
>    irqchip/gic-v3-its: Track LPI distribution on a per CPU basis
>    irqchip/gic-v3-its: Balance initial LPI affinity across CPUs
>
>   drivers/irqchip/irq-gic-v3-its.c | 170 ++++++++++++++++++++++++++-----
>   1 file changed, 143 insertions(+), 27 deletions(-)