Hi,
2015-11-16 18:02, Rahul Lakkireddy:
> Hi,
>
> I notice that the following changeset:
>
> Fixes: fd6949c55c9a ("eal: fix io permission for virtio interrupt
> handler")
>
> has moved the initialization of the interrupt thread to after the master
> lcore has been initialized. However, this causes the interrupt thread
> to _inherit_ the affinity of the master lcore. Hence, this seems to
> make all interrupts to be handled by _only_ the master lcore. Because
> of this change, it seems that now alarm interrupts would also be handled
> by master lcore only, IIUC.
>
> We are seeing a performance regression for cxgbe PMD after this commit
> since, cxgbe PMD relies on alarm to periodically transmit pending
> coalesced packets.
>
> Also, this perf degradation is only seen if there's a queue allocated
> on the master lcore, such as in l3fwd app. If the master lcore has
> been skipped, then no degradation in perf is seen since only the alarm
> will run on the master lcore.
>
> So, is the change done to make all interrupts, including alarm
> interrupts, be handled by _only_ the master lcore intended?
No it was not intended. The idea was to inherit settings (iopl) from
the device initialization into the interrupt thread.
Though a DPDK driver is not really supposed to rely on interrupt performance.
So having interrupts managed on any core was more or less a side effect.
> BTW, I have tried setting the affinity to all cpus instead in
> eal_intr_init() and this seems to restore the perf back. Perhaps it's
> better to move the master lcore initialization to after the interrupt
> thread has been initialized as well? Thoughts?
Yes, i think it's possible.
We can also imagine a command line option to set the interrupt affinity
with a default which mimics the old behaviour.
In order to make this conversation clearer, and for later references,
below is the DPDK init call tree:
start
driver constructor (if .a)
rte_eal_driver_register
main
rte_eal_init
eal_parse_args
rte_eal_pci_init
rte_eal_memory_init
eal_plugins_init
dlopen
driver constructor (if .so)
rte_eal_driver_register
eal_thread_init_master
eal_thread_set_affinity
rte_eal_dev_init
driver->init
PMD init
rte_eth_driver_register
rte_eal_intr_init
pthread_create
eal_intr_thread_main
eal_intr_handle_interrupts
pthread_create
rte_eal_pci_probe
driver->devinit
rte_eth_dev_init
rte_eth_dev_allocate
eth_drv->eth_dev_init
