Currently, when setting up the IRQ for a q_vector, we set an affinity
hint based on the v_idx of that q_vector. Meaning a loop iterates on
v_idx, which is an incremental value, and the cpumask is created based
on this value.
This is a problem in systems with multiple logical CPUs per core (like in
simultaneous multithreading (SMT) scenarios). If we disable some logical
CPUs, by turning SMT off for example, we will end up with a sparse
cpu_online_mask, i.e., only the first CPU in a core is online, and
incremental filling in q_vector cpumask might lead to multiple offline
CPUs being assigned to q_vectors.
Example: if we have a system with 8 cores each one containing 8 logical
CPUs (SMT == 8 in this case), we have 64 CPUs in total. But if SMT is
disabled, only the 1st CPU in each core remains online, so the
cpu_online_mask in this case would have only 8 bits set, in a sparse way.
In general case, when SMT is off the cpu_online_mask has only C bits set:
0, 1*N, 2*N, ..., C*(N-1) where
C == # of cores;
N == # of logical CPUs per core.
In our example, only bits 0, 8, 16, 24, 32, 40, 48, 56 would be set.
Instead, we should only assign hints for CPUs which are online. Even
better, the kernel already provides a function, cpumask_local_spread()
which takes an index and returns a CPU, spreading the interrupts across
local NUMA nodes first, and then remote ones if necessary.
Since we generally have a 1:1 mapping between vectors and CPUs, there
is no real advantage to spreading vectors to local CPUs first. In order
to avoid mismatch of the default XPS hints, we'll pass -1 so that it
spreads across all CPUs without regard to the node locality.
Note that we don't need to change the q_vector->affinity_mask as this is
initialized to cpu_possible_mask, until an actual affinity is set and
then notified back to us.
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
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