On Thu, Aug 08, 2019 at 09:04:28AM +0200, Thomas Gleixner wrote: > On Wed, 7 Aug 2019, Jon Derrick wrote: > > The current irq spreading algorithm spreads vectors amongst cpus evenly > > per node. If a node has more cpus than another node, the extra vectors > > being spread may not be reported back to the caller. > > > > This is most apparent with the NVMe driver and nr_cpus < vectors, where > > the underreporting results in the caller's WARN being triggered: > > > > irq_build_affinity_masks() > > ... > > if (nr_present < numvecs) > > WARN_ON(nr_present + nr_others < numvecs); > > > > Signed-off-by: Jon Derrick <[email protected]> > > --- > > kernel/irq/affinity.c | 7 +++++-- > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c > > index 4352b08ae48d..9beafb8c7e92 100644 > > --- a/kernel/irq/affinity.c > > +++ b/kernel/irq/affinity.c > > @@ -127,7 +127,8 @@ static int __irq_build_affinity_masks(unsigned int > > startvec, > > } > > > > for_each_node_mask(n, nodemsk) { > > - unsigned int ncpus, v, vecs_to_assign, vecs_per_node; > > + unsigned int ncpus, v, vecs_to_assign, total_vecs_to_assign, > > + vecs_per_node; > > > > /* Spread the vectors per node */ > > vecs_per_node = (numvecs - (curvec - firstvec)) / nodes; > > @@ -141,14 +142,16 @@ static int __irq_build_affinity_masks(unsigned int > > startvec, > > > > /* Account for rounding errors */ > > extra_vecs = ncpus - vecs_to_assign * (ncpus / vecs_to_assign); > > + total_vecs_to_assign = vecs_to_assign + extra_vecs; > > > > - for (v = 0; curvec < last_affv && v < vecs_to_assign; > > + for (v = 0; curvec < last_affv && v < total_vecs_to_assign; > > curvec++, v++) { > > cpus_per_vec = ncpus / vecs_to_assign; > > > > /* Account for extra vectors to compensate rounding > > errors */ > > if (extra_vecs) { > > cpus_per_vec++; > > + v++; > > --extra_vecs; > > } > > irq_spread_init_one(&masks[curvec].mask, nmsk, > > --
This looks like it will break the spread to non-present CPUs since it's not accurately reporting how many vectors were assigned for the present spread. I think the real problem is the spread's vecs_per_node doesn't account which nodes contribute more CPUs than others. For example: Node 0 has 32 CPUs Node 1 has 8 CPUs Assign 32 vectors The current algorithm assigns 16 vectors to node 0 because vecs_per_node is calculated as 32 vectors / 2 nodes on the first iteration. The subsequent iteration for node 1 gets 8 vectors because it has only 8 CPUs, leaving 8 vectors unassigned. A more fair spread would give node 0 the remaining 8 vectors. This optimization, however, is a bit more complex than the current algorithm, which is probably why it wasn't done, so I think the warning should just be removed.
