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SubjectRe: [PATCH V2 2/3] genirq/affinity: Spread vectors on node according to nr_cpu ratio
On Tue, Aug 13, 2019 at 05:26:51PM +0800, Ming Lei wrote:
> On Tue, Aug 13, 2019 at 03:41:12PM +0800, Ming Lei wrote:
> > On Mon, Aug 12, 2019 at 09:27:18AM -0600, Keith Busch wrote:
> > > On Mon, Aug 12, 2019 at 05:57:08PM +0800, Ming Lei wrote:
> > > > Now __irq_build_affinity_masks() spreads vectors evenly per node, and
> > > > all vectors may not be spread in case that each numa node has different
> > > > CPU number, then the following warning in irq_build_affinity_masks() can
> > > > be triggered:
> > > >
> > > > if (nr_present < numvecs)
> > > > WARN_ON(nr_present + nr_others < numvecs);
> > > >
> > > > Improve current spreading algorithm by assigning vectors according to
> > > > the ratio of node's nr_cpu to nr_remaining_cpus, meantime running the
> > > > assignment from smaller nodes to bigger nodes to guarantee that every
> > > > active node gets allocated at least one vector, then we can avoid
> > > > cross-node spread.
> > > >
> > > > Meantime the reported warning can be fixed.
> > > >
> > > > Another big goodness is that the spread approach becomes more fair if
> > > > node has different CPU number.
> > > >
> > > > For example, on the following machine:
> > > > [root@ktest-01 ~]# lscpu
> > > > ...
> > > > CPU(s): 16
> > > > On-line CPU(s) list: 0-15
> > > > Thread(s) per core: 1
> > > > Core(s) per socket: 8
> > > > Socket(s): 2
> > > > NUMA node(s): 2
> > > > ...
> > > > NUMA node0 CPU(s): 0,1,3,5-9,11,13-15
> > > > NUMA node1 CPU(s): 2,4,10,12
> > > >
> > > > When driver requests to allocate 8 vectors, the following spread can
> > > > be got:
> > > > irq 31, cpu list 2,4
> > > > irq 32, cpu list 10,12
> > > > irq 33, cpu list 0-1
> > > > irq 34, cpu list 3,5
> > > > irq 35, cpu list 6-7
> > > > irq 36, cpu list 8-9
> > > > irq 37, cpu list 11,13
> > > > irq 38, cpu list 14-15
> > > >
> > > > Without this patch, kernel warning is triggered on above situation, and
> > > > allocation result was supposed to be 4 vectors for each node.
> > > >
> > > > Cc: Christoph Hellwig <hch@lst.de>
> > > > Cc: Keith Busch <kbusch@kernel.org>
> > > > Cc: linux-nvme@lists.infradead.org,
> > > > Cc: Jon Derrick <jonathan.derrick@intel.com>
> > > > Cc: Jens Axboe <axboe@kernel.dk>
> > > > Reported-by: Jon Derrick <jonathan.derrick@intel.com>
> > > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > > ---
> > > > kernel/irq/affinity.c | 141 +++++++++++++++++++++++++++++++++++-------
> > > > 1 file changed, 117 insertions(+), 24 deletions(-)
> > > >
> > > > diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
> > > > index c7cca942bd8a..927dcbe80482 100644
> > > > --- a/kernel/irq/affinity.c
> > > > +++ b/kernel/irq/affinity.c
> > > > @@ -7,6 +7,7 @@
> > > > #include <linux/kernel.h>
> > > > #include <linux/slab.h>
> > > > #include <linux/cpu.h>
> > > > +#include <linux/sort.h>
> > > >
> > > > static void irq_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
> > > > unsigned int cpus_per_vec)
> > > > @@ -94,6 +95,87 @@ static int get_nodes_in_cpumask(cpumask_var_t *node_to_cpumask,
> > > > return nodes;
> > > > }
> > > >
> > > > +struct node_nr_vectors {
> > > > + unsigned n;
> > > > +
> > > > + union {
> > > > + unsigned nvectors;
> > > > + unsigned ncpus;
> > > > + };
> > > > +};
> > > > +
> > > > +static int ncpus_cmp_func(const void *l, const void *r)
> > > > +{
> > > > + const struct node_nr_vectors *ln = l;
> > > > + const struct node_nr_vectors *rn = r;
> > > > +
> > > > + if (ln->ncpus < rn->ncpus)
> > > > + return -1;
> > > > + if (ln->ncpus > rn->ncpus)
> > > > + return 1;
> > > > + return 0;
> > >
> > > You can collapse these to one line:
> > >
> > > return ln->ncpus - rn->ncpus;
> >
> > OK.
> >
> > >
> > > > +}
> > > > +
> > > > +static void alloc_nodes_vectors(unsigned int numvecs,
> > > > + const cpumask_var_t *node_to_cpumask,
> > > > + const struct cpumask *cpu_mask,
> > > > + const nodemask_t nodemsk,
> > > > + struct cpumask *nmsk,
> > > > + struct node_nr_vectors *node_vectors)
> > > > +{
> > > > + unsigned remaining_ncpus = 0;
> > > > + unsigned n;
> > > > +
> > > > + for (n = 0; n < nr_node_ids; n++) {
> > > > + node_vectors[n].n = n;
> > > > + node_vectors[n].ncpus = UINT_MAX;
> > > > + }
> > > > +
> > > > + for_each_node_mask(n, nodemsk) {
> > > > + unsigned ncpus;
> > > > +
> > > > + cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]);
> > > > + ncpus = cpumask_weight(nmsk);
> > > > +
> > > > + if (!ncpus)
> > > > + continue;
> > > > + remaining_ncpus += ncpus;
> > > > + node_vectors[n].ncpus = ncpus;
> > > > + }
> > > > +
> > > > + sort(node_vectors, nr_node_ids, sizeof(node_vectors[0]),
> > > > + ncpus_cmp_func, NULL);
> > > > +
> > > > + /*
> > > > + * Allocate vectors for each node according to the ratio of this
> > > > + * node's nr_cpus to remaining un-assigned ncpus. 'numvecs' is
> > > > + * bigger than number of active numa nodes. Always start the
> > > > + * allocation from the node with minimized nr_cpus.
> > > > + *
> > > > + * This way guarantees that each active node gets allocated at
> > > > + * least one vector, and the theory is simple: over-allocation
> > > > + * is only done when this node is assigned by one vector, so
> > > > + * other nodes will be allocated >= 1 vector, since 'numvecs' is
> > > > + * bigger than number of numa nodes.
> > > > + */
> > > > + for (n = 0; n < nr_node_ids; n++) {
> > > > + unsigned nvectors, ncpus;
> > > > +
> > > > + if (node_vectors[n].ncpus == UINT_MAX)
> > > > + continue;
> > > > +
> > > > + WARN_ON_ONCE(numvecs == 0);
> > > > +
> > > > + ncpus = node_vectors[n].ncpus;
> > > > + nvectors = max_t(unsigned, 1,
> > > > + numvecs * ncpus / remaining_ncpus);
> > > > +
> > > > + node_vectors[n].nvectors = nvectors;
> > > > + remaining_ncpus -= ncpus;
> > > > + numvecs -= nvectors;
> > > > + }
> > >
> > > This looks good to me.
> > >
> > > > +}
> > > > +
> > > > static int __irq_build_affinity_masks(unsigned int startvec,
> > > > unsigned int numvecs,
> > > > unsigned int firstvec,
> > > > @@ -102,10 +184,11 @@ static int __irq_build_affinity_masks(unsigned int startvec,
> > > > struct cpumask *nmsk,
> > > > struct irq_affinity_desc *masks)
> > > > {
> > > > - unsigned int n, nodes, cpus_per_vec, extra_vecs, done = 0;
> > > > + unsigned int i, n, nodes, cpus_per_vec, extra_vecs, done = 0;
> > > > unsigned int last_affv = firstvec + numvecs;
> > > > unsigned int curvec = startvec;
> > > > nodemask_t nodemsk = NODE_MASK_NONE;
> > > > + struct node_nr_vectors *node_vectors;
> > > >
> > > > if (!cpumask_weight(cpu_mask))
> > > > return 0;
> > > > @@ -126,8 +209,23 @@ static int __irq_build_affinity_masks(unsigned int startvec,
> > > > return numvecs;
> > > > }
> > > >
> > > > - for_each_node_mask(n, nodemsk) {
> > > > - unsigned int ncpus, v, vecs_to_assign, vecs_per_node;
> > > > + node_vectors = kcalloc(nr_node_ids,
> > > > + sizeof(struct node_nr_vectors),
> > > > + GFP_KERNEL);
> > > > + if (!node_vectors)
> > > > + return 0;
> > >
> > > I think we need to get this -ENOMEM condition back to the caller and
> > > have that condition handled.
> >
> > Good point.
> >
> > >
> > > > @@ -165,13 +250,21 @@ static int __irq_build_affinity_masks(unsigned int startvec,
> > > > }
> > > > irq_spread_init_one(&masks[curvec].mask, nmsk,
> > > > cpus_per_vec);
> > > > + /*
> > > > + * alloc_nodes_vectors() is intelligent enough to
> > > > + * allocate vectors on all nodes, so wrapping
> > > > + * shouldn't be triggered usually. However, if it
> > > > + * happens when allocated vectors is bigger than
> > > > + * node's CPU number becasue of round down, wraps
> > > > + * to the first vector allocated for this node, then
> > > > + * cross-node spread can be avoided.
> > > > + */
> > > > + if (curvec >= last_affv)
> > > > + curvec -= v;
> > >
> > > Could you explain again how this could happen? The round-down should mean we
> > > apply a vector to more CPUs so that the number of vectors applied to a
> > > node wthin the loop should never require wrapping to hit all those CPUs.
> > > And if that's true, the check should probably be a warn because it
> > > should never happen.
> >
> > You are right.
> >
> > We should simply spread from the 1st vector for this node if there is
> > more vectors not done.
>
> oops, the above approach is wrong, same with V3.
>
> It should happen on just the node with max CPU count, then we can't
> go ahead. There are more vectors than CPU count on this node, all
> vectors have to be spread, each CPU can be assigned to only one vector.
>
> However, can't observe such problem at all, I need to think about if
> it is possible from viewpoint of math.

Looks we can prove that vectors assigned to every node is <= nr_cpu of
this node, see the following:

1) suppose there are two nodes: A and B
- ncpu(X) is CPU count of node X
- vecs(X) is the vector count assigned to node X via this algorithm

ncpu(A) + ncpu(B) = N
vecs(A) + vecs(B) = V
vecs(A) = round_down(ncpu(A) * V / N)
vecs(B) = V - vecs(A)

N and V are integer, and 2 <= V <= N, suppose V = N - delta, then
delta is still integer, and 0 <= delta <= N - 2

let's prove:
vecs(A) <= ncpu(A)
vecs(B) <= ncpu(B)

2) prove vecs(A) <= ncpu(A)

vecs(A) =
round_down(ncpu(A) * (N - delta) / N) =
ncpu(A) - round_down(cpu(A) * delta / N)

so vecs(A) is <= ncpu(A)


3) prove vecs(B) <= ncpu(B)

vecs(B) = V - vecs(A)
N - delta - ncpu(A) + round_down(cpu(A) * delta / N) =
ncpu(B) - delta + round_down(cpu(A) * delta / N) <=
ncpu(B) - delta + cpu(A) * delta / N =
ncpu(B) - (1 - ncpu(A)/N) * delta <= ncpu(B)

because (1 - ncpu(A)/N) is > 0, and delta >= 0 too.

For nodes >= 3, it can be thought as one node and another big node given
that is exactly what this algorithm is implemented('remaining_ncpus' and
'numvecs' are updated in each loop), and finally for each node X, we can
prove vecs(X) <= ncpu(X).


thanks,
Ming

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