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Date: Sat, 17 Aug 2019 00:28:01 +0800
From: Ming Lei <tom.leiming@...il.com>
To: Keith Busch <kbusch@...nel.org>
Cc: Ming Lei <ming.lei@...hat.com>, Jens Axboe <axboe@...nel.dk>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Christoph Hellwig <hch@....de>,
"Derrick, Jonathan" <jonathan.derrick@...el.com>
Subject: Re: [PATCH V5 2/2] genirq/affinity: Spread vectors on node according
to nr_cpu ratio
On Fri, Aug 16, 2019 at 11:56 PM Keith Busch <kbusch@...nel.org> wrote:
>
> On Thu, Aug 15, 2019 at 07:28:49PM -0700, 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 warning in irq_build_affinity_masks() can
> > be triggered.
> >
> > 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 in normal situation.
> >
> > 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@...st-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@....de>
> > Cc: Keith Busch <kbusch@...nel.org>
> > Cc: linux-nvme@...ts.infradead.org,
> > Cc: Jon Derrick <jonathan.derrick@...el.com>
> > Cc: Jens Axboe <axboe@...nel.dk>
> > Reported-by: Jon Derrick <jonathan.derrick@...el.com>
> > Signed-off-by: Ming Lei <ming.lei@...hat.com>
>
> I had every intention to thoroughly test this on imbalanced node
> configurations, but that's not going to happen anytime soon. It looks
> correct to me, so I'll append my review here.
Thanks for your review!
>
> I'm not sure I'd include the detailed correctness explanation in the
> comments, though. It essentially boils down to "the sum of the parts
> doesn't exceed the whole", and the key to carving up the parts is the
> sorted iteration you've implemented.
There are two invariants reached by this approach:
1) In each node, if the active CPU number isn't zero, >=1 vector is allocated
for this node
2) For each node, the allocated vector number is <= CPU number in this node.
Both two are not obviously, however, the two points are quite important wrt.
the correctness of this approach.
That is why I add the long correctness proof to the comment.
Thanks,
Ming
>
> Reviewed-by: Keith Busch <kbusch@...nel.org>
>
> > ---
> > kernel/irq/affinity.c | 223 ++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 193 insertions(+), 30 deletions(-)
> >
> > diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
> > index c7cca942bd8a..32e07e58ce81 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 +95,155 @@ static int get_nodes_in_cpumask(cpumask_var_t *node_to_cpumask,
> > return nodes;
> > }
> >
> > +struct node_vectors {
> > + unsigned id;
> > +
> > + union {
> > + unsigned nvectors;
> > + unsigned ncpus;
> > + };
> > +};
> > +
> > +static int ncpus_cmp_func(const void *l, const void *r)
> > +{
> > + const struct node_vectors *ln = l;
> > + const struct node_vectors *rn = r;
> > +
> > + return ln->ncpus - rn->ncpus;
> > +}
> > +
> > +/*
> > + * Allocate vector number for each node, so that for each node:
> > + *
> > + * 1) the allocated number is >= 1
> > + *
> > + * 2) the allocated numbver is <= active CPU number of this node
> > + *
> > + * The actual allocated total vectors may be less than @numvecs when
> > + * active total CPU number is less than @numvecs.
> > + *
> > + * Active CPUs means the CPUs in '@..._mask AND @node_to_cpumask[]'
> > + * for each node.
> > + */
> > +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_vectors *node_vectors)
> > +{
> > + unsigned n, remaining_ncpus = 0;
> > +
> > + for (n = 0; n < nr_node_ids; n++) {
> > + node_vectors[n].id = 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;
> > + }
> > +
> > + numvecs = min_t(unsigned, remaining_ncpus, numvecs);
> > +
> > + 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.
> > + *
> > + * One perfect invariant is that number of allocated vectors for
> > + * each node is <= CPU count of this node:
> > + *
> > + * 1) suppose there are two nodes: A and B
> > + * ncpu(X) is CPU count of node X
> > + * vecs(X) is the vector count allocated to node X via this
> > + * algorithm
> > + *
> > + * ncpu(A) <= ncpu(B)
> > + * ncpu(A) + ncpu(B) = N
> > + * vecs(A) + vecs(B) = V
> > + *
> > + * vecs(A) = max(1, round_down(V * ncpu(A) / N))
> > + * vecs(B) = V - vecs(A)
> > + *
> > + * both N and V are integer, and 2 <= V <= N, suppose
> > + * V = N - delta, and 0 <= delta <= N - 2
> > + *
> > + * 2) obviously vecs(A) <= ncpu(A) because:
> > + *
> > + * if vecs(A) is 1, then vecs(A) <= ncpu(A) given
> > + * ncpu(A) >= 1
> > + *
> > + * otherwise,
> > + * vecs(A) <= V * ncpu(A) / N <= ncpu(A), given V <= N
> > + *
> > + * 3) prove how vecs(B) <= ncpu(B):
> > + *
> > + * if round_down(V * ncpu(A) / N) == 0, vecs(B) won't be
> > + * over-allocated, so vecs(B) <= ncpu(B),
> > + *
> > + * otherwise:
> > + *
> > + * vecs(A) =
> > + * round_down(V * ncpu(A) / N) =
> > + * round_down((N - delta) * ncpu(A) / N) =
> > + * round_down((N * ncpu(A) - delta * ncpu(A)) / N) >=
> > + * round_down((N * ncpu(A) - delta * N) / N) =
> > + * cpu(A) - delta
> > + *
> > + * then:
> > + *
> > + * vecs(A) - V >= ncpu(A) - delta - V
> > + * =>
> > + * V - vecs(A) <= V + delta - ncpu(A)
> > + * =>
> > + * vecs(B) <= N - ncpu(A)
> > + * =>
> > + * vecs(B) <= cpu(B)
> > + *
> > + * For nodes >= 3, it can be thought as one node and another big
> > + * node given that is exactly what this algorithm is implemented,
> > + * and we always re-calculate 'remaining_ncpus' & 'numvecs', and
> > + * finally for each node X: vecs(X) <= ncpu(X).
> > + *
> > + */
> > + 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);
> > + WARN_ON_ONCE(nvectors > ncpus);
> > +
> > + node_vectors[n].nvectors = nvectors;
> > +
> > + remaining_ncpus -= ncpus;
> > + numvecs -= nvectors;
> > + }
> > +}
> > +
> > static int __irq_build_affinity_masks(unsigned int startvec,
> > unsigned int numvecs,
> > unsigned int firstvec,
> > @@ -102,10 +252,11 @@ 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_vectors *node_vectors;
> >
> > if (!cpumask_weight(cpu_mask))
> > return 0;
> > @@ -126,53 +277,57 @@ 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_vectors),
> > + GFP_KERNEL);
> > + if (!node_vectors)
> > + return -ENOMEM;
> > +
> > + /* allocate vector number for each node */
> > + alloc_nodes_vectors(numvecs, node_to_cpumask, cpu_mask,
> > + nodemsk, nmsk, node_vectors);
> > +
> > + for (i = 0; i < nr_node_ids; i++) {
> > + unsigned int ncpus, v;
> > + struct node_vectors *nv = &node_vectors[i];
> > +
> > + if (nv->nvectors == UINT_MAX)
> > + continue;
> >
> > /* Get the cpus on this node which are in the mask */
> > - cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]);
> > + cpumask_and(nmsk, cpu_mask, node_to_cpumask[nv->id]);
> > ncpus = cpumask_weight(nmsk);
> > if (!ncpus)
> > continue;
> >
> > - /*
> > - * Calculate the number of cpus per vector
> > - *
> > - * Spread the vectors evenly per node. If the requested
> > - * vector number has been reached, simply allocate one
> > - * vector for each remaining node so that all nodes can
> > - * be covered
> > - */
> > - if (numvecs > done)
> > - vecs_per_node = max_t(unsigned,
> > - (numvecs - done) / nodes, 1);
> > - else
> > - vecs_per_node = 1;
> > -
> > - vecs_to_assign = min(vecs_per_node, ncpus);
> > + WARN_ON_ONCE(nv->nvectors > ncpus);
> >
> > /* Account for rounding errors */
> > - extra_vecs = ncpus - vecs_to_assign * (ncpus / vecs_to_assign);
> > + extra_vecs = ncpus - nv->nvectors * (ncpus / nv->nvectors);
> >
> > - for (v = 0; curvec < last_affv && v < vecs_to_assign;
> > - curvec++, v++) {
> > - cpus_per_vec = ncpus / vecs_to_assign;
> > + /* Spread allocated vectors on CPUs of the current node */
> > + for (v = 0; v < nv->nvectors; v++, curvec++) {
> > + cpus_per_vec = ncpus / nv->nvectors;
> >
> > /* Account for extra vectors to compensate rounding errors */
> > if (extra_vecs) {
> > cpus_per_vec++;
> > --extra_vecs;
> > }
> > +
> > + /*
> > + * wrapping has to be considered given 'startvec'
> > + * may start anywhere
> > + */
> > + if (curvec >= last_affv)
> > + curvec = firstvec;
> > irq_spread_init_one(&masks[curvec].mask, nmsk,
> > cpus_per_vec);
> > }
> > -
> > - done += v;
> > - if (curvec >= last_affv)
> > - curvec = firstvec;
> > - --nodes;
> > + done += nv->nvectors;
> > }
> > - return done < numvecs ? done : numvecs;
> > + kfree(node_vectors);
> > + return done;
> > }
> >
> > /*
> > @@ -208,6 +363,10 @@ static int irq_build_affinity_masks(unsigned int startvec, unsigned int numvecs,
> > nr_present = __irq_build_affinity_masks(curvec, numvecs,
> > firstvec, node_to_cpumask,
> > cpu_present_mask, nmsk, masks);
> > + if (nr_present < 0) {
> > + ret = nr_present;
> > + goto fail_build_affinity;
> > + }
> >
> > /*
> > * Spread on non present CPUs starting from the next vector to be
> > @@ -223,9 +382,13 @@ static int irq_build_affinity_masks(unsigned int startvec, unsigned int numvecs,
> > nr_others = __irq_build_affinity_masks(curvec, numvecs,
> > firstvec, node_to_cpumask,
> > npresmsk, nmsk, masks);
> > + if (nr_others < 0)
> > + ret = nr_others;
> > +
> > + fail_build_affinity:
> > put_online_cpus();
> >
> > - if (nr_present < numvecs)
> > + if (min(nr_present, nr_others) >= 0)
> > WARN_ON(nr_present + nr_others < numvecs);
> >
> > free_node_to_cpumask(node_to_cpumask);
> > --
> > 2.20.1
> >
>
> _______________________________________________
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> Linux-nvme@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-nvme
--
Ming Lei
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