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Message-ID: <20190809030424.GA17485@ming.t460p>
Date: Fri, 9 Aug 2019 11:04:25 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Keith Busch <kbusch@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>,
linux-nvme@...ts.infradead.org, Christoph Hellwig <hch@....de>,
LKML <linux-kernel@...r.kernel.org>,
Jon Derrick <jonathan.derrick@...el.com>
Subject: Re: [PATCH] genirq/affinity: report extra vectors on uneven nodes
On Thu, Aug 08, 2019 at 10:32:24AM -0600, Keith Busch wrote:
> 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 <jonathan.derrick@...el.com>
> > > ---
> > > 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.
Another policy is to assign vectors among nodes according to the
following ratio:
ncpus in this node / total ncpus in un-assigned nodes
I have tried the following patch, looks it works fine:
diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index 6fef48033f96..a598f20701a3 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -94,6 +94,28 @@ static int get_nodes_in_cpumask(cpumask_var_t *node_to_cpumask,
return nodes;
}
+static int nodes_cpus(unsigned start_node, const nodemask_t nodemsk,
+ const cpumask_var_t *node_to_cpumask,
+ const struct cpumask *cpu_mask, struct cpumask *nmsk)
+{
+ unsigned n, ncpus, total_cpus = 0;
+
+ for_each_node_mask(n, nodemsk) {
+ if (n < start_node)
+ continue;
+
+ /* Get the cpus on this node which are in the mask */
+ cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]);
+
+ /* Calculate the number of cpus per vector */
+ ncpus = cpumask_weight(nmsk);
+
+ total_cpus += ncpus;
+ }
+
+ return total_cpus;
+}
+
static int __irq_build_affinity_masks(unsigned int startvec,
unsigned int numvecs,
unsigned int firstvec,
@@ -128,15 +150,25 @@ 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;
-
- /* Spread the vectors per node */
- vecs_per_node = (numvecs - (curvec - firstvec)) / nodes;
+ unsigned int ncpus_left = nodes_cpus(n, nodemsk,
+ node_to_cpumask, cpu_mask, nmsk);
/* Get the cpus on this node which are in the mask */
cpumask_and(nmsk, cpu_mask, node_to_cpumask[n]);
/* Calculate the number of cpus per vector */
ncpus = cpumask_weight(nmsk);
+
+ /*
+ * Spread the vectors per node, and node with more CPUs will be
+ * assigned to more vectors
+ */
+ vecs_per_node = (numvecs - (curvec - firstvec)) * ncpus / ncpus_left;
+
+ /* at least assign one vector for this node */
+ if (!vecs_per_node)
+ vecs_per_node = 1;
+
vecs_to_assign = min(vecs_per_node, ncpus);
/* Account for rounding errors */
@@ -160,7 +192,6 @@ static int __irq_build_affinity_masks(unsigned int startvec,
break;
if (curvec >= last_affv)
curvec = firstvec;
- --nodes;
}
return done;
}
thanks,
Ming
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