RE: [PATCH] iommu/arm-smmu-v3: expose numa_node attribute to users in sysfs

["Song Bao Hua (Barry Song)" <song.bao.hua@hisilicon.com>]

> -----Original Message-----
> From: Robin Murphy [mailto:robin.murphy@arm.com]
> Sent: Tuesday, June 2, 2020 1:14 AM
> To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; will@kernel.org;
> hch@lst.de; m.szyprowski@samsung.com; iommu@lists.linux-foundation.org
> Cc: Linuxarm <linuxarm@huawei.com>; linux-arm-kernel@lists.infradead.org
> Subject: Re: [PATCH] iommu/arm-smmu-v3: expose numa_node attribute to
> users in sysfs
> 
> On 2020-05-30 10:15, Barry Song wrote:
> > As tests show the latency of dma_unmap can increase dramatically while
> > calling them cross NUMA nodes, especially cross CPU packages, eg.
> > 300ns vs 800ns while waiting for the completion of CMD_SYNC in an
> > empty command queue. The large latency causing by remote node will
> > in turn make contention of the command queue more serious, and enlarge
> > the latency of DMA users within local NUMA nodes.
> >
> > Users might intend to enforce NUMA locality with the consideration of
> > the position of SMMU. The patch provides minor benefit by presenting
> > this information to users directly, as they might want to know it without
> > checking hardware spec at all.
> 
> Hmm, given that dev-to_node() is a standard driver model thing, is there
> not already some generic device property that can expose it - and if
> not, should there be? Presumably if userspace cares enough to want to
> know whereabouts in the system an IOMMU is, it probably also cares where
> the actual endpoint devices are too.
> 
> At the very least, it doesn't seem right for it to be specific to one
> single IOMMU driver.

Right now pci devices have generally got the numa_node in sysfs by drivers/pci/pci-sysfs.c

static ssize_t numa_node_store(struct device *dev,
                               struct device_attribute *attr, const char *buf,
                               size_t count)
{
        ...

        add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
        pci_alert(pdev, FW_BUG "Overriding NUMA node to %d.  Contact your vendor for updates.",
                  node);

        dev->numa_node = node;
        return count;
}

static ssize_t numa_node_show(struct device *dev, struct device_attribute *attr,
                              char *buf)
{
        return sprintf(buf, "%d\n", dev->numa_node);
}
static DEVICE_ATTR_RW(numa_node);

for other devices who care about numa information, the specific drivers are doing that, for example:

drivers/dax/bus.c:      if (a == &dev_attr_numa_node.attr && !IS_ENABLED(CONFIG_NUMA))
drivers/dax/bus.c:      &dev_attr_numa_node.attr,
drivers/dma/idxd/sysfs.c:       &dev_attr_numa_node.attr,
drivers/hv/vmbus_drv.c: &dev_attr_numa_node.attr,
drivers/nvdimm/bus.c:   &dev_attr_numa_node.attr,
drivers/nvme/host/core.c:       &dev_attr_numa_node.attr,

smmu is usually a platform device, we can actually expose numa_node for platform_device, or globally expose numa_node
for general "device" if people don't opposite.

Barry

> 
> Robin.
> 
> > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > ---
> >   drivers/iommu/arm-smmu-v3.c | 40
> ++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 39 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> > index 82508730feb7..754c4d59498b 100644
> > --- a/drivers/iommu/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm-smmu-v3.c
> > @@ -4021,6 +4021,44 @@ err_reset_pci_ops: __maybe_unused;
> >   	return err;
> >   }
> >
> > +static ssize_t numa_node_show(struct device *dev,
> > +		struct device_attribute *attr, char *buf)
> > +{
> > +	return sprintf(buf, "%d\n", dev_to_node(dev));
> > +}
> > +static DEVICE_ATTR_RO(numa_node);
> > +
> > +static umode_t arm_smmu_numa_attr_visible(struct kobject *kobj, struct
> attribute *a,
> > +		int n)
> > +{
> > +	struct device *dev = container_of(kobj, typeof(*dev), kobj);
> > +
> > +	if (!IS_ENABLED(CONFIG_NUMA))
> > +		return 0;
> > +
> > +	if (a == &dev_attr_numa_node.attr &&
> > +			dev_to_node(dev) == NUMA_NO_NODE)
> > +		return 0;
> > +
> > +	return a->mode;
> > +}
> > +
> > +static struct attribute *arm_smmu_dev_attrs[] = {
> > +	&dev_attr_numa_node.attr,
> > +	NULL
> > +};
> > +
> > +static struct attribute_group arm_smmu_dev_attrs_group = {
> > +	.attrs          = arm_smmu_dev_attrs,
> > +	.is_visible     = arm_smmu_numa_attr_visible,
> > +};
> > +
> > +
> > +static const struct attribute_group *arm_smmu_dev_attrs_groups[] = {
> > +	&arm_smmu_dev_attrs_group,
> > +	NULL,
> > +};
> > +
> >   static int arm_smmu_device_probe(struct platform_device *pdev)
> >   {
> >   	int irq, ret;
> > @@ -4097,7 +4135,7 @@ static int arm_smmu_device_probe(struct
> platform_device *pdev)
> >   		return ret;
> >
> >   	/* And we're up. Go go go! */
> > -	ret = iommu_device_sysfs_add(&smmu->iommu, dev, NULL,
> > +	ret = iommu_device_sysfs_add(&smmu->iommu, dev,
> arm_smmu_dev_attrs_groups,
> >   				     "smmu3.%pa", &ioaddr);
> >   	if (ret)
> >   		return ret;
> >
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