+CC Brice.
On Sun, 5 Jul 2020 09:53:58 +0000 "Song Bao Hua (Barry Song)" <[email protected]> wrote: > > -----Original Message----- > > From: Will Deacon [mailto:[email protected]] > > Sent: Saturday, July 4, 2020 4:22 AM > > To: Song Bao Hua (Barry Song) <[email protected]> > > Cc: [email protected]; [email protected]; [email protected]; > > [email protected]; [email protected]; > > Linuxarm <[email protected]> > > Subject: Re: [PATCH] iommu/arm-smmu-v3: expose numa_node attribute to > > users in sysfs > > > > On Sat, May 30, 2020 at 09:15:05PM +1200, 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. > > > > I don't think that's a very good reason to expose things to userspace. > > I know sysfs shouldn't be treated as ABI, but the grim reality is that > > once somebody relies on this stuff then we can't change it, so I'd > > rather avoid exposing it unless it's absolutely necessary. > > Will, thanks for taking a look! > > I am not sure if it is absolutely necessary, but it is useful to users. The > whole story started > from some users who wanted to know the hardware topology very clear by > reading some > sysfs node just like they are able to do that for pci devices. The intention > is that users can > know hardware topology of various devices easily from linux since they maybe > don't know > all the hardware details. > > For pci devices, kernel has done that. And there are some other drivers out > of pci > exposing numa_node as well. It seems it is hard to say it is absolutely > necessary > for them too since sysfs shouldn't be treated as ABI. Brice, Given hwloc is probably the most demanding user of topology information currently... How useful would this info be for hwloc and hwloc users? Sort of feels like it might be useful in some cases. The very brief description of what we have here is exposing the numa node of an IOMMU. The discussion also diverted into whether it just makes sense to expose this for all platform devices or even do it at the device level. Jonathan > > I got some input from Linux users who also wanted to know the numa node for > other devices which are not PCI, for example, platform devices. And I thought > the > requirement is kind of reasonable. So I also had another patch to generally > support > this kind of requirements, with the below patch, this smmu patch is not > necessary > any more: > https://lkml.org/lkml/2020/6/18/1257 > > for platform device created by ARM ACPI/IORT and general > acpi_create_platform_device() > drivers/acpi/scan.c: > static void acpi_default_enumeration(struct acpi_device *device) > { > ... > if (!device->flags.enumeration_by_parent) { > acpi_create_platform_device(device, NULL); > acpi_device_set_enumerated(device); > } > } > > struct platform_device *acpi_create_platform_device(struct acpi_device *adev, > struct property_entry *properties) > { > ... > > pdev = platform_device_register_full(&pdevinfo); > if (IS_ERR(pdev)) > ... > else { > set_dev_node(&pdev->dev, acpi_get_node(adev->handle)); > ... > } > ... > } > numa_node is set for this kind of devices. > > Anyway, just want to explain to you the background some people want to know > the > hardware topology from Linux in same simple way. And it seems it is a > reasonable > requirement to me :-) > > > > > Thanks, > > > > Will > > Thanks > barry > _______________________________________________ > iommu mailing list > [email protected] > https://lists.linuxfoundation.org/mailman/listinfo/iommu _______________________________________________ iommu mailing list [email protected] https://lists.linuxfoundation.org/mailman/listinfo/iommu
