Re: [PATCH V2 1/3] mm: Define coherent device memory (CDM) node

From: Anshuman Khandual
Date: Tue Feb 14 2017 - 03:17:34 EST

On 02/13/2017 09:29 AM, John Hubbard wrote:
> On 02/10/2017 02:06 AM, Anshuman Khandual wrote:
>> There are certain devices like specialized accelerator, GPU cards,
>> network
>> cards, FPGA cards etc which might contain onboard memory which is
>> coherent
>> along with the existing system RAM while being accessed either from
>> the CPU
>> or from the device. They share some similar properties with that of
>> normal
>> system RAM but at the same time can also be different with respect to
>> system RAM.
>> User applications might be interested in using this kind of coherent
>> device
>> memory explicitly or implicitly along side the system RAM utilizing all
>> possible core memory functions like anon mapping (LRU), file mapping
>> (LRU),
>> page cache (LRU), driver managed (non LRU), HW poisoning, NUMA migrations
>> etc. To achieve this kind of tight integration with core memory
>> subsystem,
>> the device onboard coherent memory must be represented as a memory only
>> NUMA node. At the same time arch must export some kind of a function to
>> identify of this node as a coherent device memory not any other regular
>> cpu less memory only NUMA node.
>> After achieving the integration with core memory subsystem coherent
>> device
>> memory might still need some special consideration inside the kernel.
>> There
>> can be a variety of coherent memory nodes with different expectations
>> from
>> the core kernel memory. But right now only one kind of special
>> treatment is
>> considered which requires certain isolation.
>> Now consider the case of a coherent device memory node type which
>> requires
>> isolation. This kind of coherent memory is onboard an external device
>> attached to the system through a link where there is always a chance of a
>> link failure taking down the entire memory node with it. More over the
>> memory might also have higher chance of ECC failure as compared to the
>> system RAM. Hence allocation into this kind of coherent memory node
>> should
>> be regulated. Kernel allocations must not come here. Normal user space
>> allocations too should not come here implicitly (without user application
>> knowing about it). This summarizes isolation requirement of certain
>> kind of
>> coherent device memory node as an example. There can be different
>> kinds of
>> isolation requirement also.
>> Some coherent memory devices might not require isolation altogether after
>> all. Then there might be other coherent memory devices which might
>> require
>> some other special treatment after being part of core memory
>> representation
>> . For now, will look into isolation seeking coherent device memory
>> node not
>> the other ones.
> Hi Anshuman,
> I'd question the need to avoid kernel allocations in device memory.
> Maybe we should simply allow these pages to *potentially* participate in
> everything that N_MEMORY pages do: huge pages, kernel allocations, for
> example.

No, allowing kernel allocations on CDM has two problems.

* Kernel data structure should not go and be on CDM which is specialized
and may not be as reliable and may not have the same latency as that of
system RAM.

* It prevents seamless hot plugging of CDM node in and out of kernel

> There is a bit too much emphasis being placed on the idea that these
> devices are less reliable than system memory. It's true--they are less
> reliable. However, they are reliable enough to be allowed direct
> (coherent) addressing. And anything that allows that, is, IMHO, good
> enough to allow all allocations on it.

User space allocation not kernel at this point. Kernel is exposed to the
unreliability while accessing it coherently but not being on it. There
is a difference in the magnitude of risk and its mitigation afterwards.

> On the point of what reliability implies: I've been involved in the
> development (and debugging) of similar systems over the years, and what
> happens is: if the device has a fatal error, you have to take the
> computer down, some time in the near future. There are a few reasons for
> this:
> -- sometimes the MCE (machine check) is wired up to fire, if the
> device has errors, in which case you are all done very quickly. :)

We can still handle MCE right now that may just involve killing the user
application accessing given memory and the kernel can still continue
running uninterrupted.

> -- other times, the operating system relied upon now-corrupted data,
> that came from the device. So even if you claim "OK, the device has a
> fatal error, but the OS can continue running just fine", that's just
> wrong! You may have corrupted something important.

No, all that kernel facilitate is migration right now where it will access
the CDM memory during which it can still crash if there is a memory error
on CDM (which can be mitigated without crashing the kernel) but it does
not depend on the content of memory which might have been corrupted by now.

> -- even if the above two didn't get you, you still have a likely
> expensive computer that cannot do what you bought it for, so you've got
> to shut it down and replace the failed device.

I am afraid that is not a valid kernel design goal :) But more likely the
driver of the device can hot plug it out, repair it and plug it back on.

> Given all that, I think it is not especially worthwhile to design in a
> lot of constraints and limitations around coherent device memory.

I disagree on this because of all the points explained above.

> As for speed, we should be able to put in some hints to help with page
> placement. I'm still coming up to speed with what is already there, and
> I'm sure other people can comment on that.
> We should probably just let the allocations happen.
>> To implement the integration as well as isolation, the coherent memory
>> node
>> must be present in N_MEMORY and a new N_COHERENT_DEVICE node mask inside
>> the node_states[] array. During memory hotplug operations, the new
>> nodemask
>> N_COHERENT_DEVICE is updated along with N_MEMORY for these coherent
>> device
>> memory nodes. This also creates the following new sysfs based
>> interface to
>> list down all the coherent memory nodes of the system.
>> /sys/devices/system/node/is_coherent_node
> The naming bothers me: all nodes are coherent already. In fact, the
> Coherent Device Memory naming is a little off-base already: what is it
> *really* trying to say? Less reliable? Slower? My-special-device? :)

I can change the above interface file to "is_cdm_node" to make it more
on track. CDM conveys the fact that its a on device memory which is
coherent not same as system RAM. This can also accommodate special memory
which might be on the chip and but not same as system RAM.

> Will those things even always be true? Makes me question the whole CDM
> concept. Maybe just ZONE_MOVABLE (to handle hotplug) is the way to go.

If you think any device memory which does not fit the description mentioned
for a CDM memory, yes it can be plugged in as ZONE_MOVABLE into the kernel.
CDM framework applies for device memory which fits the description as
intended and explained.

>> Architectures must export function arch_check_node_cdm() which identifies
>> any coherent device memory node in case they enable
>> Signed-off-by: Anshuman Khandual <khandual@xxxxxxxxxxxxxxxxxx>
>> ---
>> Documentation/ABI/stable/sysfs-devices-node | 7 ++++
>> arch/powerpc/Kconfig | 1 +
>> arch/powerpc/mm/numa.c | 7 ++++
>> drivers/base/node.c | 6 +++
>> include/linux/nodemask.h | 58
>> ++++++++++++++++++++++++++++-
>> mm/Kconfig | 4 ++
>> mm/memory_hotplug.c | 3 ++
>> mm/page_alloc.c | 8 +++-
>> 8 files changed, 91 insertions(+), 3 deletions(-)
>> diff --git a/Documentation/ABI/stable/sysfs-devices-node
>> b/Documentation/ABI/stable/sysfs-devices-node
>> index 5b2d0f0..fa2f105 100644
>> --- a/Documentation/ABI/stable/sysfs-devices-node
>> +++ b/Documentation/ABI/stable/sysfs-devices-node
>> @@ -29,6 +29,13 @@ Description:
>> Nodes that have regular or high memory.
>> Depends on CONFIG_HIGHMEM.
>> +What: /sys/devices/system/node/is_coherent_device
>> +Date: January 2017
>> +Contact: Linux Memory Management list <linux-mm@xxxxxxxxx>
>> +Description:
>> + Lists the nodemask of nodes that have coherent device memory.
>> +
>> What: /sys/devices/system/node/nodeX
>> Date: October 2002
>> Contact: Linux Memory Management list <linux-mm@xxxxxxxxx>
>> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
>> index 281f4f1..1cff239 100644
>> --- a/arch/powerpc/Kconfig
>> +++ b/arch/powerpc/Kconfig
>> @@ -164,6 +164,7 @@ config PPC
>> config GENERIC_CSUM
>> diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
>> index b1099cb..14f0b98 100644
>> --- a/arch/powerpc/mm/numa.c
>> +++ b/arch/powerpc/mm/numa.c
>> @@ -41,6 +41,13 @@
>> #include <asm/setup.h>
>> #include <asm/vdso.h>
>> +inline int arch_check_node_cdm(int nid)
>> +{
>> + return 0;
>> +}
>> +#endif
> I'm not sure that we really need this exact sort of arch_ check. Seems
> like most arches could simply support the possibility of a CDM node.

No, this will be a feature supported by few architectures for now. But the
main reason to make this an arch specific call because only the architecture
can detect which nodes are CDM looking into the platform information such as
ACPI table, DT etc and we dont want that kind of detection to be performed
from the generic MM code.

> But we can probably table that question until we ensure that we want a
> new NUMA node type (vs. ZONE_MOVABLE).

Not sure whether I got this but we want the new NUMA type for isolation

>> +
>> static int numa_enabled = 1;
> [snip]
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index f3e0c69..84d61bb 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -6080,8 +6080,10 @@ static unsigned long __init
>> early_calculate_totalpages(void)
>> unsigned long pages = end_pfn - start_pfn;
>> totalpages += pages;
>> - if (pages)
>> + if (pages) {
>> + node_set_state_cdm(nid);
>> node_set_state(nid, N_MEMORY);
>> + }
>> }
>> return totalpages;
>> }
>> @@ -6392,8 +6394,10 @@ void __init free_area_init_nodes(unsigned long
>> *max_zone_pfn)
>> find_min_pfn_for_node(nid), NULL);
>> /* Any memory on that node */
>> - if (pgdat->node_present_pages)
>> + if (pgdat->node_present_pages) {
>> + node_set_state_cdm(nid);
>> node_set_state(nid, N_MEMORY);
> I like that you provide clean wrapper functions, but air-dropping them
> into all these routines (none of the other node types have to do this)
> makes it look like CDM is sort of hacked in. :)

Yeah and thats special casing CDM under a config option. These updates are
required to make CDM nodes identifiable inside the kernel.