Re: [PATCH v2 00/11] mempolicy2, mbind2, and weighted interleave

Gregory Price
2023-12-11T16:42:31Z

On Mon, Dec 11, 2023 at 01:53:40PM +0800, Huang, Ying wrote:
> Hi, Gregory,
>
> Thanks for updated version!
>
> Gregory Price <gourry.memverge@gmail.com> writes:
>
> > v2:
> > changes / adds:
> > - flattened weight matrix to an array at requested of Ying Huang
> > - Updated ABI docs per Davidlohr Bueso request
> > - change uapi structure to use aligned/fixed-length members as
> > Suggested-by: Arnd Bergmann <arnd@arndb.de>
> > - Implemented weight fetch logic in get_mempolicy2
> > - mbind2 was changed to take (iovec,len) as function arguments
> > rather than add them to the uapi structure, since they describe
> > where to apply the mempolicy - as opposed to being part of it.
> >
> > The sysfs structure is designed as follows.
> >
> > $ tree /sys/kernel/mm/mempolicy/
> > /sys/kernel/mm/mempolicy/
> > ├── possible_nodes
> > └── weighted_interleave
> > ├── nodeN
> > │  └── weight
> > └── nodeN+X
> >   └── weight
> >
> > 'mempolicy' is added to '/sys/kernel/mm/' as a control group for
> > the mempolicy subsystem.
>
> Is it good to add 'mempolicy' in '/sys/kernel/mm/numa'? The advantage
> is that 'mempolicy' here is in fact "NUMA mempolicy". The disadvantage
> is one more directory nesting. I have no strong opinion here.
>

i don't have a strong opinion here.

> > 'possible_nodes' is added to 'mm/mempolicy' to help describe the
> > expected structures under mempolicy directorys. For example,
> > possible_nodes describes what nodeN directories wille exist under
> > the weighted_interleave directory.
>
> We have '/sys/devices/system/node/possible' already. Is this just a
> duplication? If so, why? And, the possible nodes can be gotten via
> contents of 'weighted_interleave' too.
>

I'll remove it

> And it appears not necessary to make 'weighted_interleave/nodeN'
> directory. Why not just make it a file.
>

Originally I wasn't sure whether there would be more attributes, but
this is probably fine. I'll change it.

> And, can we add a way to reset weight to the default value? For example
> `echo > nodeN/weight` or `echo > nodeN`.
>

Seems reasonable.

> > =====================================================================
> > (Patches 7-10) set_mempolicy2, get_mempolicy2, mbind2
> >
> > These interfaces are the 'extended' counterpart to their relatives.
> > They use the userland 'struct mpol_args' structure to communicate a
> > complete mempolicy configuration to the kernel. This structure
> > looks very much like the kernel-internal 'struct mempolicy_args':
> >
> > struct mpol_args {
> > /* Basic mempolicy settings */
> > __u16 mode;
> > __u16 mode_flags;
> > __s32 home_node;
> > __aligned_u64 pol_nodes;
> > __u64 pol_maxnodes;
> > __u64 addr;
> > __s32 policy_node;
> > __s32 addr_node;
> > __aligned_u64 *il_weights; /* of size pol_maxnodes */
> > };
>
> This looks unnecessarily complex. I don't think that it's a good idea
> to use exact same parameter for all 3 syscalls.
>

It is exactly as complex as mempolicy is. Everything here is already
described in the existing interfaces (except il_weights).

> For example, can we use something as below?
>
> long set_mempolicy2(int mode, const unsigned long *nodemask, unsigned int *il_weights,
> unsigned long maxnode, unsigned long home_node,
> unsigned long flags);
>
> long mbind2(unsigned long start, unsigned long len,
> int mode, const unsigned long *nodemask, unsigned int *il_weights,
> unsigned long maxnode, unsigned long home_node,
> unsigned long flags);
>

Your definition of mbind2 is impossible.

Neither of these interfaces solve the extensibility issue. If a new
policy which requires a new format of data arrives, we can look forward
to set_mempolicy3 and mbind3.

> A struct may be defined to hold mempolicy iteself.
>
> struct mpol {
> int mode;
> unsigned int home_node;
> const unsigned long *nodemask;
> unsigned int *il_weights;
> unsigned int maxnode;
> };
>

addr could be pulled out for get_mempolicy2, so i will do that

'addr_node' and 'policy_node' are warts that came from the original
get_mempolicy. Removing them increases the complexity of handling
arguments in the common get_mempolicy code.

I could probably just drop support for retrieving the addr_node from
get_mempolicy2, since it's already possible with get_mempolicy. So I
will do that.

~Gregory