Re: [PATCH v4 00/11] mempolicy2, mbind2, and weighted interleave

[Huang, Ying]

Gregory Price <gregory.price@xxxxxxxxxxxx> writes:

> On Tue, Dec 19, 2023 at 11:04:05AM +0800, Huang, Ying wrote:
>> Gregory Price <gourry.memverge@xxxxxxxxx> writes:
>> 
>> > This patch set extends the mempolicy interface to enable new
>> > mempolicies which may require extended data to operate.
>> >
>> > MPOL_WEIGHTED_INTERLEAVE is included as an example extension.
>> 
>> Per my understanding, it's better to describe why we need this patchset
>> at the beginning.  Per my understanding, weighted interleave is used to
>> expand DRAM bandwidth for workloads with real high memory bandwidth
>> requirements.  Without it, DRAM bandwidth will be saturated, which leads
>> to poor performance.
>> 
>
> Will add more details, thanks.
>
>> > struct mempolicy_args {
>> >     unsigned short mode;            /* policy mode */
>> >     unsigned short mode_flags;      /* policy mode flags */
>> >     int home_node;                  /* mbind: use MPOL_MF_HOME_NODE */
>> >     nodemask_t *policy_nodes;       /* get/set/mbind */
>> >     unsigned char *il_weights;      /* for mode MPOL_WEIGHTED_INTERLEAVE */
>> >     int policy_node;                /* get: policy node information */
>> > };
>> 
>> Because we use more and more parameters to describe the mempolicy, I
>> think it's a good idea to replace some parameters with struct.  But I
>> don't think it's a good idea to put unrelated stuff into the struct.
>> For example,
>> 
>> struct mempolicy_param {
>>     unsigned short mode;            /* policy mode */
>>     unsigned short mode_flags;      /* policy mode flags */
>>     int home_node;                  /* mbind: use MPOL_MF_HOME_NODE */
>>     nodemask_t *policy_nodes;
>>     unsigned char *il_weights;      /* for mode MPOL_WEIGHTED_INTERLEAVE */
>> };
>> 
>> describe the parameters to create the mempolicy.  It can be used by
>> set/get_mempolicy() and mbind().  So, I think that it's a good
>> abstraction.  But "policy_node" has nothing to do with set_mempolicy()
>> and mbind().  So I think that we shouldn't add it into the struct.  It's
>> totally OK to use different parameters for different functions.  For
>> example,
>> 
>> long do_set_mempolicy(struct mempolicy_param *mparam);
>> long do_mbind(unsigned long start, unsigned long len,
>>                 struct mempolicy_param *mparam, unsigned long flags);
>> long do_get_task_mempolicy(struct mempolicy_param *mparam, int
>>                 *policy_node);
>> 
>> This isn't the full list.  My point is to use separate parameter for
>> something specific for some function.
>>
>
> this is the internal structure, but i get the point, we can drop it from
> the structure and extend the arg list internally.
>
> I'd originally thought to just remove the policy_node stuff all
> together from get_mempolicy2().  Do you prefer to have a separate struct
> for set/get interfaces so that the get interface struct can be extended?
>
> All the MPOL_F_NODE "alternate data fetch" mechanisms from
> get_mempolicy() feel like more of a wart than a feature.  And presently
> the only data returned in policy_node is the next allocation node for
> interleave.  That's not even particularly useful, so I'm of a mind to
> remove it.
>
> Assuming we remove policy_node altogether... do we still break up the
> set/get interface into separate structures to avoid this in the future?

I have no much experience at ABI definition.  So, I want to get guidance
from more experienced people on this.

Is it good to implement all functionality of get_mempolicy() with
get_mempolicy2(), so we can deprecate get_mempolicy() and remove it
finally?  So, users don't need to use 2 similar syscalls?

And, IIUC, we will not get policy_node, addr_node, and policy config at
the same time, is it better to use a union instead of struct in
get_mempolicy2()?

>> > struct mpol_args {
>> >         /* Basic mempolicy settings */
>> >         __u16 mode;
>> >         __u16 mode_flags;
>> >         __s32 home_node;
>> >         __aligned_u64 pol_nodes;
>> >         __aligned_u64 *il_weights;      /* of size pol_maxnodes */
>> >         __u64 pol_maxnodes;
>> >         __s32 policy_node;
>> > };
>> 
>> Same as my idea above.  I think we shouldn't add policy_node for
>> set_mempolicy2()/mbind2().  That will make users confusing.  We can use
>> a different struct for get_mempolicy2().
>> 
>
> See above.

--
Best Regards,
Huang, Ying