Re: [PATCH 6/6] hugetlb: update hugetlb documentation for mempolicy based management.

David Rientjes
2009-09-11T22:27:30Z

On Thu, 10 Sep 2009, Mel Gorman wrote:

> > Would you explain why introducing a new mempolicy flag, MPOL_F_HUGEPAGES,
> > and only using the new behavior when this is set would be inconsistent or
> > inadvisible?
>
> I already explained this. The interface in numactl would look weird. There
> would be an --interleave switch and a --hugepages-interleave that only
> applies to nr_hugepages. The smarts could be in hugeadm to apply the mask
> when --pool-pages-min is specified but that wouldn't help scripts that are
> still using echo.
>

I don't think we need to address the scripts that are currently using echo
since they're (hopefully) written to the kernel implementation, i.e. no
mempolicy restriction on writing to nr_hugepages.

> I hate to have to do this, but how about nr_hugepages which acts
> system-wide as it did traditionally and nr_hugepages_mempolicy that obeys
> policies? Something like the following untested patch. It would be fairly
> trivial for me to implement a --obey-mempolicies switch for hugeadm which
> works in conjunction with --pool--pages-min and less likely to cause confusion
> than --hugepages-interleave in numactl.
>

I like it.

> Sorry the patch is untested. I can't hold of a NUMA machine at the moment
> and fake NUMA support sucks far worse than I expected it to.
>

Hmm, I rewrote most of fake NUMA a couple years ago. What problems are
you having with it?

> ==== BEGIN PATCH ====
>
> [PATCH] Optionally use a memory policy when tuning the size of the static hugepage pool
>
> Patch "derive huge pages nodes allowed from task mempolicy" brought
> huge page support more in line with the core VM in that tuning the size
> of the static huge page pool would obey memory policies. Using this,
> administrators could interleave allocation of huge pages from a subset
> of nodes. This is consistent with how dynamic hugepage pool resizing
> works and how hugepages get allocated to applications at run-time.
>
> However, it was pointed out that scripts may exist that depend on being
> able to drain all hugepages via /proc/sys/vm/nr_hugepages from processes
> that are running within a memory policy. This patch adds
> /proc/sys/vm/nr_hugepages_mempolicy which when written to will obey
> memory policies. /proc/sys/vm/nr_hugepages continues then to be a
> system-wide tunable regardless of memory policy.
>
> Signed-off-by: Mel Gorman <mel@csn.ul.ie>
> ---
> include/linux/hugetlb.h | 1 +
> kernel/sysctl.c | 11 +++++++++++
> mm/hugetlb.c | 35 ++++++++++++++++++++++++++++++++---
> 3 files changed, 44 insertions(+), 3 deletions(-)
>

It'll need an update to Documentation/vm/hugetlb.txt, but this can
probably be done in one of Lee's patches that edits the same file when he
reposts.

> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index fcb1677..fc3a659 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -21,6 +21,7 @@ static inline int is_vm_hugetlb_page(struct vm_area_struct *vma)
>
> void reset_vma_resv_huge_pages(struct vm_area_struct *vma);
> int hugetlb_sysctl_handler(struct ctl_table *, int, void __user *, size_t *, loff_t *);
> +int hugetlb_mempolicy_sysctl_handler(struct ctl_table *, int, void __user *, size_t *, loff_t *);
> int hugetlb_overcommit_handler(struct ctl_table *, int, void __user *, size_t *, loff_t *);
> int hugetlb_treat_movable_handler(struct ctl_table *, int, void __user *, size_t *, loff_t *);
> int copy_hugetlb_page_range(struct mm_struct *, struct mm_struct *, struct vm_area_struct *);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8bac3f5..0637655 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1171,6 +1171,17 @@ static struct ctl_table vm_table[] = {
> .extra1 = (void *)&hugetlb_zero,
> .extra2 = (void *)&hugetlb_infinity,
> },
> +#ifdef CONFIG_NUMA
> + {
> + .procname = "nr_hugepages_mempolicy",
> + .data = NULL,
> + .maxlen = sizeof(unsigned long),
> + .mode = 0644,
> + .proc_handler = &hugetlb_mempolicy_sysctl_handler,
> + .extra1 = (void *)&hugetlb_zero,
> + .extra2 = (void *)&hugetlb_infinity,
> + },
> +#endif
> {
> .ctl_name = VM_HUGETLB_GROUP,
> .procname = "hugetlb_shm_group",
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 83decd6..68abef0 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -1244,6 +1244,7 @@ static int adjust_pool_surplus(struct hstate *h, nodemask_t *nodes_allowed,
> return ret;
> }
>
> +#define NUMA_NO_NODE_OBEY_MEMPOLICY (-2)
> #define persistent_huge_pages(h) (h->nr_huge_pages - h->surplus_huge_pages)
> static unsigned long set_max_huge_pages(struct hstate *h, unsigned long count,
> int nid)

I think it would be possible to avoid adding NUMA_NO_NODE_OBEY_MEMPOLICY
if the nodemask was allocated in the sysctl handler instead and passing it
into set_max_huge_pages() instead of a nid. Lee, what do you think?

Other than that, I like this approach because it avoids the potential for
userspace breakage while adding the new feature in way that avoids
confusion.