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

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Sep 11, 2009 at 03:27:30PM -0700, David Rientjes wrote:
> 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.
> 

Ok.

> > 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.
> 

Ok, when I get this tested, I'll sent it as a follow-on patch to Lee's
for proper incorporation.

> > 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?
> 

On PPC64, the parameters behave differently. I couldn't convince it to
create more than one NUMA node. On x86-64, the NUMA nodes appeared to
exist and would be visible on /proc/buddyinfo for example but the sysfs
directories for the fake nodes were not created so nr_hugepages couldn't
be examined on a per-node basis for example.

> > ==== 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.
> 

Agreed.

> > 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.
> 

Indeed. While the addition of another proc tunable sucks, it seems like
the only available compromise.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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