Re: [PATCH 5/5] [hugetlb] Try to grow pool for MAP_SHARED mappings

Nish Aravamudan
2007-07-18T21:16:24Z

On 7/18/07, Lee Schermerhorn <Lee.Schermerhorn@hp.com> wrote:
> On Wed, 2007-07-18 at 08:17 -0700, Nish Aravamudan wrote:
> > On 7/18/07, Lee Schermerhorn <Lee.Schermerhorn@hp.com> wrote:
> > > On Tue, 2007-07-17 at 16:42 -0700, Nish Aravamudan wrote:
> > > > On 7/13/07, Paul Jackson <pj@sgi.com> wrote:
> > > > > Adam wrote:
> > > > > > To be honest, I just don't think a global hugetlb pool and cpusets are
> > > > > > compatible, period.
> > > > >
> > > > > It's not an easy fit, that's for sure ;).
> > > >
> > > > In the context of my patches to make the hugetlb pool's interleave
> > > > work with memoryless nodes, I may have pseudo-solution for growing the
> > > > pool while respecting cpusets.
> > > >
> > > > Essentially, given that GFP_THISNODE allocations stay on the node
> > > > requested (which is the case after Christoph's set of memoryless node
> > > > patches go in), we invoke:
> > > >
> > > > pol = mpol_new(MPOL_INTERLEAVE, &node_states[N_MEMORY])
> > > >
> > > > in the two callers of alloc_fresh_huge_page(pol) in hugetlb.c.
> > > > alloc_fresh_huge_page() in turn invokes interleave_nodes(pol) so that
> > > > we request hugepages in an interleaved fashion over all nodes with
> > > > memory.
> > > >
> > > > Now, what I'm wondering is why interleave_nodes() is not cpuset aware?
> > > > Or is it expected that the caller do the right thing with the policy
> > > > beforehand? If so, I think I could just make those two callers do
> > > >
> > > > pol = mpol_new(MPOL_INTERLEAVE, cpuset_mems_allowed(current))
> > > >
> > > > ?
> > > >
> > > > Or am I way off here?
> > >
> > >
> > > Nish:
> > >
> > > I have always considered the huge page pool, as populated by
> > > alloc_fresh_huge_page() in response to changes in nr_hugepages, to be a
> > > system global resource. I think the system "does the right
> > > thing"--well, almost--with Christoph's memoryless patches and your
> > > hugetlb patches. Certaintly, the huge pages allocated at boot time,
> > > based on the command line parameter, are system-wide. cpusets have not
> > > been set up at that time.
> >
> > I fully agree that hugepages are a global resource.
> >
> > > It requires privilege to write to the nr_hugepages sysctl, so allowing
> > > it to spread pages across all available nodes [with memory], regardless
> > > of cpusets, makes sense to me. Altho' I don't expect many folks are
> > > currently changing nr_hugepages from within a constrained cpuset, I
> > > wouldn't want to see us change existing behavior, in this respect. Your
> > > per node attributes will provide the mechanism to allocate different
> > > numbers of hugepages for, e.g., nodes in cpusets that have applications
> > > that need them.
> >
> > The issue is that with Adam's patches, the hugepage pool will grow on
> > demand, presuming the process owner's mlock limit is sufficiently
> > high. If said process were running within a constrained cpuset, it
> > seems slightly out-of-whack to allow it grow the pool on other nodes
> > to satisfy the demand.
>
> Ah, I see. In that case, it might make sense to grow just for the
> cpuset. A couple of things come to mind tho':
>
> 1) we might want a per cpuset control to enable/disable hugetlb pool
> growth on demand, or to limit the max size of the pool--especially if
> the memories are not exclusively owned by the cpuset. Otherwise,
> non-privileged processes could grow the hugetlb pool in memories shared
> with other cpusets [maybe the root cpuset?], thereby reducing the amount
> of normal, managed pages available to the other cpusets. Probably want
> such a control in the absense of cpusets as well, if on-demand hugetlb
> pool growth is implemented.

Well, the current restriction is on a per-process basis for locked
memory. But it might make sense to add a separate rlimit for hugepages
and then just allow cpusets to restrict that rlimit for processes
contained therein?

Similar would probably hold for the non-cpuset case?

But that seems like special casing for hugetlb pages where small pages
don't have the same restriction. If two cpusets share the same node,
can't one exhaust the node and thus starve the other cpuset? At that
point you need more than cpusets (arguably) and want resource
management at some level.

> 2) per cpuset, on-demand hugetlb pool growth shouldn't affect the
> behavior of the nr_hugepages sysctl--IMO, anyway.

Right, it doesn't as of right now. But we have an existing issue
(independent of hugetlb pool growth, just made more apparent that way)
for cpusets and run-time growth of the pool (which is more likely to
succeed (and perhaps happen) with Mel's patches, recently added to
Linus' tree). So I'm just trying to decide if it will be sufficient to
just obey the cpuset's allocation restrictions, if they have any.

> 3) managed "superpages" keeps sounding better and better ;-)

Preaching to the choir ... Still, have customers to support with the
current solution and want to do right by them, of course.

> > > Re: the "well, almost": nr_hugepages is still "broken" for me on some
> > > of my platforms where the interleaved, dma-only pseudo-node contains
> > > sufficient memory to satisfy a hugepage request. I'll end up with a few
> > > hugepages consuming most of the dma memory. Consuming the dma isn't the
> > > issue--there should be enough remaining for any dma needs. I just want
> > > more control over what gets placed on the interleaved pseudo-node by
> > > default. I think that Paul Mundt [added to cc list] has similar
> > > concerns about default policies on the sh platforms. I have some ideas,
> > > but I'm waiting for the memoryless nodes and your patches to stabilize
> > > in the mm tree.
> >
> > And well, we're already 'broken' as far as I can tell with cpusets and
> > the hugepage pool. I'm just trying to decide if it's fixable as is, or
> > if we need extra cleverness. A simple hack would be to just modify the
> > interleave call with a callback that uses the appropriate mask if
> > CPUSETS is on or off (I don't want to always use cpuset_mems_allowed()
> > unconditionally, becuase it returns node_possible_map if !CPUSETS.
>
> Maybe you want/need a cpuset_hugemems_allowed() that does "the right
> thing" with and without cpusets?

Yeah, that would be the callback I'd use (most likely). Would make
sense to just put it with the other cpuset code, though, good idea.

> > Thanks for the feedback. If folks are ok with the way things are, then
> > so be it. I was just hoping Paul might have some thoughts on how best
> > to avoid violating cpuset constraints with Adam's patches in the
> > context of my patches.
>
> I'm not trying to discourage you, here. I agree that cpusets, as useful
> as I find them, do make things, uh, "interesting"--especially with
> shared resources.

Definitely. Just wanted to get some input. Will hopefully get around
to making my callback change suggested above to restrict pool growth
and will test on some NUMA boxen today or tomorrow.

Thanks,
Nish