On Thu, Feb 11, 2021 at 04:20:31PM +0000, Chuck Lever wrote:
> > On Feb 11, 2021, at 4:12 AM, Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > <SNIP>
> >
> > Parameters to __rmqueue_pcplist are garbage as the parameter order changed.
> > I'm surprised it didn't blow up in a spectacular fashion. Again, this
> > hasn't been near any testing and passing a list with high orders to
> > free_pages_bulk() will corrupt lists too. Mostly it's a curiousity to see
> > if there is justification for reworking the allocator to fundamentally
> > deal in batches and then feed batches to pcp lists and the bulk allocator
> > while leaving the normal GFP API as single page "batches". While that
> > would be ideal, it's relatively high risk for regressions. There is still
> > some scope for adding a basic bulk allocator before considering a major
> > refactoring effort.
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index f8353ea7b977..8f3fe7de2cf7 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -5892,7 +5892,7 @@ __alloc_pages_bulk_nodemask(gfp_t gfp_mask, unsigned int order,
> > pcp_list = &pcp->lists[migratetype];
> >
> > while (nr_pages) {
> > - page = __rmqueue_pcplist(zone, gfp_mask, migratetype,
> > + page = __rmqueue_pcplist(zone, migratetype, alloc_flags,
> > pcp, pcp_list);
> > if (!page)
> > break;
>
> The NFS server is considerably more stable now. Thank you!
>
Thanks for testing!
> I confirmed that my patch is requesting and getting multiple pages.
> The new NFSD code and the API seem to be working as expected.
>
> The results are stunning. Each svc_alloc_arg() call here allocates
> 65 pages to satisfy a 256KB NFS READ request.
>
> Before:
>
> nfsd-972 [000] 584.513817: funcgraph_entry: + 35.385 us | svc_alloc_arg();
> nfsd-979 [002] 584.513870: funcgraph_entry: + 29.051 us | svc_alloc_arg();
> nfsd-980 [001] 584.513951: funcgraph_entry: + 29.178 us | svc_alloc_arg();
> nfsd-983 [000] 584.514014: funcgraph_entry: + 29.211 us | svc_alloc_arg();
> nfsd-976 [002] 584.514059: funcgraph_entry: + 29.315 us | svc_alloc_arg();
> nfsd-974 [001] 584.514127: funcgraph_entry: + 29.237 us | svc_alloc_arg();
>
> After:
>
> nfsd-977 [002] 87.049425: funcgraph_entry: 4.293 us | svc_alloc_arg();
> nfsd-981 [000] 87.049478: funcgraph_entry: 4.059 us | svc_alloc_arg();
> nfsd-988 [001] 87.049549: funcgraph_entry: 4.474 us | svc_alloc_arg();
> nfsd-983 [003] 87.049612: funcgraph_entry: 3.819 us | svc_alloc_arg();
> nfsd-976 [000] 87.049619: funcgraph_entry: 3.869 us | svc_alloc_arg();
> nfsd-980 [002] 87.049738: funcgraph_entry: 4.124 us | svc_alloc_arg();
> nfsd-975 [000] 87.049769: funcgraph_entry: 3.734 us | svc_alloc_arg();
>
Uhhhh, that is much better than I expected given how lame the
implementation is. Sure -- it works, but it has more overhead than it
should with the downside that reducing it requires fairly deep surgery. It
may be enough to tidy this up to handle order-0 pages only to start with
and see how far it gets. That's a fairly trivial modification.
> There appears to be little cost change for single-page allocations
> using the bulk allocator (nr_pages=1):
>
> Before:
>
> nfsd-985 [003] 572.324517: funcgraph_entry: 0.332 us | svc_alloc_arg();
> nfsd-986 [001] 572.324531: funcgraph_entry: 0.311 us | svc_alloc_arg();
> nfsd-985 [003] 572.324701: funcgraph_entry: 0.311 us | svc_alloc_arg();
> nfsd-986 [001] 572.324727: funcgraph_entry: 0.424 us | svc_alloc_arg();
> nfsd-985 [003] 572.324760: funcgraph_entry: 0.332 us | svc_alloc_arg();
> nfsd-986 [001] 572.324786: funcgraph_entry: 0.390 us | svc_alloc_arg();
>
> After:
>
> nfsd-989 [002] 75.043226: funcgraph_entry: 0.322 us | svc_alloc_arg();
> nfsd-988 [001] 75.043436: funcgraph_entry: 0.368 us | svc_alloc_arg();
> nfsd-989 [002] 75.043464: funcgraph_entry: 0.424 us | svc_alloc_arg();
> nfsd-988 [001] 75.043490: funcgraph_entry: 0.317 us | svc_alloc_arg();
> nfsd-989 [002] 75.043517: funcgraph_entry: 0.425 us | svc_alloc_arg();
> nfsd-988 [001] 75.050025: funcgraph_entry: 0.407 us | svc_alloc_arg();
>
That is not too surprising given that there would be some additional
overhead to manage a list of 1 page. I would hope that users of the bulk
allocator are not routinely calling it with nr_pages == 1.
--
Mel Gorman
SUSE Labs
