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SubjectRe: [PATCH 3/3] resource: Introduce resource cache
On Tue, Jun 18, 2019 at 12:40 AM Nadav Amit <namit@vmware.com> wrote:
>
> > On Jun 17, 2019, at 10:33 PM, Nadav Amit <namit@vmware.com> wrote:
> >
> >> On Jun 17, 2019, at 9:57 PM, Andrew Morton <akpm@linux-foundation.org> wrote:
> >>
> >> On Wed, 12 Jun 2019 21:59:03 -0700 Nadav Amit <namit@vmware.com> wrote:
> >>
> >>> For efficient search of resources, as needed to determine the memory
> >>> type for dax page-faults, introduce a cache of the most recently used
> >>> top-level resource. Caching the top-level should be safe as ranges in
> >>> that level do not overlap (unlike those of lower levels).
> >>>
> >>> Keep the cache per-cpu to avoid possible contention. Whenever a resource
> >>> is added, removed or changed, invalidate all the resources. The
> >>> invalidation takes place when the resource_lock is taken for write,
> >>> preventing possible races.
> >>>
> >>> This patch provides relatively small performance improvements over the
> >>> previous patch (~0.5% on sysbench), but can benefit systems with many
> >>> resources.
> >>
> >>> --- a/kernel/resource.c
> >>> +++ b/kernel/resource.c
> >>> @@ -53,6 +53,12 @@ struct resource_constraint {
> >>>
> >>> static DEFINE_RWLOCK(resource_lock);
> >>>
> >>> +/*
> >>> + * Cache of the top-level resource that was most recently use by
> >>> + * find_next_iomem_res().
> >>> + */
> >>> +static DEFINE_PER_CPU(struct resource *, resource_cache);
> >>
> >> A per-cpu cache which is accessed under a kernel-wide read_lock looks a
> >> bit odd - the latency getting at that rwlock will swamp the benefit of
> >> isolating the CPUs from each other when accessing resource_cache.
> >>
> >> On the other hand, if we have multiple CPUs running
> >> find_next_iomem_res() concurrently then yes, I see the benefit. Has
> >> the benefit of using a per-cpu cache (rather than a kernel-wide one)
> >> been quantified?
> >
> > No. I am not sure how easy it would be to measure it. On the other hander
> > the lock is not supposed to be contended (at most cases). At the time I saw
> > numbers that showed that stores to “exclusive" cache lines can be as
> > expensive as atomic operations [1]. I am not sure how up to date these
> > numbers are though. In the benchmark I ran, multiple CPUs ran
> > find_next_iomem_res() concurrently.
> >
> > [1] http://sigops.org/s/conferences/sosp/2013/papers/p33-david.pdf
>
> Just to clarify - the main motivation behind the per-cpu variable is not
> about contention, but about the fact the different processes/threads that
> run concurrently might use different resources.

IIUC, the underlying problem is that dax relies heavily on ioremap(),
and ioremap() on x86 takes too long because it relies on
find_next_iomem_res() via the __ioremap_caller() ->
__ioremap_check_mem() -> walk_mem_res() path.

The fact that x86 is the only arch that does this much work in
ioremap() makes me wonder. Is there something unique about x86
mapping attributes that requires this extra work, or is there some way
this could be reworked to avoid searching the resource map in the
first place?

Bjorn

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