[PATCH 0/3] resource: find_next_iomem_res() improvements

[Nadav Amit <namit@vmware.com>]

Running some microbenchmarks on dax keeps showing find_next_iomem_res()
as a place in which significant amount of time is spent. It appears that
in order to determine the cacheability that is required for the PTE,
lookup_memtype() is called, and this one traverses the resources list in
an inefficient manner. This patch-set tries to improve this situation.

The first patch fixes what appears to be unsafe locking in
find_next_iomem_res().

The second patch improves performance by searching the top level first,
to find a matching range, before going down to the children. The third
patch improves the performance by caching the top level resource of the
last found resource in find_next_iomem_res().

Both of these optimizations are based on the ranges in the top level not
overlapping each other.

Running sysbench on dax (Haswell, pmem emulation, with write_cache
disabled):

  sysbench fileio --file-total-size=3G --file-test-mode=rndwr \
   --file-io-mode=mmap --threads=4 --file-fsync-mode=fdatasync run

Provides the following results:

		events (avg/stddev)
		-------------------
  5.2-rc3:	1247669.0000/16075.39
  +patches:	1293408.5000/7720.69 (+3.5%)

Cc: Borislav Petkov <bp@suse.de>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ingo Molnar <mingo@kernel.org>

Nadav Amit (3):
  resource: Fix locking in find_next_iomem_res()
  resource: Avoid unnecessary lookups in find_next_iomem_res()
  resource: Introduce resource cache

 kernel/resource.c | 96 ++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 79 insertions(+), 17 deletions(-)

-- 
2.20.1