[PATCH 0/5] Avoid pessimistic scatter-gather allocation

From: Tvrtko Ursulin
Date: Fri Oct 21 2016 - 10:13:19 EST

From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>

We can decrease the i915 kernel memory usage by doing more sg list
coallescing and avoiding the pessimistic list allocation.

At the moment we got two places in our code, the main shmemfs backed
object allocator, and the userptr object allocator, which both can
allocate sg list size pessimistically, and in the latter case also do
not exploit entry coallescing when it is possible.

This results in between one to six megabytes of memory wasted on unused
sg list entries under some common workloads:

* Logging into KDE there is 1-2 MiB of unused sg entries.
* Running the T-Rex benchamrk aroun 3 Mib.
* Similarly for Manhattan 5-6 MiB.

To remove this wastage this series starts with some cleanups in the
sg_alloc_table_from_pages implementation and then adds and exports a new
__sg_alloc_table_from_pages function.

This then gets used by the i915 driver to achieve the described savings.

Tvrtko Ursulin (5):
lib/scatterlist: Fix offset type in sg_alloc_table_from_pages
lib/scatterlist: Avoid potential scatterlist entry overflow
lib/scatterlist: Introduce and export __sg_alloc_table_from_pages
drm/i915: Use __sg_alloc_table_from_pages for allocating object
backing store
drm/i915: Use __sg_alloc_table_from_pages for userptr allocations

drivers/gpu/drm/i915/i915_drv.h | 9 +++
drivers/gpu/drm/i915/i915_gem.c | 77 +++++++++++--------------
drivers/gpu/drm/i915/i915_gem_userptr.c | 29 +++-------
drivers/media/v4l2-core/videobuf2-dma-contig.c | 4 +-
drivers/rapidio/devices/rio_mport_cdev.c | 4 +-
include/linux/scatterlist.h | 11 ++--
lib/scatterlist.c | 78 ++++++++++++++++++++------
7 files changed, 120 insertions(+), 92 deletions(-)