This patch updates the kernel documentation. Signed-off-by: Aravinda Prasad <aravinda.prasad@xxxxxxxxx> --- Documentation/mm/damon/design.rst | 42 +++++++++++++++++++++++++++++++ 1 file changed, 42 insertions(+) diff --git a/Documentation/mm/damon/design.rst b/Documentation/mm/damon/design.rst index 5620aab9b385..59014ecbb551 100644 --- a/Documentation/mm/damon/design.rst +++ b/Documentation/mm/damon/design.rst @@ -139,6 +139,48 @@ the interference is the responsibility of sysadmins. However, it solves the conflict with the reclaim logic using ``PG_idle`` and ``PG_young`` page flags, as Idle page tracking does. +Profiling enhancement for virtual address space +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +For virtual address space tracking, relying on checking Accessed bit(s) only +at the leaf level of the page table is inefficient. Hardware architectures +have supported Accessed bit(s) at all levels of the page table tree by +updating them during the page table walk. Hence, DAMON dynamically +profiles different levels (PMD/PUD/P4D) of a multi-level page table tree. + +DAMON leverages the following key insight: a data page that is accessed +should also have the Accessed bit set at PMD, PUD, P4D, and PGD entry. +Similarly, if the Accessed bit in a PGD entry (or a PUD/PMD entry) is +not set, then none of the data pages under the PGD entry (or PUD/PMD +entry) subtree are accessed. DAMON profiles Accessed bits at the highest +possible level of the page table tree to identify the regions that are +accessed. + +For example, consider a region and the sampling address (SA) in the below +figure. The address range of a PUD entry corresponding to SA is within +region bounds and hence PUD is picked for checking and setting the +Accessed bits. However, this not true if P4D is picked for profiling. +Hence in this case PUD is the highest possible level that can be picked +for profiling. + ....... + + P4D + + ....... + / \ + / \ + / \ + / \ + / \ + / ....... \ + / + PUD + \ + / ....... \ + / / \ \ +- - - - - +-----*---*--+====+-*------+- -*- - - + + # SA # + + + # # + +- - - - - +------------+====+--------+- - - - - + + | ----- DAMON region ------| + Core Logics =========== -- 2.21.3
