On Fri, Jul 26, 2019 at 11:23:19AM -0400, Joel Fernandes (Google) wrote: > This patch updates the documentation with the new page_idle tracking > feature which uses virtual address indexing. > > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> One nit below, otherwise Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxx> > --- > .../admin-guide/mm/idle_page_tracking.rst | 43 ++++++++++++++++--- > 1 file changed, 36 insertions(+), 7 deletions(-) > > diff --git a/Documentation/admin-guide/mm/idle_page_tracking.rst b/Documentation/admin-guide/mm/idle_page_tracking.rst > index df9394fb39c2..1eeac78c94a7 100644 > --- a/Documentation/admin-guide/mm/idle_page_tracking.rst > +++ b/Documentation/admin-guide/mm/idle_page_tracking.rst > @@ -19,10 +19,14 @@ It is enabled by CONFIG_IDLE_PAGE_TRACKING=y. > > User API > ======== > +There are 2 ways to access the idle page tracking API. One uses physical > +address indexing, another uses a simpler virtual address indexing scheme. > > -The idle page tracking API is located at ``/sys/kernel/mm/page_idle``. > -Currently, it consists of the only read-write file, > -``/sys/kernel/mm/page_idle/bitmap``. > +Physical address indexing > +------------------------- > +The idle page tracking API for physical address indexing using page frame > +numbers (PFN) is located at ``/sys/kernel/mm/page_idle``. Currently, it > +consists of the only read-write file, ``/sys/kernel/mm/page_idle/bitmap``. > > The file implements a bitmap where each bit corresponds to a memory page. The > bitmap is represented by an array of 8-byte integers, and the page at PFN #i is > @@ -74,6 +78,31 @@ See :ref:`Documentation/admin-guide/mm/pagemap.rst <pagemap>` for more > information about ``/proc/pid/pagemap``, ``/proc/kpageflags``, and > ``/proc/kpagecgroup``. > > +Virtual address indexing > +------------------------ > +The idle page tracking API for virtual address indexing using virtual page > +frame numbers (VFN) is located at ``/proc/<pid>/page_idle``. It is a bitmap > +that follows the same semantics as ``/sys/kernel/mm/page_idle/bitmap`` > +except that it uses virtual instead of physical frame numbers. Can you please make it more explicit that VFNs are in the <pid>'s address space? > + > +This idle page tracking API does not need deal with PFN so it does not require > +prior lookups of ``pagemap`` in order to find if page is idle or not. This is > +an advantage on some systems where looking up PFN is considered a security > +issue. Also in some cases, this interface could be slightly more reliable to > +use than physical address indexing, since in physical address indexing, address > +space changes can occur between reading the ``pagemap`` and reading the > +``bitmap``, while in virtual address indexing, the process's ``mmap_sem`` is > +held for the duration of the access. > + > +To estimate the amount of pages that are not used by a workload one should: > + > + 1. Mark all the workload's pages as idle by setting corresponding bits in > + ``/proc/<pid>/page_idle``. > + > + 2. Wait until the workload accesses its working set. > + > + 3. Read ``/proc/<pid>/page_idle`` and count the number of bits set. > + > .. _impl_details: > > Implementation Details > @@ -99,10 +128,10 @@ When a dirty page is written to swap or disk as a result of memory reclaim or > exceeding the dirty memory limit, it is not marked referenced. > > The idle memory tracking feature adds a new page flag, the Idle flag. This flag > -is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` (see the > -:ref:`User API <user_api>` > -section), and cleared automatically whenever a page is referenced as defined > -above. > +is set manually, by writing to ``/sys/kernel/mm/page_idle/bitmap`` for physical > +addressing or by writing to ``/proc/<pid>/page_idle`` for virtual > +addressing (see the :ref:`User API <user_api>` section), and cleared > +automatically whenever a page is referenced as defined above. > > When a page is marked idle, the Accessed bit must be cleared in all PTEs it is > mapped to, otherwise we will not be able to detect accesses to the page coming > -- > 2.22.0.709.g102302147b-goog > -- Sincerely yours, Mike.
