[PATCH v3 0/8] padata: parallelize deferred page init

From: Daniel Jordan
Date: Wed May 27 2020 - 13:38:18 EST

Thanks to Alex for his continued review and Josh for running v2! Please
continue to review and test, and acks for the padata parts would be



Deferred struct page init is a bottleneck in kernel boot--the biggest
for us and probably others. Optimizing it maximizes availability for
large-memory systems and allows spinning up short-lived VMs as needed
without having to leave them running. It also benefits bare metal
machines hosting VMs that are sensitive to downtime. In projects such
as VMM Fast Restart[1], where guest state is preserved across kexec
reboot, it helps prevent application and network timeouts in the guests.

So, multithread deferred init to take full advantage of system memory

Extend padata, a framework that handles many parallel singlethreaded
jobs, to handle multithreaded jobs as well by adding support for
splitting up the work evenly, specifying a minimum amount of work that's
appropriate for one helper thread to do, load balancing between helpers,
and coordinating them. More documentation in patches 4 and 8.

This series is the first step in a project to address other memory
proportional bottlenecks in the kernel such as pmem struct page init,
vfio page pinning, hugetlb fallocate, and munmap. Deferred page init
doesn't require concurrency limits, resource control, or priority
adjustments like these other users will because it happens during boot
when the system is otherwise idle and waiting for page init to finish.

This has been run on a variety of x86 systems and speeds up kernel boot
by 4% to 49%, saving up to 1.6 out of 4 seconds. Patch 6 has more

The powerpc and s390 lists are included in case they want to give this a
try, they had enabled this feature when it was configured per arch.

Series based on v5.7-rc7 plus these three from mmotm


and it's available here:

git://oss.oracle.com/git/linux-dmjordan.git padata-mt-definit-v3

and the future users and related features are available as

git://oss.oracle.com/git/linux-dmjordan.git padata-mt-wip-v0.5

- Remove nr_pages accounting as suggested by Alex, adding a new patch
- Align deferred init ranges up not down, simplify surrounding code (Alex)
- Add Josh's T-b's from v2 (Josh's T-b's for v1 lost in rebase, apologies!)
- Move padata.h include up in init/main.c to reduce patch collisions (Andrew)
- Slightly reword Documentation patch
- Rebase on v5.7-rc7 and retest

- Improve the problem statement (Andrew, Josh, Pavel)
- Add T-b's to unchanged patches (Josh)
- Fully initialize max-order blocks to avoid buddy issues (Alex)
- Parallelize on section-aligned boundaries to avoid potential
false sharing (Alex)
- Return the maximum thread count from a function that architectures
can override, with the generic version returning 1 (current
behavior). Override for x86 since that's the only arch this series
has been tested on so far. Other archs can test with more threads
by dropping patch 6.
- Rebase to v5.7-rc6, rerun tests

RFC v4 [2] -> v1:
- merged with padata (Peter)
- got rid of the 'task' nomenclature (Peter, Jon)

future work branch:
- made lockdep-aware (Jason, Peter)
- adjust workqueue worker priority with renice_or_cancel() (Tejun)
- fixed undo problem in VFIO (Alex)

The remaining feedback, mainly resource control awareness (cgroup etc),
is TODO for later series.

[1] https://static.sched.com/hosted_files/kvmforum2019/66/VMM-fast-restart_kvmforum2019.pdf

[2] https://lore.kernel.org/linux-mm/20181105165558.11698-1-daniel.m.jordan@xxxxxxxxxx/

Daniel Jordan (8):
padata: remove exit routine
padata: initialize earlier
padata: allocate work structures for parallel jobs from a pool
padata: add basic support for multithreaded jobs
mm: don't track number of pages during deferred initialization
mm: parallelize deferred_init_memmap()
mm: make deferred init's max threads arch-specific
padata: document multithreaded jobs

Documentation/core-api/padata.rst | 41 +++--
arch/x86/mm/init_64.c | 12 ++
include/linux/memblock.h | 3 +
include/linux/padata.h | 43 ++++-
init/main.c | 2 +
kernel/padata.c | 277 ++++++++++++++++++++++++------
mm/Kconfig | 6 +-
mm/page_alloc.c | 59 +++++--
8 files changed, 361 insertions(+), 82 deletions(-)

base-commit: 9cb1fd0efd195590b828b9b865421ad345a4a145
prerequisite-patch-id: 4ad522141e1119a325a9799dad2bd982fbac8b7c
prerequisite-patch-id: 169273327e56f5461101a71dfbd6b4cfd4570cf0
prerequisite-patch-id: 0f34692c8a9673d4c4f6a3545cf8ec3a2abf8620