From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: David Hildenbrand <david@kernel.org>
Cc: linux-kernel@vger.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
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Hugh Dickins <hughd@google.com>
Subject: [RFC PATCH v25 00/14] Hierarchical Percpu Counters for RSS and tmpfs
Date: Wed, 11 Nov 2026 15:36:12 -0500 [thread overview]
Message-ID: <20261111203626.95838-1-mathieu.desnoyers@efficios.com> (raw)
Hi David,
Here is the hierarchical per-cpu counters series, based on v7.4-rc2.
After your review of v24 [3], v25 adds a second user in patches 8 to
11. This user is the tmpfs size limit. It is still an RFC because of the
questions at the end.
The problem
-----------
The RSS counters are struct percpu_counter since
commit f1a7941243c1 ("mm: convert mm's rss stats into percpu_counter").
Their batch size is max(32, 2 * nr_cpus). So their error grows with
nr_cpus^2. At 384 logical CPUs, it reaches 1.1GB per counter.
/proc/PID/status, /proc/PID/statm and the OOM killer now use precise
sums (commits 82241a83cd15 and 5898aa8f9a0b). Other readers still use
the approximation:
- the RSS high watermark (VmHWM, ru_maxrss, taskstats);
- /proc/PID/stat (ps, top, monitoring agents);
- the taskstats RSS-time integral (coremem);
- NUMA balancing and the mark_victim tracepoint.
On the test machine below, /proc/PID/stat was off by 17MB for a
single-threaded process. It was off by up to 134MB with 64 threads.
Precise sums cannot fix the high watermark or the integral. They are
sampled on every unmap and at each tick. A precise sum reads every
possible CPU: about 4us per counter at 384 logical CPUs. A smaller batch
takes the counter spinlock on every carry. At batch 32, an update of a
shared mm costs 4.4 to 69us with one thread per CPU. With this series,
it costs 26ns to 1.2us.
The tmpfs size limit has the same problem in another form.
percpu_counter_limited_add() takes no lock only while the mount is more
than batch * nr_cpus pages below its size. This is 1.1GB at 384 logical
CPUs. A smaller mount, such as a 64MB /dev/shm, takes the counter
spinlock and sums all CPUs for each page.
The series
----------
Patches 1 to 5 use hierarchical per-cpu counters for the RSS counters.
Per-cpu bytes carry into a tree of atomic counters. The tree is shaped
at boot from nr_cpu_ids. The batch is 32 and the error bound is in
O(n*log(n)). There is no global lock. A follow-up will use the
comparison functions for the OOM killer task selection.
Patches 6 and 7 keep the RSS counters of single-threaded processes
exact, without per-cpu memory. Until a second thread shares the mm, the
counters are a plain bias and an atomic bias. Creating the second
thread allocates the per-cpu counters and activates the tree. Most
processes never need that per-cpu memory, so process creation gets
cheaper. In a 384-vCPU guest, posix_spawn() of a static program gains
22 to 42% over patches 1 to 5.
Patch 8 adds limit counters on the tree. A limit counter splits the
value into two counters which only grow: the additions and the
subtractions. Their carries are ordered so that each sum errs in one
direction only. An addition then checks the limit without a lock. It
sums the CPUs only within the accuracy of the limit. The limit stays
strict. Patch 9 tests the limit counters, and patch 10 checks their
memory ordering with litmus tests. Patch 11 uses a limit counter for
the tmpfs size limit.
Patches 12 to 14 are tools to check the tree shape on other hardware:
two boot parameters and a KUnit benchmark. I measured the defaults on
one machine only, so results from other hardware are welcome.
Results
-------
The machine is a 2-socket AMD EPYC 9654 with 384 logical CPUs. The
tree is 8,8,8 with batch 32. Patches 3, 7 and 11 have the details. The
bare-metal results were measured on v7.3.
Accuracy per counter, bound and largest error seen, while a process
hops across all CPUs (384-vCPU guest pinned to the host):
hpcc upstream
bound 158.5MB 1.1GB
measured, single-threaded 0 (exact) 841MB
measured, with a second thread 96MB 845MB
Update cost, ns per update, one thread per CPU, bare metal (mm: the 4
counters of one mm; private: one counter per thread):
upstream upstream
hpcc batch 768 batch 32
mm, +1 26 288 4370
mm, random +-32 135 385 69171
mm, random +-512 1174 20229 69186
private, random +-32 31 20 -
Memory per mm, in bytes (guest, v7.4-rc1):
single-threaded threaded upstream
16 CPUs 1820 1948 2076
384 CPUs 1928 8145 8072
1024 CPUs 2185 17204 18569
tmpfs, fallocate() of one page at a time, in thousands of calls per
second (384-vCPU guest, v7.4-rc2):
size= processes upstream hpcc
64MB 1 208 483
64MB 384 126 54275
1GB 384 125 93879
2GB 384 93851 93476
Behavior changes
----------------
- The per-cpu counters are allocated by the second thread, not by
fork(). So clone() with CLONE_VM can now fail with -ENOMEM.
- mm_struct gains rss_owner and rss_stat_mutex. rss_owner is the task
running on the mm. It is never dereferenced.
- The precise sums take no lock. Under concurrent updates, they can miss
a carry on its way up the tree. They are exact for a process at rest.
- The rss_stat tracepoint reports the approximation (patch 5).
- tmpfs (patch 11): an allocation near a full mount can fail although
it fits. This happens while a concurrent allocation which does not
fit is undone. A remount keeps the batch size of the first size.
Caveats
-------
- From 33 to 39 CPUs, the RSS counters are up to 1.37 times less
accurate than upstream.
- Just above a power of 2 CPUs, a threaded process uses up to 1.50
times the memory of upstream for its RSS counters.
- A multi-threaded process which ran on few CPUs can hold more error
than upstream.
- With 384 threads created at once, the per-cpu allocator serializes
the allocations, as it does upstream at every fork.
- tmpfs: in the last 1/4 of the size, each allocation sums all CPUs.
With one process, this is 37% slower than upstream.
Changes since v24 [2]
---------------------
- Rebased onto v7.4-rc2.
- New patches 8 to 11: the tmpfs size limit. In my reply to your review
[3], I wrote that mounts of 2GB and above lose 1 to 2%. Measured
again on v7.4-rc2, they are within 1%.
- New litmus tests in patch 10, in tools/testing/hpcc/litmus. They check
the memory ordering of the limit counters, of the carries and of the
publication of the tree. Variants without an ordering show that it is
needed.
- Patch 6 (Sashiko): the activation race test releases its workers with
release/acquire.
- Patches 12 to 14 were patches 8 to 10, unchanged.
The v24 cover letter [2] lists the earlier changes, since v22 [1].
These tests pass on v7.4-rc2:
- Every commit builds without warnings on x86_64 (SMP and UP), arm64,
ppc64le, 32-bit powerpc and sparc64. Sparse adds no warnings.
- KUnit passes in guests with 1, 16 and 384 vCPUs. It also passes with
lockdep with 16 and 64 vCPUs.
- The userspace test passes on simulated topologies up to 65536 CPUs,
as a 64-bit and a 32-bit build.
- The RSS counters match a page table walk from 1 to 64 threads.
- herd7 checks the memory ordering of the limit counters with litmus
tests.
I wrote v22. Starting from it, an LLM coding assistant drove the
implementation of this revision from my design decisions. It wrote the
code, the tests, the benchmark and the measurement scripts. It ran the
measurements in guests and prepared the bare-metal ones. It drafted the
comments, the changelogs and this letter. I made the design decisions,
reviewed every change and directed the rework through my review
comments.
Questions
---------
David answered the questions of v24 [3]. These remain open:
1. Fork and exit reviewers: is mm->rss_owner acceptable (patch 7)? The
alternative is current->mm == mm and an "activated" flag.
2. Martin: the rss_stat tracepoint now reports the approximation (patch
5). Is that fine for its users, such as Perfetto?
3. Who could run the benchmark of patch 14 on non-AMD, arm64 or POWER
machines? The defaults were only measured on one machine.
4. tmpfs maintainers: are the behavior changes of patch 11 acceptable?
Thanks,
Mathieu
[1] https://lore.kernel.org/lkml/20260227153730.1556542-1-mathieu.desnoyers@efficios.com/
[2] https://lore.kernel.org/lkml/20261105203639.1459524-1-mathieu.desnoyers@efficios.com/
[3] https://lore.kernel.org/lkml/88a96126-11be-41cc-ab0d-372d32189e79@kernel.org/
Mathieu Desnoyers (14):
lib: introduce hierarchical per-cpu counters
lib: test hierarchical per-cpu counters
mm: use hierarchical per-cpu counters for the RSS counters
tools/testing/hpcc: add userspace test of hierarchical per-CPU
counters
mm: report the approximate RSS in the rss_stat tracepoint
lib: percpu_counter_tree: add lazy tree activation
mm: keep the RSS counters of single-threaded processes exact
lib: percpu_counter_tree: add limit counters
lib: percpu_counter_tree: test the limit counters
tools/testing/hpcc: add litmus tests of the memory ordering
mm: shmem: use a limit counter for the tmpfs size limit
lib: percpu_counter_tree: add the arity boot parameter
mm: add the rss_stat_batch boot parameter
lib: add a benchmark of hierarchical per-cpu counters
.../admin-guide/kernel-parameters.txt | 24 +
Documentation/core-api/index.rst | 1 +
.../core-api/percpu-counter-tree.rst | 145 ++
MAINTAINERS | 12 +
fs/exec.c | 5 +
include/linux/mm.h | 28 +-
include/linux/mm_types.h | 19 +-
include/linux/percpu_counter_tree.h | 445 +++++
include/linux/shmem_fs.h | 3 +-
include/trace/events/kmem.h | 3 +-
init/main.c | 3 +
kernel/exit.c | 3 +
kernel/fork.c | 61 +-
lib/Kconfig | 27 +
lib/Makefile | 1 +
lib/percpu_counter_tree.c | 1776 +++++++++++++++++
lib/tests/Makefile | 3 +
lib/tests/percpu_counter_tree_bench.c | 610 ++++++
lib/tests/percpu_counter_tree_kunit.c | 1240 ++++++++++++
mm/shmem.c | 33 +-
tools/testing/hpcc/.gitignore | 2 +
tools/testing/hpcc/Makefile | 18 +
tools/testing/hpcc/include/hpcc_shim.h | 253 +++
tools/testing/hpcc/include/kunit/visibility.h | 2 +
tools/testing/hpcc/include/linux/args.h | 4 +
tools/testing/hpcc/include/linux/atomic.h | 2 +
tools/testing/hpcc/include/linux/bug.h | 2 +
tools/testing/hpcc/include/linux/cache.h | 2 +
tools/testing/hpcc/include/linux/cpumask.h | 2 +
tools/testing/hpcc/include/linux/errno.h | 3 +
tools/testing/hpcc/include/linux/limits.h | 2 +
tools/testing/hpcc/include/linux/math.h | 2 +
.../testing/hpcc/include/linux/moduleparam.h | 12 +
tools/testing/hpcc/include/linux/percpu.h | 2 +
.../hpcc/include/linux/percpu_counter_tree.h | 2 +
tools/testing/hpcc/include/linux/preempt.h | 2 +
tools/testing/hpcc/include/linux/slab.h | 2 +
tools/testing/hpcc/litmus/README | 61 +
.../leave-first-precise-sum-relaxed.litmus | 37 +
.../litmus/leave-first-precise-sum.litmus | 41 +
.../litmus/limit-arrive-first-acquire.litmus | 38 +
.../limit-arrive-first-unordered.litmus | 37 +
.../hpcc/litmus/limit-arrive-first.litmus | 41 +
.../hpcc/litmus/limit-free-level0-nomb.litmus | 40 +
.../hpcc/litmus/limit-free-level0.litmus | 43 +
.../hpcc/litmus/limit-free-root.litmus | 42 +
.../litmus/limit-last-checker-nomb.litmus | 28 +
.../hpcc/litmus/limit-last-checker.litmus | 33 +
.../hpcc/litmus/limit-undo-level0.litmus | 35 +
.../hpcc/litmus/limit-undo-root-rmb.litmus | 30 +
.../litmus/limit-undo-root-unordered.litmus | 29 +
.../hpcc/litmus/limit-undo-root.litmus | 32 +
.../litmus/publish-approximate-sum.litmus | 40 +
.../hpcc/litmus/publish-carry-normb.litmus | 36 +
.../testing/hpcc/litmus/publish-carry.litmus | 43 +
.../publish-precise-sum-noacquire.litmus | 38 +
.../hpcc/litmus/publish-precise-sum.litmus | 44 +
.../litmus/publish-update-norelease.litmus | 34 +
.../testing/hpcc/litmus/publish-update.litmus | 40 +
tools/testing/hpcc/main.c | 892 +++++++++
60 files changed, 6467 insertions(+), 23 deletions(-)
create mode 100644 Documentation/core-api/percpu-counter-tree.rst
create mode 100644 include/linux/percpu_counter_tree.h
create mode 100644 lib/percpu_counter_tree.c
create mode 100644 lib/tests/percpu_counter_tree_bench.c
create mode 100644 lib/tests/percpu_counter_tree_kunit.c
create mode 100644 tools/testing/hpcc/.gitignore
create mode 100644 tools/testing/hpcc/Makefile
create mode 100644 tools/testing/hpcc/include/hpcc_shim.h
create mode 100644 tools/testing/hpcc/include/kunit/visibility.h
create mode 100644 tools/testing/hpcc/include/linux/args.h
create mode 100644 tools/testing/hpcc/include/linux/atomic.h
create mode 100644 tools/testing/hpcc/include/linux/bug.h
create mode 100644 tools/testing/hpcc/include/linux/cache.h
create mode 100644 tools/testing/hpcc/include/linux/cpumask.h
create mode 100644 tools/testing/hpcc/include/linux/errno.h
create mode 100644 tools/testing/hpcc/include/linux/limits.h
create mode 100644 tools/testing/hpcc/include/linux/math.h
create mode 100644 tools/testing/hpcc/include/linux/moduleparam.h
create mode 100644 tools/testing/hpcc/include/linux/percpu.h
create mode 100644 tools/testing/hpcc/include/linux/percpu_counter_tree.h
create mode 100644 tools/testing/hpcc/include/linux/preempt.h
create mode 100644 tools/testing/hpcc/include/linux/slab.h
create mode 100644 tools/testing/hpcc/litmus/README
create mode 100644 tools/testing/hpcc/litmus/leave-first-precise-sum-relaxed.litmus
create mode 100644 tools/testing/hpcc/litmus/leave-first-precise-sum.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-arrive-first-acquire.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-arrive-first-unordered.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-arrive-first.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-free-level0-nomb.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-free-level0.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-free-root.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-last-checker-nomb.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-last-checker.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-undo-level0.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-undo-root-rmb.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-undo-root-unordered.litmus
create mode 100644 tools/testing/hpcc/litmus/limit-undo-root.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-approximate-sum.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-carry-normb.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-carry.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-precise-sum-noacquire.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-precise-sum.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-update-norelease.litmus
create mode 100644 tools/testing/hpcc/litmus/publish-update.litmus
create mode 100644 tools/testing/hpcc/main.c
base-commit: 7475ea8b58616b33c04afb976cf0fbaad9dd63fb
--
2.47.3
next reply other threads:[~2026-11-11 20:36 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-11 20:36 Mathieu Desnoyers [this message]
2026-11-11 20:36 ` [RFC PATCH v25 01/14] lib: introduce hierarchical per-cpu counters Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 02/14] lib: test " Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 03/14] mm: use hierarchical per-cpu counters for the RSS counters Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 04/14] tools/testing/hpcc: add userspace test of hierarchical per-CPU counters Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 05/14] mm: report the approximate RSS in the rss_stat tracepoint Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 06/14] lib: percpu_counter_tree: add lazy tree activation Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 07/14] mm: keep the RSS counters of single-threaded processes exact Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 08/14] lib: percpu_counter_tree: add limit counters Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 09/14] lib: percpu_counter_tree: test the " Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 10/14] tools/testing/hpcc: add litmus tests of the memory ordering Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 11/14] mm: shmem: use a limit counter for the tmpfs size limit Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 12/14] lib: percpu_counter_tree: add the arity boot parameter Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 13/14] mm: add the rss_stat_batch " Mathieu Desnoyers
2026-11-11 20:36 ` [RFC PATCH v25 14/14] lib: add a benchmark of hierarchical per-cpu counters Mathieu Desnoyers
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