--- /dev/null
+.. SPDX-License-Identifier: GPL-2.0
+
+=============
+False Sharing
+=============
+
+What is False Sharing
+=====================
+False sharing is related with cache mechanism of maintaining the data
+coherence of one cache line stored in multiple CPU's caches; then
+academic definition for it is in [1]_. Consider a struct with a
+refcount and a string::
+
+ struct foo {
+ refcount_t refcount;
+ ...
+ char name[16];
+ } ____cacheline_internodealigned_in_smp;
+
+Member 'refcount'(A) and 'name'(B) _share_ one cache line like below::
+
+ +-----------+ +-----------+
+ | CPU 0 | | CPU 1 |
+ +-----------+ +-----------+
+ / |
+ / |
+ V V
+ +----------------------+ +----------------------+
+ | A B | Cache 0 | A B | Cache 1
+ +----------------------+ +----------------------+
+ | |
+ ---------------------------+------------------+-----------------------------
+ | |
+ +----------------------+
+ | |
+ +----------------------+
+ Main Memory | A B |
+ +----------------------+
+
+'refcount' is modified frequently, but 'name' is set once at object
+creation time and is never modified. When many CPUs access 'foo' at
+the same time, with 'refcount' being only bumped by one CPU frequently
+and 'name' being read by other CPUs, all those reading CPUs have to
+reload the whole cache line over and over due to the 'sharing', even
+though 'name' is never changed.
+
+There are many real-world cases of performance regressions caused by
+false sharing. One of these is a rw_semaphore 'mmap_lock' inside
+mm_struct struct, whose cache line layout change triggered a
+regression and Linus analyzed in [2]_.
+
+There are two key factors for a harmful false sharing:
+
+* A global datum accessed (shared) by many CPUs
+* In the concurrent accesses to the data, there is at least one write
+ operation: write/write or write/read cases.
+
+The sharing could be from totally unrelated kernel components, or
+different code paths of the same kernel component.
+
+
+False Sharing Pitfalls
+======================
+Back in time when one platform had only one or a few CPUs, hot data
+members could be purposely put in the same cache line to make them
+cache hot and save cacheline/TLB, like a lock and the data protected
+by it. But for recent large system with hundreds of CPUs, this may
+not work when the lock is heavily contended, as the lock owner CPU
+could write to the data, while other CPUs are busy spinning the lock.
+
+Looking at past cases, there are several frequently occurring patterns
+for false sharing:
+
+* lock (spinlock/mutex/semaphore) and data protected by it are
+ purposely put in one cache line.
+* global data being put together in one cache line. Some kernel
+ subsystems have many global parameters of small size (4 bytes),
+ which can easily be grouped together and put into one cache line.
+* data members of a big data structure randomly sitting together
+ without being noticed (cache line is usually 64 bytes or more),
+ like 'mem_cgroup' struct.
+
+Following 'mitigation' section provides real-world examples.
+
+False sharing could easily happen unless they are intentionally
+checked, and it is valuable to run specific tools for performance
+critical workloads to detect false sharing affecting performance case
+and optimize accordingly.
+
+
+How to detect and analyze False Sharing
+========================================
+perf record/report/stat are widely used for performance tuning, and
+once hotspots are detected, tools like 'perf-c2c' and 'pahole' can
+be further used to detect and pinpoint the possible false sharing
+data structures. 'addr2line' is also good at decoding instruction
+pointer when there are multiple layers of inline functions.
+
+perf-c2c can capture the cache lines with most false sharing hits,
+decoded functions (line number of file) accessing that cache line,
+and in-line offset of the data. Simple commands are::
+
+ $ perf c2c record -ag sleep 3
+ $ perf c2c report --call-graph none -k vmlinux
+
+When running above during testing will-it-scale's tlb_flush1 case,
+perf reports something like::
+
+ Total records : 1658231
+ Locked Load/Store Operations : 89439
+ Load Operations : 623219
+ Load Local HITM : 92117
+ Load Remote HITM : 139
+
+ #----------------------------------------------------------------------
+ 4 0 2374 0 0 0 0xff1100088366d880
+ #----------------------------------------------------------------------
+ 0.00% 42.29% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff81373b7b 0 231 129 5312 64 [k] __mod_lruvec_page_state [kernel.vmlinux] memcontrol.h:752 1
+ 0.00% 13.10% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff81374718 0 226 97 3551 64 [k] folio_lruvec_lock_irqsave [kernel.vmlinux] memcontrol.h:752 1
+ 0.00% 11.20% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff812c29bf 0 170 136 555 64 [k] lru_add_fn [kernel.vmlinux] mm_inline.h:41 1
+ 0.00% 7.62% 0.00% 0.00% 0.00% 0x8 1 1 0xffffffff812c3ec5 0 175 108 632 64 [k] release_pages [kernel.vmlinux] mm_inline.h:41 1
+ 0.00% 23.29% 0.00% 0.00% 0.00% 0x10 1 1 0xffffffff81372d0a 0 234 279 1051 64 [k] __mod_memcg_lruvec_state [kernel.vmlinux] memcontrol.c:736 1
+
+A nice introduction for perf-c2c is [3]_.
+
+'pahole' decodes data structure layouts delimited in cache line
+granularity. Users can match the offset in perf-c2c output with
+pahole's decoding to locate the exact data members. For global
+data, users can search the data address in System.map.
+
+
+Possible Mitigations
+====================
+False sharing does not always need to be mitigated. False sharing
+mitigations should balance performance gains with complexity and
+space consumption. Sometimes, lower performance is OK, and it's
+unnecessary to hyper-optimize every rarely used data structure or
+a cold data path.
+
+False sharing hurting performance cases are seen more frequently with
+core count increasing. Because of these detrimental effects, many
+patches have been proposed across variety of subsystems (like
+networking and memory management) and merged. Some common mitigations
+(with examples) are:
+
+* Separate hot global data in its own dedicated cache line, even if it
+ is just a 'short' type. The downside is more consumption of memory,
+ cache line and TLB entries.
+
+ - Commit 91b6d3256356 ("net: cache align tcp_memory_allocated, tcp_sockets_allocated")
+
+* Reorganize the data structure, separate the interfering members to
+ different cache lines. One downside is it may introduce new false
+ sharing of other members.
+
+ - Commit 802f1d522d5f ("mm: page_counter: re-layout structure to reduce false sharing")
+
+* Replace 'write' with 'read' when possible, especially in loops.
+ Like for some global variable, use compare(read)-then-write instead
+ of unconditional write. For example, use::
+
+ if (!test_bit(XXX))
+ set_bit(XXX);
+
+ instead of directly "set_bit(XXX);", similarly for atomic_t data::
+
+ if (atomic_read(XXX) == AAA)
+ atomic_set(XXX, BBB);
+
+ - Commit 7b1002f7cfe5 ("bcache: fixup bcache_dev_sectors_dirty_add() multithreaded CPU false sharing")
+ - Commit 292648ac5cf1 ("mm: gup: allow FOLL_PIN to scale in SMP")
+
+* Turn hot global data to 'per-cpu data + global data' when possible,
+ or reasonably increase the threshold for syncing per-cpu data to
+ global data, to reduce or postpone the 'write' to that global data.
+
+ - Commit 520f897a3554 ("ext4: use percpu_counters for extent_status cache hits/misses")
+ - Commit 56f3547bfa4d ("mm: adjust vm_committed_as_batch according to vm overcommit policy")
+
+Surely, all mitigations should be carefully verified to not cause side
+effects. To avoid introducing false sharing when coding, it's better
+to:
+
+* Be aware of cache line boundaries
+* Group mostly read-only fields together
+* Group things that are written at the same time together
+* Separate frequently read and frequently written fields on
+ different cache lines.
+
+and better add a comment stating the false sharing consideration.
+
+One note is, sometimes even after a severe false sharing is detected
+and solved, the performance may still have no obvious improvement as
+the hotspot switches to a new place.
+
+
+Miscellaneous
+=============
+One open issue is that kernel has an optional data structure
+randomization mechanism, which also randomizes the situation of cache
+line sharing of data members.
+
+
+.. [1] https://en.wikipedia.org/wiki/False_sharing
+.. [2] https://lore.kernel.org/lkml/CAHk-=whoqV=cX5VC80mmR9rr+Z+yQ6fiQZm36Fb-izsanHg23w@mail.gmail.com/
+.. [3] https://joemario.github.io/blog/2016/09/01/c2c-blog/
projects / platform / kernel / linux-rpi.git / commitdiff