Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 206 insertions, 0 deletions

diff --git a/Documentation/kernel-hacking/false-sharing.rst b/Documentation/kernel-hacking/false-sharing.rst
new file mode 100644
index 000000000..eb0596734
--- /dev/null
+++ b/Documentation/kernel-hacking/false-sharing.rst
@@ -0,0 +1,206 @@
+.. 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 the kernel has an optional data structure
+randomization mechanism, which also randomizes the situation of cache
+line sharing among 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/