Re: [PATCH v3] Documentation: Add document for false sharing

[Tim Chen]

Thanks for the update. Looks good.  Some minor nits to improve readability.

On Tue, 2023-04-04 at 13:22 +0800, Feng Tang wrote:
> 
> +
> +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 analysis False Sharing

s/analysis/analyse

> +========================================
> +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 need to balance performance gains with complexity and

s/need to/should/

> +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.  And to avoid false sharing in advance during coding, it's

Maybe say
To avoid introducing false sharing when coding

> +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 known read-mostly and written-mostly fields

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@xxxxxxxxxxxxxx/
> +.. [3] https://joemario.github.io/blog/2016/09/01/c2c-blog/
> diff --git a/Documentation/kernel-hacking/index.rst b/Documentation/kernel-hacking/index.rst
> index f53027652290..79c03bac99a2 100644
> --- a/Documentation/kernel-hacking/index.rst
> +++ b/Documentation/kernel-hacking/index.rst
> @@ -9,3 +9,4 @@ Kernel Hacking Guides
>  
>     hacking
>     locking
> +   false-sharing

Thanks.

Tim