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

Feng Tang
2023-04-06T01:20:37Z

Hi Tim,

On Wed, Apr 05, 2023 at 03:38:20PM -0700, Tim Chen wrote:
> Thanks for the update. Looks good. Some minor nits to improve readability.

Many thanks for the review!

>
> 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

Will correct it.

> > +========================================
> > +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.
[...]
> > +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/

Will change.

> > +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

Yes, this is better.

> > +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

Will change. Thanks!

- Feng

> > +
> > +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/
> > 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
>