Hi Bagas Sanjaya,
Many thanks for the reviews!
On Fri, Mar 24, 2023 at 07:45:28PM +0700, Bagas Sanjaya wrote:
> On Fri, Mar 24, 2023 at 03:13:16PM +0800, Feng Tang wrote:
> > +There are many real-world cases of performance regressions caused by
> > +false sharing, and one is a rw_semaphore 'mmap_lock' inside struct
> "... . One of these is rw_semaphore 'mmap_lock' ..."
OK, will use this.
> But I think in English we commonly name things as "foobar struct"
> instead of "struct foobar" (that is, common noun follow the proper noun
> that names something).
I can change that. And IIRC, I saw 'struct XXX' and 'XXX struct' both
frequently used in kernel. I just run '# git log | grep -w struct'
and the majority use 'struct XXX'
> > +* A global datum accessed (shared) by many CPUs
> Global data?
In RFC version, I used 'data' and Randy suggested 'datum'. TBH, I
looked it up in a dictionary :), and found:
"Data" is the Latin plural form of "datum"
> > +Following 'mitigation' section provides real-world examples.
> "The real-world examples are given in 'Possible mitigations' sections."
Will use this, thanks.
> > + #perf c2c record -ag sleep 3
> > + #perf c2c report --call-graph none -k vmlinux
>
> Are these commands really run as root?
You are right, people can run it as 'root' or a normal user. And I
guess this won't confuse kernel developers.
My original version is kind of too long and full of explainations,
and some kernel developer suggested that this doc is under
'kernel-hacking' and its audience is kernel developers, and I should
make it clear and short, and not make it look like a wiki page or
man page.
> > +
> > +Run it when testing will-it-scale's tlb_flush1 case, and the report
> > +has pieces like::
>
> "When running above during testing ..., perf reports something like::"
This is more logical, will change.
> > +False sharing hurting performance cases are seen more frequently with
> > +core count increasing, and there have been many patches merged to
> > +solve it, like in networking and memory management subsystems. Some
> > +common mitigations (with examples) are:
>
> "... Because of these detrimental effects, many patches have been
> proposed across variety of subsystems (like networking and memory
> management) and merged."
This is much better, thanks
> > +
> > +* 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:
> "... For example, write::"
The following is a coding pattern (for bit operation, atomic, etc.),
and I think 'use' may also be good?
> > +
> > + if (!test_bit(XXX))
> > + set_bit(XXX);
> > +
> > + instead of directly "set_bit(XXX);", similarly for atomic_t data.
> > +
> > + 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")
>
> IMO it's odd to jump to specifying example commits without some sort of
> conjuction (e.g. "for example, see commit <commit>").
I agree, and I had the same concern, but I was also afraid of that
too many repeating of this, so the previous
"Following 'mitigation' section provides real-world examples."
in last section (which you helped to improve) was added trying
to address this.
> > +
> > +Surely, all mitigations should be carefully verified to not cause side
> > +effects. And to avoid false sharing in advance during 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 known read-mostly and written-mostly fields
>
> Proactively prevent false sharing with above tips?
You are right. And most of these bullets are directly taken from
Dave Hansen's reviews (thanks to Dave)
Thanks,
Feng
> Thanks.
>
> --
> An old man doll... just what I always wanted! - Clara
