Re: [PATCH v1] Documentation: Add document for false sharing

[Feng Tang]

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