Do not compile in the really expensive entropy harvesting unless it is requested.

[markm]

Summary

High-performance folks have objected to my harvesting of entropy in the zone allocator (UMA). This work will set that harvesting to optional, with the default to "off". The code concerned is compiled out, so there are NO instructions added when it is not present. The call lines remain, but these will be null macros by default.

Folks who need this entropy may turn it on with a kernel build option, and SDT probes are set in place to assist the measurements needed in making the decision.

[markm]

ScottL, JeffR - this is not complete yet, but does the general idea meet your requirements?

[imp]

sys/vm/uma_core.c

2138–2141 ↗

(On Diff #7292)

Not entirely sure that this long comment is needed...

But if you must, you should make it a one liner:

/* Enable entropy collection for RANDOM_ENABLE_UMA kernel option */

or something. Here and below.

[markm]

Shorten some comments.

[imp]

Glad you liked my suggestion.

[delphij]

I like the idea, but it looks like the new version doesn't have the random.h changes. If combined with the sys/conf/NOTES, sys/conf/files and sys/sys/random.h changes in the first version of changeset, then consider this as an "Accept".

[jmg]

Please post a complete patch that includes both sets of changes, and adds proper documentation.

sys/conf/NOTES

2987 ↗

(On Diff #7292)

Comment here is necessary for people to understand what this does.

sys/sys/random.h

93 ↗

(On Diff #7292)

I don't like creating a new function for this, just for two points... putting ifdefs in the uma code to me seems better.

Also, these new functions are undocumented, and need to be documented if added.

[markm]

In D3197#64019, @delphij wrote:

I like the idea, but it looks like the new version doesn't have the random.h changes. If combined with the sys/conf/NOTES, sys/conf/files and sys/sys/random.h changes in the first version of changeset, then consider this as an "Accept".

Oops - yes - it is both together. I messed up using arcanist. :-]

[markm]

• Make the UMA harvesting go away completely if not wanted. Default to "not wanted".
• Fix comment as per review suggestion.

[markm]

This comment was removed by markm.

sys/sys/random.h

93 ↗

(On Diff #7343)

I would be prepared to put in comments where these function wrappers are used, except I was just asked to remove this.

Wrapping them in ifdefs will be a nasty mess, and documenting them as if they were API functions when they are only used in one place is a waste of time.

[markm]

sys/conf/NOTES

2987 ↗

(On Diff #7343)

On its way. This is part of the WIP.

[markm]

Massage this into a more commit-worthy version; document what is going on and provide SDT probes to assist adopters.