Re: [GIT PULL] RCU changes for v5.10

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Oct 12, 2020 at 7:14 AM Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > Please pull the latest core/rcu git tree from:
> >
> > RCU changes for v5.10:
> >
> >  - Debugging for smp_call_function()
> >  - RT raw/non-raw lock ordering fixes
> >  - Strict grace periods for KASAN
> >  - New smp_call_function() torture test
> >  - Torture-test updates
> >  - Documentation updates
> >  - Miscellaneous fixes
> 
> I am *very* unhappy with this pull request.
> 
> It doesn't even mention the big removal of CONFIR_PREEMPT, that I felt 
> was still under discussion.

Not mentioning the unconditional PREEMPT_COUNT enabling aspect was 100% my 
fault in summarizing the changes insufficiently, as I (mistakenly) thought 
them to be uncontroversial. My apologies for that!

Here's a second attempt to properly justify these changes:

Regarding the performance aspect of the change, I was relying on these 
performance measurements:

  "Freshly conducted benchmarks did not reveal any measurable impact from 
   enabling preempt count unconditionally. On kernels with 
   CONFIG_PREEMPT_NONE or CONFIG_PREEMPT_VOLUNTARY the preempt count is only 
   incremented and decremented but the result of the decrement is not 
   tested. Contrary to that enabling CONFIG_PREEMPT which tests the result 
   has a small but measurable impact due to the conditional branch/call."

FWIW, to inject some hard numbers into this discussion, here's also the 
code generation impact of an unconditional PREEMPT_COUNT, on x86-defconfig:

      text       data        bss    filename
  19675937    5591036    1433672    vmlinux.ubuntu.vanilla          # 856deb866d16: ("Linux 5.9-rc5")
  19682382    5590964    1425480    vmlinux.ubuntu.PREEMPT_COUNT=y  # 7681205ba49d: ("preempt: Make preempt count unconditional")

So this is a pretty small, +0.03% increase (+6k) in generated code in the 
core kernel, and it doesn't add widespread new control dependencies either.

I also measured the core kernel code generation impact on the kernel config 
from a major Linux distribution that uses PREEMPT_VOLUNTARY=y (Ubuntu):

  kepler:~/tip> grep PREEMPT .config
  # CONFIG_PREEMPT_NONE is not set
  CONFIG_PREEMPT_VOLUNTARY=y
  # CONFIG_PREEMPT is not set
  CONFIG_PREEMPT_COUNT=y
  CONFIG_PREEMPT_NOTIFIERS=y

     text       data        bss      filename
  15754341    13790786    5242880    vmlinux.ubuntu.vanilla          # 856deb866d16: ("Linux 5.9-rc5")
  15754790    13791018    5242880    vmlinux.ubuntu.PREEMPT_COUNT=y  # 7681205ba49d: ("preempt: Make preempt count unconditional")
  15754771    13791018    5242880    vmlinux.ubuntu.full_cleanups    # 849b9c5446cc: ("kvfree_rcu(): Fix ifnullfree.cocci warnings")

In this test the changes result in very little generated code increase in 
the core kernel, just +449 bytes, or +0.003%.

In fact the impact was so low on this config that I initially disbelieved 
it and double-checked the result and re-ran the build with all =m's turned 
into =y's, to get a whole-kernel measurement of the generated code impact:

      text       data        bss      filename
  84594448    61819613    42000384    vmlinux.ubuntu.vanilla          # 856deb866d16: ("Linux 5.9-rc5")
  84594129    61819777    42000384    vmlinux.ubuntu.PREEMPT_COUNT=y  # 7681205ba49d: ("preempt: Make preempt count unconditional")

Note how the full ~84 MB image actually *shrunk*, possibly due to random 
function & section alignment noise.

So to get a truly sensitive measurement of the impact of the PREEMPT_COUNT 
change I built with CONFIG_CC_OPTIMIZE_FOR_SIZE=y, to get tight instruction 
packing and no alignment padding artifacts:

      text        data         bss    filename
  69460329    60932573    40411136    vmlinux.ubuntu.vanilla          # 856deb866d16: ("Linux 5.9-rc5")
  69460739    60936853    40411136    vmlinux.ubuntu.PREEMPT_COUNT=y  # 7681205ba49d: ("preempt: Make preempt count unconditional")

This shows a 410 bytes (+0.0005%) increase.

  ( Side note: it's rather impressive that -Os saves 21% of text size - if 
    only GCC wasn't so stupid with the final 2-3% size optimizations... )

So there's even less relative impact on the whole 84 MB kernel image - 
modules don't do much direct preempt_count manipulation.

Just for completeness' sake I re-ran the original defconfig build as well, 
this time with -Os:

     text       data        bss     filename
  16091696    5565988    2928696    vmlinux.defconfig.Os.vanilla          # 856deb866d16: ("Linux 5.9-rc5")
  16095525    5570156    2928696    vmlinux.defconfig.Os.PREEMPT_COUNT=y  # 7681205ba49d: ("preempt: Make preempt count unconditional")

3.8k, or +0.025% - similar to the initial +0.03% result.

So even though I'm normally fiercely anti-bloat, if we combine the 
performance and code generation measurements with these maintainability 
arguments:

   "It's about time to make essential functionality of the kernel consistent 
    across the various preemption models.

    Enable CONFIG_PREEMPT_COUNT unconditionally. Follow up changes will 
    remove the #ifdeffery and remove the config option at the end."

I think the PREEMPT_COUNT=y change to reduce the schizm between the various 
preemption models is IMHO justified - and reducing the code base distance 
to -rt is the icing on the cake.

Thanks,

	Ingo