Re: RFC: Intervals to schedule the worker for mix_interrupt_randomness().

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On 2022-02-28 19:58:05 [+0100], Jason A. Donenfeld wrote:
> Hi Sebastian,
Hi Jason,

> I'm actually trying quite hard not to change the details of entropy
> gathering for 5.18. There are lots of little arguments for why each
…
> random.c. So I'd like to minimize changes to the semantics. Right now,
> those semantics are:
> 
> A) crng_init==0: pre_init_inject after 64 interrupts.
> B) crng_init!=0: mix_pool_bytes after 64 interrupts OR after 1 second
> has elapsed.

Yes. I double checked, that it was not a recent change during that
rework. So yes, lets keep it as is, I just wanted to point that out.
…
> But all this brings me to what I'm really wondering when reading your
> email: do your observations matter? Are you observing a performance or
> reliability issue or something like that with those workqueues
> pending? Is this whole workqueue approach a mistake and we should
> revert it? Or is it still okay, but you were just idly wondering about
> that time limit? As you can tell, I'm mostly concerned with not
> breaking something by accident.

I noticed it because I backported the required patches (not everything
from your queue, just the patches I needed to drop mine and have
everything working). During testing I noticed that the worker is
scheduled more often than I expected and I looked that it is scheduled
and not accidentally stops due to a backport that went wrong. And since
I got the condition wrong…

But you are asking for performance. I run b.sh which does:
- unpack a kernel to /dev/shm
- build allmodconfig

and then invoked it with "perf stat -r 5 -a --table ./b.sh" to get some
numbers. I applied your complete queue on top of v5.17-rc6, and the
result was:

| Performance counter stats for 'system wide' (5 runs):
|
|     45.502.822,32 msec cpu-clock                 #   32,014 CPUs utilized            ( +-  0,05% )
|         9.479.371      context-switches          #  208,419 /sec                     ( +-  0,08% )
|           839.380      cpu-migrations            #   18,455 /sec                     ( +-  0,38% )
|       624.839.341      page-faults               #   13,738 K/sec                    ( +-  0,00% )
|105.297.794.633.131      cycles                    #    2,315 GHz                      ( +-  0,01% )
|77.238.191.940.405      stalled-cycles-frontend   #   73,37% frontend cycles idle     ( +-  0,01% )
|56.724.314.805.475      stalled-cycles-backend    #   53,89% backend cycles idle      ( +-  0,02% )
|69.889.082.499.264      instructions              #    0,66  insn per cycle         
|                                                  #    1,10  stalled cycles per insn  ( +-  0,00% )
|14.670.304.314.265      branches                  #  322,550 M/sec                    ( +-  0,00% )
|   561.326.606.978      branch-misses             #    3,83% of all branches          ( +-  0,02% )
|
|          # Table of individual measurements:
|          1419,113 (-2,247) #
|          1422,552 (+1,192) #
|          1420,773 (-0,587) #
|          1422,362 (+1,002) #
|          1422,001 (+0,641) #
|
|          # Final result:
|          1421,360 +- 0,641 seconds time elapsed  ( +-  0,05% )

Checked a few commit earlier, before the workqueue rework started
  "random: rewrite header introductory comment":
| Performance counter stats for 'system wide' (5 runs):
|
|     45.508.013,44 msec cpu-clock                 #   32,034 CPUs utilized            ( +-  0,05% )
|         9.456.280      context-switches          #  208,017 /sec                     ( +-  0,11% )
|           837.148      cpu-migrations            #   18,415 /sec                     ( +-  0,30% )
|       624.851.749      page-faults               #   13,745 K/sec                    ( +-  0,00% )
|105.289.268.852.002      cycles                    #    2,316 GHz                      ( +-  0,01% )
|77.233.457.186.415      stalled-cycles-frontend   #   73,38% frontend cycles idle     ( +-  0,02% )
|56.740.014.447.074      stalled-cycles-backend    #   53,91% backend cycles idle      ( +-  0,02% )
|69.882.802.096.982      instructions              #    0,66  insn per cycle
|                                                  #    1,10  stalled cycles per insn  ( +-  0,00% )
|14.670.395.601.080      branches                  #  322,716 M/sec                    ( +-  0,00% )
|   560.846.203.691      branch-misses             #    3,82% of all branches          ( +-  0,01% )
|
|          # Table of individual measurements:
|          1418,288 (-2,347) #
|          1420,425 (-0,210) #
|          1420,633 (-0,001) #
|          1421,665 (+1,030) #
|          1422,162 (+1,528) #
|
|          # Final result:
|          1420,635 +- 0,669 seconds time elapsed  ( +-  0,05% )

and then on v5.17-rc6:
| Performance counter stats for 'system wide' (5 runs):
|
|     45.459.406,05 msec cpu-clock                 #   32,009 CPUs utilized            ( +-  0,04% )
|         9.460.380      context-switches          #  208,171 /sec                     ( +-  0,10% )
|           837.571      cpu-migrations            #   18,430 /sec                     ( +-  0,30% )
|       624.859.326      page-faults               #   13,750 K/sec                    ( +-  0,00% )
|105.247.262.852.106      cycles                    #    2,316 GHz                      ( +-  0,01% )
|77.185.603.119.285      stalled-cycles-frontend   #   73,34% frontend cycles idle     ( +-  0,01% )
|56.688.996.383.094      stalled-cycles-backend    #   53,87% backend cycles idle      ( +-  0,02% )
|69.883.077.705.602      instructions              #    0,66  insn per cycle         
|                                                  #    1,10  stalled cycles per insn  ( +-  0,00% )
|14.670.347.661.094      branches                  #  322,813 M/sec                    ( +-  0,00% )
|   561.066.414.554      branch-misses             #    3,82% of all branches          ( +-  0,01% )
|
|          # Table of individual measurements:
|          1418,142 (-2,061) #
|          1420,187 (-0,016) #
|          1421,242 (+1,039) #
|          1420,800 (+0,597) #
|          1420,644 (+0,441) #
|
|          # Final result:
|          1420,203 +- 0,542 seconds time elapsed  ( +-  0,04% )

It does not appear that something stands out.

> Regards,
> Jason

Sebastian




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