Re: [PATCH 0/9] rcu: Support for lazy callbacks on !CONFIG_RCU_NOCB_CPU

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, May 31, 2023 at 12:17:27PM +0200, Frederic Weisbecker a écrit :
> Hi,
> 
> Here is a first attempt at providing support for lazy callbacks on
> non-offloaded CPUs. I have measured interesting results on my machine
> when it is mostly idle (just an idle ssh connection kept alive) with
> a gain of +10% idle time. This needs more serious testing though,
> especially to make sure that no performance regression is introduced.

So after refining my measurements, it actually doesn't seem to bring
much.

Here's what I measured (it's an average after 100 runs):

* 0.01% more idle time during a "sleep 10"
* introduce 3% performance regression during an "scp" on the tools directory
  of the kernel source.
* Brings less than 1% performance improvement with a "make" on tools/perf

It only brings unconditional improvement when rcu boost is enabled (callbacks
offloaded to rcuc) but still below 1%.

The reason for the performance penalty is unclear, I initially thought it
was due to the batch execution delaying other softirqs vectors but it doesn't
look that way after all.

So for now I'm only going to re-iterate the cleanups in the beginning
of the patchset and I'll wait for a good reason before ever proceeding with the
rest.

Thanks.