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    SubjectRe: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y
    On Fri, Apr 8, 2022 at 10:22 AM Paul E. McKenney <paulmck@kernel.org> wrote:
    >
    > On Thu, Apr 07, 2022 at 09:07:33PM +0000, Joel Fernandes wrote:
    > > On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
    > > which ends up not offloading any CPU. This patch removes yet another
    > > dependency from the bootloader having to know about RCU, about how many
    > > CPUs the system has, and about how to provide the mask. Basically, I
    > > think we should stop pretending that the user knows what they are doing :).
    > > In other words, if NO_CB_CPU is enabled, lets make use of it.
    > >
    > > My goal is to make RCU as zero-config as possible with sane defaults. If
    > > user wants to provide rcu_nocbs= or nohz_full= options, then those will
    > > take precedence and this patch will have no effect.
    > >
    > > I tested providing rcu_nocbs= option, ensuring that is preferred over this.
    >
    > Unless something has changed, this would change behavior relied upon
    > the enterprise distros. Last I checked, they want to supply a single
    > binary, as evidenced by the recent CONFIG_PREEMPT_DYNAMIC Kconfig option,
    > and they also want the default to be non-offloaded. That is, given a
    > kernel built with CONFIG_RCU_NOCB_CPU=y and without either a nohz_full
    > or a nocbs_cpu boot parameter, all of the CPUs must be non-offloaded.

    Just curious, do you have information (like data, experiment results)
    on why they want default non-offloaded? Or maybe they haven't tried
    the recent work done in NOCB code?

    Another option I think is to make it enforce NOCB if NR_CPUS <= 32 if
    that makes sense.

    > So for me to push this to mainline, I need an ack from someone from each
    > of the enterprise distros, and each of those someones needs to understand
    > the single-binary strategy used by the corresponding distro.

    Ok.

    > And is it really all -that- hard to specify an additional boot parameter
    > across ChromeOS devices? Android seems to manage it. ;-)

    That's not the hard part I think. The hard part is to make sure a
    future Linux user who is not an RCU expert does not forget to turn it
    on. ChromeOS is not the only OS that I've seen someone forget to do it
    ;-D. AFAIR, there were Android devices too in the past where I saw
    this forgotten. I don't think we should rely on the users doing the
    right thing (as much as possible).

    The single kernel binary point makes sense but in this case, I think
    the bigger question that I'd have is what is the default behavior and
    what do *most* users of RCU want. So we can keep sane defaults for the
    majority and reduce human errors related to configuration.

    thanks,

    -Joel

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