Re: [RFC][PATCH] avoid cpu hot removal if busy take3

[Nathan Lynch <ntl@pobox.com>]

KAMEZAWA Hiroyuki wrote:
> 
> Now, cpu hot remove migrates all tasks on target cpu by force.
> 
> During cpu-hot-remove, if tsk->cpus_allowed contains the only target
> cpu of removal, tsk->cpus_allowd is disposed and the kernel migrate it to
> any cpu at random. It's obvious that user-land configuration before cpu hot
> removal was bad. This looks a realisitc workaround, but this is not good in
> carefully scheduled environment.
> 
> In this case,
> 1. ignore bad configuration in user-land just do warnings.
> 2. cancel cpu hot removal and warn users to fix the problem and retry.
> seems to be a realisitc workaround. Killing the problematic process may
> cause some trouble in user-land (dead-lock etc..)
> 
> This patch adds sysctl cpu_removal_migration.
> If cpu_removal_migration == 1, all tasks are migrated by force.
> If cpu_removal_migration == 0, cpu_hotremoval can fail because of not-migratable
> tasks.
> 
> Note: cpu scheduler's notifier chain has the highest priority. then, this
>       failure detection will be done at first.

I'm still not convinced that this is a good thing to do.  I reiterate:
this can be implemented in userspace (probably with fewer lines of
code, even).  Why should this policy be in the kernel?