Re: [PATCHSET] cpuhog: implement and use cpuhog

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-03-09 at 00:53 +0900, Tejun Heo wrote:
> Hello, all.
> 
> This patchset implements cpuhog which is a simplistic cpu
> monopolization mechanism and reimplements stop_machine() and replaces
> migration_thread with it.
> 
> This allows stop_machine() to be simpler and much more efficient on
> very large machines without using more resources while also making the
> rather messy overloaded migration_thread usages cleaner.
> 
> This should solve the slow boot problem[1] caused by repeated
> stop_machine workqueue creation/destruction reported by Dimitri
> Sivanich.
> 
> The patchset is currently on top of v2.6.33 and contains the following
> patches.
> 
>  0001-cpuhog-implement-cpuhog.patch
>  0002-stop_machine-reimplement-using-cpuhog.patch
>  0003-scheduler-replace-migration_thread-with-cpuhog.patch
>  0004-scheduler-kill-paranoia-check-in-synchronize_sched_e.patch
> 
> 0001 implements cpuhog.  0002 converts stop_machine.  0003 converts
> migration users and 0004 removes paranoia checks in
> synchronize_sched_expedited().  0004 is done separately so that 0003
> can serve as a debug/bisection point.
> 
> Tested cpu on/offlining, shutdown, all migration usage paths including
> RCU torture test at 0003 and 004 and everything seems to work fine
> here.  Dimitri, can you please test whether this solves the problem
> you're seeing there?

cpuhog as a name doesn't work for me, stop-machine had a name that
described its severity and impact, cpuhog makes me think of while(1);.

Can't we keep the stop_machine name and make that a workqueue interface
like you propose?

That way we'd end up with something like:

kernel/stop_machine.c
  int stop_cpu(int cpu, stop_fn_t fn, void *arg)
  int stop_machine(struct cpumask *mask, stop_fn_t fn, void *arg)

alternatively, something like schedule_primary_work*() might work I
guess.