[PATCH] powerpc/64: Use tick accounting by default

[Nicholas Piggin]

On Mon, 22 May 2017 14:15:57 +1000
Michael Ellerman <mpe at ellerman.id.au> wrote:

> Anton Blanchard <anton at ozlabs.org> writes:
> 
> > From: Anton Blanchard <anton at samba.org>
> >
> > ppc64 is the only architecture that turns on VIRT_CPU_ACCOUNTING_NATIVE
> > by default. The overhead of this option is extremely high - a context
> > switch microbenchmark using sched_yield() is almost 20% slower.  
> 
> Running on what? It should all be nop'ed out unless you're on a platform
> that needs it (SPLPAR).

Not ACCOUNT_CPU_USER_ENTRY/EXIT, which adds a fair cost to kernel
entry/exit (my notes say ~70 cycles on getppid() out of 3-4 hundred).

> 
> > To get finer grained user/hardirq/softirq statitics, the
> > IRQ_TIME_ACCOUNTING option can be used instead, which has much lower
> > overhead.  
> 
> Can it? We don't select HAVE_IRQ_TIME_ACCOUNTING, so AFAICS it can't be
> enabled.
> 
> Doesn't dropping this mean we never count stolen time?
> 
> cheers