Re: [Xen-devel] [PATCH v2 2/2] sched_credit2.c: runqueue_per_core code

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Wed, 2015-03-18 at 16:08 +0000, George Dunlap wrote:
> On 03/18/2015 03:59 PM, Jan Beulich wrote:
> >>>> On 18.03.15 at 16:26, <george.dunlap@xxxxxxxxxxxxx> wrote:
> >> In both cases there's a slight risk in using system_state to determine
> >> whether to rely on cpu_data[] or not, because there's actually a window
> >> for each processor after system_state == SYS_STATE_smp_boot where
> >> cpu_data[] is *not* initialized, but it's not obvious from looking at
> >> the data itself.  If the callback mechanisms ever change order with the
> >> cpu_data[] initializations in the future, we risk a situation where
> >> credit2 silently regresses to using a single massive runqueue.
> > 
> > But such an ordering change is extremely unlikely, since the
> > CPU_STARTING notification specifically tells you that the given
> > CPU is now ready for normal use, which includes it having its
> > topology related data set up.

> Even if they look at init_pcpu() in credit2, they're
> unlikely to know that cpu_to_socket() isn't valid until after
> boot_secondary() has happened; and if they try it and boot it,
> everything will seem to work perfectly for credit2 -- except that there
> will be a single global runqueue.
> 
Agreed... and I'm still replying to your (George's) email, but just
wanted to say that, for this, having system_state referenced from
Credit2's code would help making it clear the fact that code has
dependencies with the boot process, wouldn't it?

> If you, Dario and I don't happen to catch it -- or don't happen to
> remember the exact details of the constraints -- nobody may notice until
> a year later.
> 
:-)

> This is the point of having ASSERTs -- so that we don't have to worry
> about remembering and catching all these crazy constraints.
> 
I'm all for finding a way for ASSERT()ing something meaningful to this
effect.

Regards,
Dario

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