Re: [PATCH] xen/arm: Fix rtds scheduler for arm

[Ian Campbell <Ian.Campbell@citrix.com>]

On Mon, 2015-02-02 at 11:40 +0000, Jan Beulich wrote:
> >>> On 02.02.15 at 12:14, <Ian.Campbell@citrix.com> wrote:
> > On Mon, 2015-02-02 at 12:49 +0200, Denys Drozdov wrote:
> >> The issue observed on credit2 scheduler is similar to the rt scheduler
> >> on arm platform. The root cause is that interrupts are disabled in the
> >> beginning of arm context_switch, thus spin_lock_irq is failing in
> >> ASSERT(local_irq_is_enabled()). I propose to change both credit2 and
> >> rt scheduler to run on arm platform as well and re-run the regression
> >> with scheduler patches.
> > 
> > I'd like to hear from the scheduler and other $arch folks regarding
> > whether we think the rtds and credit2 schedulers are wrong or whether it
> > is actually the ARM arch code which needs fixing before considering any
> > change.
> 
> The aspect to be understood first is why ARM needs IRQs disabled
> past __context_switch() into schedule_tail() (and there until after
> ctxt_switch_from() and ctxt_switch_to()). If that's indeed necessary,
> there's no question that the schedulers need to be adjusted to
> accommodate for this.

I don't think it's *necessary*, but we do seem to have ended up with the
context switch having that requirement today (and relying on it in
several places in switch from/to (mostly to).

I'm pretty sure we could rework things more along the lines of how x86
does it if needed, but it would be a non-trivial refactoring I think.

>  X86 calls context_saved() before
> schedule_tail() and has no need for IRQs to be disabled after
> __context_switch() returned.

In fact from the comment ("...which may fault...") I think it requires
that they are not disabled?

>  Otoh some roughly equivalent stuff
> ARM does in ctxt_switch_{from,to}() is being done in
> __context_switch() on x86 (and in the context here the restore
> parts seem to be of most interest) - maybe the call to
> context_saved() could be deferred on ARM until after IRQs got
> re-enabled?

Is it allowable to have the context_saved of prev be after (most of) the
state of next has been restored?

Ian.