On Mon, 2018-07-30 at 18:26 +0200, Peter Zijlstra wrote: > On Mon, Jul 30, 2018 at 10:30:11AM -0400, Rik van Riel wrote: > > > > What happened to the rework I did there? That not only avoided > > > fiddling > > > with active_mm, but also avoids grab/drop cycles for the other > > > architectures when doing task->kthread->kthread->task things. > > > > I don't think I saw that. I only saw your email from > > July 20th with this fragment of code, which does not > > appear to avoid the grab/drop cycles, and still fiddles > > with active_mm: > > Yeah, that's it. Note how it doesn't do a grab+drop for kernel- > >kernel, > where the current could would have. > > And also note that it only fiddles with active_mm if it does the > grab+drop thing (the below should have s/ifdef/ifndef/ to make more > sense maybe).
I'll kick off a test with your variant. I don't think we
will see any performance difference on x86 (due to not
using a refcount at all any more), but unless Ingo is in
a hurry I guess there's no issue rewriting this part of
the patch series :)
Do the other patches look ok to you and Andy?
> So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
> ->active_mm == ->mm.
>
> > + /*
> > + * kernel -> kernel lazy + transfer active
> > + * user -> kernel lazy + mmgrab() active
> > + *
> > + * kernel -> user switch + mmdrop() active
> > + * user -> user switch
> > + */
> > + if (!next->mm) { // to
> > kernel
> > + enter_lazy_tlb(prev->active_mm, next);
> > +
>
> #ifndef ARCH_NO_ACTIVE_MM
> > + next->active_mm = prev->active_mm;
> > + if (prev->mm) // from
> > user
> > + mmgrab(prev->active_mm);
>
> else
> prev->active_mm = NULL;
> > +#endif
> > + } else { // to user
> > + switch_mm_irqs_off(prev->active_mm, next->mm,
> > next);
> > +
>
> #ifndef ARCH_NO_ACTIVE_MM
> > + if (!prev->mm) { // from
> > kernel
> > + /* will mmdrop() in finish_task_switch().
> > */
> > + rq->prev_mm = prev->active_mm;
> > + prev->active_mm = NULL;
> > + }
> > +#endif
>
>
>
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