On Wed, Mar 10, 2021 at 02:10:12PM -0800, Mike Kravetz wrote:
> On 3/10/21 1:49 PM, Paul E. McKenney wrote:
> > On Wed, Mar 10, 2021 at 10:11:22PM +0100, Michal Hocko wrote:
> >> On Wed 10-03-21 10:56:08, Mike Kravetz wrote:
> >>> On 3/10/21 7:19 AM, Michal Hocko wrote:
> >>>> On Mon 08-03-21 18:28:02, Muchun Song wrote:
> >>>> [...]
> >>>>> @@ -1447,7 +1486,7 @@ void free_huge_page(struct page *page)
> >>>>> /*
> >>>>> * Defer freeing if in non-task context to avoid hugetlb_lock deadlock.
> >>>>> */
> >>>>> - if (!in_task()) {
> >>>>> + if (in_atomic()) {
> >>>>
> >>>> As I've said elsewhere in_atomic doesn't work for CONFIG_PREEMPT_COUNT=n.
> >>>> We need this change for other reasons and so it would be better to pull
> >>>> it out into a separate patch which also makes HUGETLB depend on
> >>>> PREEMPT_COUNT.
> >>>
> >>> Yes, the issue of calling put_page for hugetlb pages from any context
> >>> still needs work. IMO, that is outside the scope of this series. We
> >>> already have code in this path which blocks/sleeps.
> >>>
> >>> Making HUGETLB depend on PREEMPT_COUNT is too restrictive. IIUC,
> >>> PREEMPT_COUNT will only be enabled if we enable:
> >>> PREEMPT "Preemptible Kernel (Low-Latency Desktop)"
> >>> PREEMPT_RT "Fully Preemptible Kernel (Real-Time)"
> >>> or, other 'debug' options. These are not enabled in 'more common'
> >>> kernels. Of course, we do not want to disable HUGETLB in common
> >>> configurations.
> >>
> >> I haven't tried that but PREEMPT_COUNT should be selectable even without
> >> any change to the preemption model (e.g. !PREEMPT).
> >
> > It works reliably for me, for example as in the diff below. So,
> > as Michal says, you should be able to add "select PREEMPT_COUNT" to
> > whatever Kconfig option you need to.
> >
>
> Thanks Paul.
>
> I may have been misreading Michal's suggestion of "make HUGETLB depend on
> PREEMPT_COUNT". We could "select PREEMPT_COUNT" if HUGETLB is enabled.
> However, since HUGETLB is enabled in most configs, then this would
> result in PREEMPT_COUNT also being enabled in most configs. I honestly
> do not know how much this will cost us? I assume that if it was free or
> really cheap it would already be always on?
There are a -lot- of configs out there, so are you sure that HUGETLB is
really enabled in most of them? ;-)
More seriously, I was going by earlier emails in this and related threads
plus Michal's "PREEMPT_COUNT should be selectable". But there are other
situations that would like PREEMPT_COUNT. And to your point, some who
would rather PREEMPT_COUNT not be universally enabled. I haven't seen
any performance or kernel-size numbers from any of them, however.
Thanx, Paul
> --
> Mike Kravetz
>
> > Thanx, Paul
> >
> > diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> > index 3128b7c..7d9f989 100644
> > --- a/kernel/rcu/Kconfig
> > +++ b/kernel/rcu/Kconfig
> > @@ -8,6 +8,7 @@ menu "RCU Subsystem"
> > config TREE_RCU
> > bool
> > default y if SMP
> > + select PREEMPT_COUNT
> > help
> > This option selects the RCU implementation that is
> > designed for very large SMP system with hundreds or
> >
