Re: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page

[Michal Hocko <mhocko@suse.com>]

On Wed 10-03-21 15:28:51, Paul E. McKenney wrote:
> 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?  ;-)

It certainly is enabled for all distribution kernels and many are
!PREEMPT so I believe this is what Mike was concerned about.

> 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.

Yeah per cpu preempt counting shouldn't be noticeable but I have to
confess I haven't benchmarked it.
-- 
Michal Hocko
SUSE Labs