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    SubjectRe: [PATCH v18 4/9] mm: hugetlb: alloc the vmemmap pages associated with each HugeTLB page
    On Thu 11-03-21 09:40:57, Michal Hocko wrote:
    > 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.

    But all this seems moot now http://lkml.kernel.org/r/YEoA08n60+jzsnAl@hirez.programming.kicks-ass.net

    --
    Michal Hocko
    SUSE Labs

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