Re: [PATCH 1/1] mm/khugepaged: bypassing unnecessary scans with MMF_DISABLE_THP check

[Yang Shi]

On Wed, Jan 31, 2024 at 1:30 AM Lance Yang <ioworker0@xxxxxxxxx> wrote:
>
> Updating the change log.
>
> khugepaged scans the entire address space in the
> background for each given mm, looking for
> opportunities to merge sequences of basic pages
> into huge pages. However, when an mm is inserted
> to the mm_slots list, and the MMF_DISABLE_THP
> flag is set later, this scanning process becomes
> unnecessary for that mm and can be skipped to
> avoid redundant operations, especially in scenarios
> with a large address space.
>
> This commit introduces a check before each scanning
> process to test the MMF_DISABLE_THP flag for the
> given mm; if the flag is set, the scanning process is
> bypassed, thereby improving the efficiency of khugepaged.
>
> This optimization is not a correctness issue but rather an
> enhancement to save expensive checks on each VMA
> when userspace cannot prctl itself before spawning
> into the new process.

If this is an optimization, you'd better show some real numbers to help justify.

>
> On some servers within our company, we deploy a
> daemon responsible for monitoring and updating local
> applications. Some applications prefer not to use THP,
> so the daemon calls prctl to disable THP before fork/exec.
> Conversely, for other applications, the daemon calls prctl
> to enable THP before fork/exec.

If your daemon calls prctl with MMF_DISABLE_THP before fork, then you
end up having the child mm on the hash list in the first place, I
think it should be a bug in khugepaged_fork() IIUC. khugepaged_fork()
should check this flag and bail out if it is set. Did I miss
something?

>
> Ideally, the daemon should invoke prctl after the fork,
> but its current implementation follows the described
> approach. In the Go standard library, there is no direct
> encapsulation of the fork system call; instead, fork and
> execve are combined into one through syscall.ForkExec.
>
> Thanks,
> Lance
>
> On Mon, Jan 29, 2024 at 1:46 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> >
> > khugepaged scans the entire address space in the
> > background for each given mm, looking for
> > opportunities to merge sequences of basic pages
> > into huge pages. However, when an mm is inserted
> > to the mm_slots list, and the MMF_DISABLE_THP flag
> > is set later, this scanning process becomes
> > unnecessary for that mm and can be skipped to avoid
> > redundant operations, especially in scenarios with
> > a large address space.
> >
> > This commit introduces a check before each scanning
> > process to test the MMF_DISABLE_THP flag for the
> > given mm; if the flag is set, the scanning process
> > is bypassed, thereby improving the efficiency of
> > khugepaged.
> >
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >  mm/khugepaged.c | 18 ++++++++++++------
> >  1 file changed, 12 insertions(+), 6 deletions(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 2b219acb528e..d6a700834edc 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -410,6 +410,12 @@ static inline int hpage_collapse_test_exit(struct mm_struct *mm)
> >         return atomic_read(&mm->mm_users) == 0;
> >  }
> >
> > +static inline int hpage_collapse_test_exit_or_disable(struct mm_struct *mm)
> > +{
> > +       return hpage_collapse_test_exit(mm) ||
> > +              test_bit(MMF_DISABLE_THP, &mm->flags);
> > +}
> > +
> >  void __khugepaged_enter(struct mm_struct *mm)
> >  {
> >         struct khugepaged_mm_slot *mm_slot;
> > @@ -1422,7 +1428,7 @@ static void collect_mm_slot(struct khugepaged_mm_slot *mm_slot)
> >
> >         lockdep_assert_held(&khugepaged_mm_lock);
> >
> > -       if (hpage_collapse_test_exit(mm)) {
> > +       if (hpage_collapse_test_exit_or_disable(mm)) {
> >                 /* free mm_slot */
> >                 hash_del(&slot->hash);
> >                 list_del(&slot->mm_node);
> > @@ -2360,7 +2366,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                 goto breakouterloop_mmap_lock;
> >
> >         progress++;
> > -       if (unlikely(hpage_collapse_test_exit(mm)))
> > +       if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
> >                 goto breakouterloop;
> >
> >         vma_iter_init(&vmi, mm, khugepaged_scan.address);
> > @@ -2368,7 +2374,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                 unsigned long hstart, hend;
> >
> >                 cond_resched();
> > -               if (unlikely(hpage_collapse_test_exit(mm))) {
> > +               if (unlikely(hpage_collapse_test_exit_or_disable(mm))) {
> >                         progress++;
> >                         break;
> >                 }
> > @@ -2390,7 +2396,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                         bool mmap_locked = true;
> >
> >                         cond_resched();
> > -                       if (unlikely(hpage_collapse_test_exit(mm)))
> > +                       if (unlikely(hpage_collapse_test_exit_or_disable(mm)))
> >                                 goto breakouterloop;
> >
> >                         VM_BUG_ON(khugepaged_scan.address < hstart ||
> > @@ -2408,7 +2414,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >                                 fput(file);
> >                                 if (*result == SCAN_PTE_MAPPED_HUGEPAGE) {
> >                                         mmap_read_lock(mm);
> > -                                       if (hpage_collapse_test_exit(mm))
> > +                                       if (hpage_collapse_test_exit_or_disable(mm))
> >                                                 goto breakouterloop;
> >                                         *result = collapse_pte_mapped_thp(mm,
> >                                                 khugepaged_scan.address, false);
> > @@ -2450,7 +2456,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >          * Release the current mm_slot if this mm is about to die, or
> >          * if we scanned all vmas of this mm.
> >          */
> > -       if (hpage_collapse_test_exit(mm) || !vma) {
> > +       if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
> >                 /*
> >                  * Make sure that if mm_users is reaching zero while
> >                  * khugepaged runs here, khugepaged_exit will find
> > --
> > 2.33.1
> >