Re: [PATCH v17 17/21] mm/lru: replace pgdat lru_lock with lruvec lock

[Alexander Duyck <alexander.duyck@gmail.com>]

On Tue, Jul 28, 2020 at 8:40 AM Alex Shi <alex.shi@linux.alibaba.com> wrote:
>
>
>
> 在 2020/7/28 上午7:34, Alexander Duyck 写道:
> > It might make more sense to look at modifying
> > compact_unlock_should_abort and compact_lock_irqsave (which always
> > returns true so should probably be a void) to address the deficiencies
> > they have that make them unusable for you.
>
> One of possible reuse for the func compact_unlock_should_abort, could be
> like the following, the locked parameter reused different in 2 places.
> but, it's seems no this style usage in kernel, isn't it?
>
> Thanks
> Alex
>
> From 41d5ce6562f20f74bc6ac2db83e226ac28d56e90 Mon Sep 17 00:00:00 2001
> From: Alex Shi <alex.shi@linux.alibaba.com>
> Date: Tue, 28 Jul 2020 21:19:32 +0800
> Subject: [PATCH] compaction polishing
>
> Signed-off-by: Alex Shi <alex.shi@linux.alibaba.com>
> ---
>  mm/compaction.c | 71 ++++++++++++++++++++++++---------------------------------
>  1 file changed, 30 insertions(+), 41 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c28a43481f01..36fce988de3e 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -479,20 +479,20 @@ static bool test_and_set_skip(struct compact_control *cc, struct page *page,
>   *
>   * Always returns true which makes it easier to track lock state in callers.
>   */
> -static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
> +static void compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
>                                                 struct compact_control *cc)
>         __acquires(lock)
>  {
>         /* Track if the lock is contended in async mode */
>         if (cc->mode == MIGRATE_ASYNC && !cc->contended) {
>                 if (spin_trylock_irqsave(lock, *flags))
> -                       return true;
> +                       return;
>
>                 cc->contended = true;
>         }
>
>         spin_lock_irqsave(lock, *flags);
> -       return true;
> +       return;
>  }
>
>  /*
> @@ -511,11 +511,11 @@ static bool compact_lock_irqsave(spinlock_t *lock, unsigned long *flags,
>   *             scheduled)
>   */
>  static bool compact_unlock_should_abort(spinlock_t *lock,
> -               unsigned long flags, bool *locked, struct compact_control *cc)
> +               unsigned long flags, void **locked, struct compact_control *cc)

Instead of passing both a void pointer and the lock why not just pass
the pointer to the lock pointer? You could combine lock and locked
into a single argument and save yourself some extra effort.

>  {
>         if (*locked) {
>                 spin_unlock_irqrestore(lock, flags);
> -               *locked = false;
> +               *locked = NULL;
>         }
>
>         if (fatal_signal_pending(current)) {
> @@ -543,7 +543,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>         int nr_scanned = 0, total_isolated = 0;
>         struct page *cursor;
>         unsigned long flags = 0;
> -       bool locked = false;
> +       struct compact_control *locked = NULL;
>         unsigned long blockpfn = *start_pfn;
>         unsigned int order;
>
> @@ -565,7 +565,7 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>                  */
>                 if (!(blockpfn % SWAP_CLUSTER_MAX)
>                     && compact_unlock_should_abort(&cc->zone->lock, flags,
> -                                                               &locked, cc))
> +                                                       (void**)&locked, cc))
>                         break;
>
>                 nr_scanned++;
> @@ -599,8 +599,8 @@ static unsigned long isolate_freepages_block(struct compact_control *cc,
>                  * recheck as well.
>                  */
>                 if (!locked) {
> -                       locked = compact_lock_irqsave(&cc->zone->lock,
> -                                                               &flags, cc);
> +                       compact_lock_irqsave(&cc->zone->lock, &flags, cc);
> +                       locked = cc;
>
>                         /* Recheck this is a buddy page under lock */
>                         if (!PageBuddy(page))

If you have to provide a pointer you might as well just provide a
pointer to the zone lock since that is the thing that is actually
holding the lock at this point and would be consistent with your other
uses of the locked value. One possibility would be to change the
return type so that you return a pointer to the lock you are using.
Then the code would look closer to the lruvec code you are already
using.

> @@ -787,7 +787,7 @@ static bool too_many_isolated(pg_data_t *pgdat)
>         unsigned long nr_scanned = 0, nr_isolated = 0;
>         struct lruvec *lruvec;
>         unsigned long flags = 0;
> -       struct lruvec *locked_lruvec = NULL;
> +       struct lruvec *locked = NULL;
>         struct page *page = NULL, *valid_page = NULL;
>         unsigned long start_pfn = low_pfn;
>         bool skip_on_failure = false;
> @@ -847,21 +847,11 @@ static bool too_many_isolated(pg_data_t *pgdat)
>                  * contention, to give chance to IRQs. Abort completely if
>                  * a fatal signal is pending.
>                  */
> -               if (!(low_pfn % SWAP_CLUSTER_MAX)) {
> -                       if (locked_lruvec) {
> -                               unlock_page_lruvec_irqrestore(locked_lruvec,
> -                                                                       flags);
> -                               locked_lruvec = NULL;
> -                       }
> -
> -                       if (fatal_signal_pending(current)) {
> -                               cc->contended = true;
> -
> -                               low_pfn = 0;
> -                               goto fatal_pending;
> -                       }
> -
> -                       cond_resched();
> +               if (!(low_pfn % SWAP_CLUSTER_MAX)
> +                   && compact_unlock_should_abort(&locked->lru_lock, flags,
> +                                               (void**)&locked, cc)) {

An added advantage to making locked a pointer to a spinlock is that
you could reduce the number of pointers you have to pass. Instead of
messing with &locked->lru_lock you would just pass the pointer to
locked resulting in fewer arguments being passed and if it is NULL you
skip the whole unlock pass.

> +                       low_pfn = 0;
> +                       goto fatal_pending;
>                 }
>
>                 if (!pfn_valid_within(low_pfn))
> @@ -932,9 +922,9 @@ static bool too_many_isolated(pg_data_t *pgdat)
>                          */
>                         if (unlikely(__PageMovable(page)) &&
>                                         !PageIsolated(page)) {
> -                               if (locked_lruvec) {
> -                                       unlock_page_lruvec_irqrestore(locked_lruvec, flags);
> -                                       locked_lruvec = NULL;
> +                               if (locked) {
> +                                       unlock_page_lruvec_irqrestore(locked, flags);
> +                                       locked = NULL;
>                                 }
>
>                                 if (!isolate_movable_page(page, isolate_mode))
> @@ -979,13 +969,13 @@ static bool too_many_isolated(pg_data_t *pgdat)
>                 lruvec = mem_cgroup_page_lruvec(page, pgdat);
>
>                 /* If we already hold the lock, we can skip some rechecking */
> -               if (lruvec != locked_lruvec) {
> -                       if (locked_lruvec)
> -                               unlock_page_lruvec_irqrestore(locked_lruvec,
> +               if (lruvec != locked) {
> +                       if (locked)
> +                               unlock_page_lruvec_irqrestore(locked,
>                                                                         flags);
>
>                         compact_lock_irqsave(&lruvec->lru_lock, &flags, cc);
> -                       locked_lruvec = lruvec;
> +                       locked = lruvec;
>                         rcu_read_unlock();
>
>                         lruvec_memcg_debug(lruvec, page);
> @@ -1041,9 +1031,9 @@ static bool too_many_isolated(pg_data_t *pgdat)
>
>  isolate_fail_put:
>                 /* Avoid potential deadlock in freeing page under lru_lock */
> -               if (locked_lruvec) {
> -                       unlock_page_lruvec_irqrestore(locked_lruvec, flags);
> -                       locked_lruvec = NULL;
> +               if (locked) {
> +                       unlock_page_lruvec_irqrestore(locked, flags);
> +                       locked = NULL;
>                 }
>                 put_page(page);
>
> @@ -1057,10 +1047,9 @@ static bool too_many_isolated(pg_data_t *pgdat)
>                  * page anyway.
>                  */
>                 if (nr_isolated) {
> -                       if (locked_lruvec) {
> -                               unlock_page_lruvec_irqrestore(locked_lruvec,
> -                                                                       flags);
> -                               locked_lruvec = NULL;
> +                       if (locked) {
> +                               unlock_page_lruvec_irqrestore(locked, flags);
> +                               locked = NULL;
>                         }
>                         putback_movable_pages(&cc->migratepages);
>                         cc->nr_migratepages = 0;
> @@ -1087,8 +1076,8 @@ static bool too_many_isolated(pg_data_t *pgdat)
>         page = NULL;
>
>  isolate_abort:
> -       if (locked_lruvec)
> -               unlock_page_lruvec_irqrestore(locked_lruvec, flags);
> +       if (locked)
> +               unlock_page_lruvec_irqrestore(locked, flags);
>         if (page) {
>                 SetPageLRU(page);
>                 put_page(page);
> --
> 1.8.3.1
>