On Mon, 9 Jun 2014, Vlastimil Babka wrote:
> Async compaction aborts when it detects zone lock contention or need_resched()
> is true. David Rientjes has reported that in practice, most direct async
> compactions for THP allocation abort due to need_resched(). This means that a
> second direct compaction is never attempted, which might be OK for a page
> fault, but hugepaged is intended to attempt a sync compaction in such case and
> in these cases it won't.
>
> This patch replaces "bool contended" in compact_control with an enum that
> distinguieshes between aborting due to need_resched() and aborting due to lock
> contention. This allows propagating the abort through all compaction functions
> as before, but declaring the direct compaction as contended only when lock
> contantion has been detected.
>
> As a result, hugepaged will proceed with second sync compaction as intended,
> when the preceding async compaction aborted due to need_resched().
>
s/hugepaged/khugepaged/ on the changelog.
> Reported-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> Cc: Michal Nazarewicz <mina86@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> ---
> mm/compaction.c | 20 ++++++++++++++------
> mm/internal.h | 15 +++++++++++----
> 2 files changed, 25 insertions(+), 10 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index b73b182..d37f4a8 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -185,9 +185,14 @@ static void update_pageblock_skip(struct compact_control *cc,
> }
> #endif /* CONFIG_COMPACTION */
>
> -static inline bool should_release_lock(spinlock_t *lock)
> +enum compact_contended should_release_lock(spinlock_t *lock)
> {
> - return need_resched() || spin_is_contended(lock);
> + if (need_resched())
> + return COMPACT_CONTENDED_SCHED;
> + else if (spin_is_contended(lock))
> + return COMPACT_CONTENDED_LOCK;
> + else
> + return COMPACT_CONTENDED_NONE;
> }
>
> /*
I think eventually we're going to remove the need_resched() heuristic
entirely and so enum compact_contended might be overkill, but do we need
to worry about spin_is_contended(lock) && need_resched() reporting
COMPACT_CONTENDED_SCHED here instead of COMPACT_CONTENDED_LOCK?
> @@ -202,7 +207,9 @@ static inline bool should_release_lock(spinlock_t *lock)
> static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
> bool locked, struct compact_control *cc)
> {
> - if (should_release_lock(lock)) {
> + enum compact_contended contended = should_release_lock(lock);
> +
> + if (contended) {
> if (locked) {
> spin_unlock_irqrestore(lock, *flags);
> locked = false;
> @@ -210,7 +217,7 @@ static bool compact_checklock_irqsave(spinlock_t *lock, unsigned long *flags,
>
> /* async aborts if taking too long or contended */
> if (cc->mode == MIGRATE_ASYNC) {
> - cc->contended = true;
> + cc->contended = contended;
> return false;
> }
>
> @@ -236,7 +243,7 @@ static inline bool compact_should_abort(struct compact_control *cc)
> /* async compaction aborts if contended */
> if (need_resched()) {
> if (cc->mode == MIGRATE_ASYNC) {
> - cc->contended = true;
> + cc->contended = COMPACT_CONTENDED_SCHED;
> return true;
> }
>
> @@ -1095,7 +1102,8 @@ static unsigned long compact_zone_order(struct zone *zone, int order,
> VM_BUG_ON(!list_empty(&cc.freepages));
> VM_BUG_ON(!list_empty(&cc.migratepages));
>
> - *contended = cc.contended;
> + /* We only signal lock contention back to the allocator */
> + *contended = cc.contended == COMPACT_CONTENDED_LOCK;
> return ret;
> }
>
Hmm, since the only thing that matters for cc->contended is
COMPACT_CONTENDED_LOCK, it may make sense to just leave this as a bool
within struct compact_control instead of passing the actual reason around
when it doesn't matter.
> diff --git a/mm/internal.h b/mm/internal.h
> index 7f22a11f..4659e8e 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -117,6 +117,13 @@ extern int user_min_free_kbytes;
>
> #if defined CONFIG_COMPACTION || defined CONFIG_CMA
>
> +/* Used to signal whether compaction detected need_sched() or lock contention */
> +enum compact_contended {
> + COMPACT_CONTENDED_NONE = 0, /* no contention detected */
> + COMPACT_CONTENDED_SCHED, /* need_sched() was true */
> + COMPACT_CONTENDED_LOCK, /* zone lock or lru_lock was contended */
> +};
> +
> /*
> * in mm/compaction.c
> */
> @@ -144,10 +151,10 @@ struct compact_control {
> int order; /* order a direct compactor needs */
> int migratetype; /* MOVABLE, RECLAIMABLE etc */
> struct zone *zone;
> - bool contended; /* True if a lock was contended, or
> - * need_resched() true during async
> - * compaction
> - */
> + enum compact_contended contended; /* Signal need_sched() or lock
> + * contention detected during
> + * compaction
> + */
> };
>
> unsigned long
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