On Thursday, January 19, 2017 6:08 PM Mel Gorman wrote:
>
> If it's definitely required and is proven to fix the
> infinite-loop-without-oom workload then I'll back off and withdraw my
> objections. However, I'd at least like the following untested patch to
> be considered as an alternative. It has some weaknesses and would be
> slower to OOM than your patch but it avoids reintroducing zone counters
>
> ---8<---
> mm, vmscan: Wait on a waitqueue when too many pages are isolated
>
> When too many pages are isolated, direct reclaim waits on congestion to clear
> for up to a tenth of a second. There is no reason to believe that too many
> pages are isolated due to dirty pages, reclaim efficiency or congestion.
> It may simply be because an extremely large number of processes have entered
> direct reclaim at the same time. However, it is possible for the situation
> to persist forever and never reach OOM.
>
> This patch queues processes a waitqueue when too many pages are isolated.
> When parallel reclaimers finish shrink_page_list, they wake the waiters
> to recheck whether too many pages are isolated.
>
> The wait on the queue has a timeout as not all sites that isolate pages
> will do the wakeup. Depending on every isolation of LRU pages to be perfect
> forever is potentially fragile. The specific wakeups occur for page reclaim
> and compaction. If too many pages are isolated due to memory failure,
> hotplug or directly calling migration from a syscall then the waiting
> processes may wait the full timeout.
>
> Note that the timeout allows the use of waitqueue_active() on the basis
> that a race will cause the full timeout to be reached due to a missed
> wakeup. This is relatively harmless and still a massive improvement over
> unconditionally calling congestion_wait.
>
> Direct reclaimers that cannot isolate pages within the timeout will consider
> return to the caller. This is somewhat clunky as it won't return immediately
> and make go through the other priorities and slab shrinking. Eventually,
> it'll go through a few iterations of should_reclaim_retry and reach the
> MAX_RECLAIM_RETRIES limit and consider going OOM.
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 91f69aa0d581..3dd617d0c8c4 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -628,6 +628,7 @@ typedef struct pglist_data {
> int node_id;
> wait_queue_head_t kswapd_wait;
> wait_queue_head_t pfmemalloc_wait;
> + wait_queue_head_t isolated_wait;
> struct task_struct *kswapd; /* Protected by
> mem_hotplug_begin/end() */
> int kswapd_order;
> diff --git a/mm/compaction.c b/mm/compaction.c
> index 43a6cf1dc202..1b1ff6da7401 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1634,6 +1634,10 @@ static enum compact_result compact_zone(struct zone *zone, struct compact_contro
> count_compact_events(COMPACTMIGRATE_SCANNED, cc->total_migrate_scanned);
> count_compact_events(COMPACTFREE_SCANNED, cc->total_free_scanned);
>
> + /* Page reclaim could have stalled due to isolated pages */
> + if (waitqueue_active(&zone->zone_pgdat->isolated_wait))
> + wake_up(&zone->zone_pgdat->isolated_wait);
> +
> trace_mm_compaction_end(start_pfn, cc->migrate_pfn,
> cc->free_pfn, end_pfn, sync, ret);
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 8ff25883c172..d848c9f31bff 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5823,6 +5823,7 @@ static void __paginginit free_area_init_core(struct pglist_data *pgdat)
> #endif
> init_waitqueue_head(&pgdat->kswapd_wait);
> init_waitqueue_head(&pgdat->pfmemalloc_wait);
> + init_waitqueue_head(&pgdat->isolated_wait);
> #ifdef CONFIG_COMPACTION
> init_waitqueue_head(&pgdat->kcompactd_wait);
> #endif
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2281ad310d06..c93f299fbad7 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1603,16 +1603,16 @@ int isolate_lru_page(struct page *page)
> * the LRU list will go small and be scanned faster than necessary, leading to
> * unnecessary swapping, thrashing and OOM.
> */
> -static int too_many_isolated(struct pglist_data *pgdat, int file,
> +static bool safe_to_isolate(struct pglist_data *pgdat, int file,
> struct scan_control *sc)
I prefer the current function name.
> {
> unsigned long inactive, isolated;
>
> if (current_is_kswapd())
> - return 0;
> + return true;
>
> - if (!sane_reclaim(sc))
> - return 0;
> + if (sane_reclaim(sc))
> + return true;
We only need a one-line change.
>
> if (file) {
> inactive = node_page_state(pgdat, NR_INACTIVE_FILE);
> @@ -1630,7 +1630,7 @@ static int too_many_isolated(struct pglist_data *pgdat, int file,
> if ((sc->gfp_mask & (__GFP_IO | __GFP_FS)) == (__GFP_IO | __GFP_FS))
> inactive >>= 3;
>
> - return isolated > inactive;
> + return isolated < inactive;
> }
>
> static noinline_for_stack void
> @@ -1719,12 +1719,28 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> struct pglist_data *pgdat = lruvec_pgdat(lruvec);
> struct zone_reclaim_stat *reclaim_stat = &lruvec->reclaim_stat;
>
> - while (unlikely(too_many_isolated(pgdat, file, sc))) {
> - congestion_wait(BLK_RW_ASYNC, HZ/10);
> + while (!safe_to_isolate(pgdat, file, sc)) {
> + long ret;
> +
> + ret = wait_event_interruptible_timeout(pgdat->isolated_wait,
> + safe_to_isolate(pgdat, file, sc), HZ/10);
>
> /* We are about to die and free our memory. Return now. */
> - if (fatal_signal_pending(current))
> - return SWAP_CLUSTER_MAX;
> + if (fatal_signal_pending(current)) {
> + nr_reclaimed = SWAP_CLUSTER_MAX;
> + goto out;
> + }
> +
> + /*
> + * If we reached the timeout, this is direct reclaim, and
> + * pages cannot be isolated then return. If the situation
Please add something that we would rather shrink slab than go
another round of nap.
> + * persists for a long time then it'll eventually reach
> + * the no_progress limit in should_reclaim_retry and consider
> + * going OOM. In this case, do not wake the isolated_wait
> + * queue as the wakee will still not be able to make progress.
> + */
> + if (!ret && !current_is_kswapd() && !safe_to_isolate(pgdat, file, sc))
> + return 0;
> }
>
> lru_add_drain();
> @@ -1839,6 +1855,10 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
> stat.nr_activate, stat.nr_ref_keep,
> stat.nr_unmap_fail,
> sc->priority, file);
> +
> +out:
> + if (waitqueue_active(&pgdat->isolated_wait))
> + wake_up(&pgdat->isolated_wait);
> return nr_reclaimed;
> }
>
Is it also needed to check isolated_wait active before kswapd
takes nap?
thanks
Hillf
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