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Date: Sun, 11 Dec 2022 11:59:48 +0800
From: Chen Wandun <chenwandun@...wei.com>
To: Yu Zhao <yuzhao@...gle.com>
CC: Johannes Weiner <hannes@...xchg.org>,
Jonathan Corbet <corbet@....net>,
Michael Larabel <Michael@...haellarabel.com>,
Michal Hocko <mhocko@...nel.org>,
Mike Rapoport <rppt@...nel.org>,
Roman Gushchin <roman.gushchin@...ux.dev>,
Suren Baghdasaryan <surenb@...gle.com>, <linux-mm@...ck.org>,
<linux-kernel@...r.kernel.org>, <linux-mm@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH mm-unstable v1 3/8] mm: multi-gen LRU: remove eviction
fairness safeguard
在 2022/12/2 6:39, Yu Zhao 写道:
> Recall that the eviction consumes the oldest generation: first it
> bucket-sorts folios whose gen counters were updated by the aging and
> reclaims the rest; then it increments lrugen->min_seq.
>
> The current eviction fairness safeguard for global reclaim has a
> dilemma: when there are multiple eligible memcgs, should it continue
> or stop upon meeting the reclaim goal? If it continues, it overshoots
> and increases direct reclaim latency; if it stops, it loses fairness
> between memcgs it has taken memory away from and those it has yet to.
>
> With memcg LRU, the eviction, while ensuring eventual fairness, will
> stop upon meeting its goal. Therefore the current eviction fairness
> safeguard for global reclaim will not be needed.
>
> Note that memcg LRU only applies to global reclaim. For memcg reclaim,
> the eviction will continue, even if it is overshooting. This becomes
> unconditional due to code simplification.
>
> Signed-off-by: Yu Zhao <yuzhao@...gle.com>
> ---
> mm/vmscan.c | 81 +++++++++++++++--------------------------------------
> 1 file changed, 23 insertions(+), 58 deletions(-)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ebab1ec3d400..d714a777c88b 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -449,6 +449,11 @@ static bool cgroup_reclaim(struct scan_control *sc)
> return sc->target_mem_cgroup;
> }
>
> +static bool global_reclaim(struct scan_control *sc)
> +{
> + return !sc->target_mem_cgroup || mem_cgroup_is_root(sc->target_mem_cgroup);
> +}
> +
> /**
> * writeback_throttling_sane - is the usual dirty throttling mechanism available?
> * @sc: scan_control in question
> @@ -499,6 +504,11 @@ static bool cgroup_reclaim(struct scan_control *sc)
> return false;
> }
>
> +static bool global_reclaim(struct scan_control *sc)
> +{
> + return true;
> +}
> +
> static bool writeback_throttling_sane(struct scan_control *sc)
> {
> return true;
> @@ -4991,8 +5001,7 @@ static int isolate_folios(struct lruvec *lruvec, struct scan_control *sc, int sw
> return scanned;
> }
>
> -static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swappiness,
> - bool *need_swapping)
> +static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swappiness)
> {
> int type;
> int scanned;
> @@ -5081,9 +5090,6 @@ static int evict_folios(struct lruvec *lruvec, struct scan_control *sc, int swap
> goto retry;
> }
>
> - if (need_swapping && type == LRU_GEN_ANON)
> - *need_swapping = true;
> -
> return scanned;
> }
>
> @@ -5122,67 +5128,26 @@ static unsigned long get_nr_to_scan(struct lruvec *lruvec, struct scan_control *
> return min_seq[!can_swap] + MIN_NR_GENS <= max_seq ? nr_to_scan : 0;
> }
>
> -static bool should_abort_scan(struct lruvec *lruvec, unsigned long seq,
> - struct scan_control *sc, bool need_swapping)
> +static unsigned long get_nr_to_reclaim(struct scan_control *sc)
> {
> - int i;
> - DEFINE_MAX_SEQ(lruvec);
> + /* don't abort memcg reclaim to ensure fairness */
> + if (!global_reclaim(sc))
> + return -1;
The return type of the function is unsigned long. Does the return of - 1
mean something else?
>
> - if (!current_is_kswapd()) {
> - /* age each memcg at most once to ensure fairness */
> - if (max_seq - seq > 1)
> - return true;
> + /* discount the previous progress for kswapd */
> + if (current_is_kswapd())
> + return sc->nr_to_reclaim + sc->last_reclaimed;
>
> - /* over-swapping can increase allocation latency */
> - if (sc->nr_reclaimed >= sc->nr_to_reclaim && need_swapping)
> - return true;
> -
> - /* give this thread a chance to exit and free its memory */
> - if (fatal_signal_pending(current)) {
> - sc->nr_reclaimed += MIN_LRU_BATCH;
> - return true;
> - }
> -
> - if (cgroup_reclaim(sc))
> - return false;
> - } else if (sc->nr_reclaimed - sc->last_reclaimed < sc->nr_to_reclaim)
> - return false;
> -
> - /* keep scanning at low priorities to ensure fairness */
> - if (sc->priority > DEF_PRIORITY - 2)
> - return false;
> -
> - /*
> - * A minimum amount of work was done under global memory pressure. For
> - * kswapd, it may be overshooting. For direct reclaim, the allocation
> - * may succeed if all suitable zones are somewhat safe. In either case,
> - * it's better to stop now, and restart later if necessary.
> - */
> - for (i = 0; i <= sc->reclaim_idx; i++) {
> - unsigned long wmark;
> - struct zone *zone = lruvec_pgdat(lruvec)->node_zones + i;
> -
> - if (!managed_zone(zone))
> - continue;
> -
> - wmark = current_is_kswapd() ? high_wmark_pages(zone) : low_wmark_pages(zone);
> - if (wmark > zone_page_state(zone, NR_FREE_PAGES))
> - return false;
> - }
> -
> - sc->nr_reclaimed += MIN_LRU_BATCH;
> -
> - return true;
> + return max(sc->nr_to_reclaim, compact_gap(sc->order));
> }
>
> static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc)
> {
> struct blk_plug plug;
> bool need_aging = false;
> - bool need_swapping = false;
> unsigned long scanned = 0;
> unsigned long reclaimed = sc->nr_reclaimed;
> - DEFINE_MAX_SEQ(lruvec);
> + unsigned long nr_to_reclaim = get_nr_to_reclaim(sc);
>
> lru_add_drain();
>
> @@ -5206,7 +5171,7 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc
> if (!nr_to_scan)
> goto done;
>
> - delta = evict_folios(lruvec, sc, swappiness, &need_swapping);
> + delta = evict_folios(lruvec, sc, swappiness);
> if (!delta)
> goto done;
>
> @@ -5214,7 +5179,7 @@ static void lru_gen_shrink_lruvec(struct lruvec *lruvec, struct scan_control *sc
> if (scanned >= nr_to_scan)
> break;
>
> - if (should_abort_scan(lruvec, max_seq, sc, need_swapping))
> + if (sc->nr_reclaimed >= nr_to_reclaim)
> break;
>
> cond_resched();
> @@ -5661,7 +5626,7 @@ static int run_eviction(struct lruvec *lruvec, unsigned long seq, struct scan_co
> if (sc->nr_reclaimed >= nr_to_reclaim)
> return 0;
>
> - if (!evict_folios(lruvec, sc, swappiness, NULL))
> + if (!evict_folios(lruvec, sc, swappiness))
> return 0;
>
> cond_resched();
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