Date:   Thu, 9 Mar 2023 18:11:30 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     Aaron Tomlin <atomlin@...mlin.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Russell King <linux@...linux.org.uk>,
        Huacai Chen <chenhuacai@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>, x86@...nel.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v4 01/12] mm/vmstat: remove remote node draining

On 3/5/23 14:36, Marcelo Tosatti wrote:
> Draining of pages from the local pcp for a remote zone should not be
> necessary, since once the system is low on memory (or compaction on a
> zone is in effect), drain_all_pages should be called freeing any unused
> pcps.

Hmm I can't easily say this is a good idea without a proper evaluation. It's
true that drain_all_pages() will be called from
__alloc_pages_direct_reclaim(), but if you look closely, it's only after
__perform_reclaim() and subsequent get_page_from_freelist() failure, which
means very low on memory. There's also kcompactd_do_work() caller, but that
shouldn't respond to order-0 pressure.

Maybe part of the problem is that pages sitting in pcplist are not counted
as NR_FREE_PAGES, so watermark checks will not "see" them as available. That
would be true for both local and remote nodes as we process the zonelist,
but if pages are stuck on remote pcplists, it could make the problem worse
than if they stayed only in local ones.

But the good news should be that for this series you shouldn't actually need
this patch, AFAIU. Last year Mel did the "Drain remote per-cpu directly"
series and followups, so remote draining of pcplists is today done without
sending an IPI to the remote CPU.

BTW I kinda like the implementation that relies on "per-cpu spin lock"
that's used mostly locally thus uncontended, and only rarely remotely. Much
simpler than complicated cmpxchg schemes, wonder if those are really still
such a win these days, e.g. in SLUB or here in vmstat...

> For reference, the original commit which introduces remote node
> draining is 4037d452202e34214e8a939fa5621b2b3bbb45b7.
> 
> Acked-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Marcelo Tosatti <mtosatti@...hat.com>
> 
> Index: linux-vmstat-remote/include/linux/mmzone.h
> ===================================================================
> --- linux-vmstat-remote.orig/include/linux/mmzone.h
> +++ linux-vmstat-remote/include/linux/mmzone.h
> @@ -679,9 +679,6 @@ struct per_cpu_pages {
>  	int high;		/* high watermark, emptying needed */
>  	int batch;		/* chunk size for buddy add/remove */
>  	short free_factor;	/* batch scaling factor during free */
> -#ifdef CONFIG_NUMA
> -	short expire;		/* When 0, remote pagesets are drained */
> -#endif
>  
>  	/* Lists of pages, one per migrate type stored on the pcp-lists */
>  	struct list_head lists[NR_PCP_LISTS];
> Index: linux-vmstat-remote/mm/vmstat.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/vmstat.c
> +++ linux-vmstat-remote/mm/vmstat.c
> @@ -803,20 +803,16 @@ static int fold_diff(int *zone_diff, int
>   *
>   * The function returns the number of global counters updated.
>   */
> -static int refresh_cpu_vm_stats(bool do_pagesets)
> +static int refresh_cpu_vm_stats(void)
>  {
>  	struct pglist_data *pgdat;
>  	struct zone *zone;
>  	int i;
>  	int global_zone_diff[NR_VM_ZONE_STAT_ITEMS] = { 0, };
>  	int global_node_diff[NR_VM_NODE_STAT_ITEMS] = { 0, };
> -	int changes = 0;
>  
>  	for_each_populated_zone(zone) {
>  		struct per_cpu_zonestat __percpu *pzstats = zone->per_cpu_zonestats;
> -#ifdef CONFIG_NUMA
> -		struct per_cpu_pages __percpu *pcp = zone->per_cpu_pageset;
> -#endif
>  
>  		for (i = 0; i < NR_VM_ZONE_STAT_ITEMS; i++) {
>  			int v;
> @@ -826,44 +822,8 @@ static int refresh_cpu_vm_stats(bool do_
>  
>  				atomic_long_add(v, &zone->vm_stat[i]);
>  				global_zone_diff[i] += v;
> -#ifdef CONFIG_NUMA
> -				/* 3 seconds idle till flush */
> -				__this_cpu_write(pcp->expire, 3);
> -#endif
>  			}
>  		}
> -#ifdef CONFIG_NUMA
> -
> -		if (do_pagesets) {
> -			cond_resched();
> -			/*
> -			 * Deal with draining the remote pageset of this
> -			 * processor
> -			 *
> -			 * Check if there are pages remaining in this pageset
> -			 * if not then there is nothing to expire.
> -			 */
> -			if (!__this_cpu_read(pcp->expire) ||
> -			       !__this_cpu_read(pcp->count))
> -				continue;
> -
> -			/*
> -			 * We never drain zones local to this processor.
> -			 */
> -			if (zone_to_nid(zone) == numa_node_id()) {
> -				__this_cpu_write(pcp->expire, 0);
> -				continue;
> -			}
> -
> -			if (__this_cpu_dec_return(pcp->expire))
> -				continue;
> -
> -			if (__this_cpu_read(pcp->count)) {
> -				drain_zone_pages(zone, this_cpu_ptr(pcp));
> -				changes++;
> -			}
> -		}
> -#endif
>  	}
>  
>  	for_each_online_pgdat(pgdat) {
> @@ -880,8 +840,7 @@ static int refresh_cpu_vm_stats(bool do_
>  		}
>  	}
>  
> -	changes += fold_diff(global_zone_diff, global_node_diff);
> -	return changes;
> +	return fold_diff(global_zone_diff, global_node_diff);
>  }
>  
>  /*
> @@ -1867,7 +1826,7 @@ int sysctl_stat_interval __read_mostly =
>  #ifdef CONFIG_PROC_FS
>  static void refresh_vm_stats(struct work_struct *work)
>  {
> -	refresh_cpu_vm_stats(true);
> +	refresh_cpu_vm_stats();
>  }
>  
>  int vmstat_refresh(struct ctl_table *table, int write,
> @@ -1877,6 +1836,8 @@ int vmstat_refresh(struct ctl_table *tab
>  	int err;
>  	int i;
>  
> +	drain_all_pages(NULL);
> +
>  	/*
>  	 * The regular update, every sysctl_stat_interval, may come later
>  	 * than expected: leaving a significant amount in per_cpu buckets.
> @@ -1931,7 +1892,7 @@ int vmstat_refresh(struct ctl_table *tab
>  
>  static void vmstat_update(struct work_struct *w)
>  {
> -	if (refresh_cpu_vm_stats(true)) {
> +	if (refresh_cpu_vm_stats()) {
>  		/*
>  		 * Counters were updated so we expect more updates
>  		 * to occur in the future. Keep on running the
> @@ -1994,7 +1955,7 @@ void quiet_vmstat(void)
>  	 * it would be too expensive from this path.
>  	 * vmstat_shepherd will take care about that for us.
>  	 */
> -	refresh_cpu_vm_stats(false);
> +	refresh_cpu_vm_stats();
>  }
>  
>  /*
> Index: linux-vmstat-remote/mm/page_alloc.c
> ===================================================================
> --- linux-vmstat-remote.orig/mm/page_alloc.c
> +++ linux-vmstat-remote/mm/page_alloc.c
> @@ -3176,26 +3176,6 @@ static int rmqueue_bulk(struct zone *zon
>  	return allocated;
>  }
>  
> -#ifdef CONFIG_NUMA
> -/*
> - * Called from the vmstat counter updater to drain pagesets of this
> - * currently executing processor on remote nodes after they have
> - * expired.
> - */
> -void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
> -{
> -	int to_drain, batch;
> -
> -	batch = READ_ONCE(pcp->batch);
> -	to_drain = min(pcp->count, batch);
> -	if (to_drain > 0) {
> -		spin_lock(&pcp->lock);
> -		free_pcppages_bulk(zone, to_drain, pcp, 0);
> -		spin_unlock(&pcp->lock);
> -	}
> -}
> -#endif
> -
>  /*
>   * Drain pcplists of the indicated processor and zone.
>   */
> 
> 
> 

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