linux-kernel - Re: [PATCH v8] mm: Proactive compaction

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Date:   Wed, 17 Jun 2020 13:53:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nitin Gupta <nigupta@...dia.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Oleksandr Natalenko <oleksandr@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        David Rientjes <rientjes@...gle.com>,
        Nitin Gupta <ngupta@...ingupta.dev>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v8] mm: Proactive compaction

On Tue, 16 Jun 2020 13:45:27 -0700 Nitin Gupta <nigupta@...dia.com> wrote:

> For some applications, we need to allocate almost all memory as
> hugepages. However, on a running system, higher-order allocations can
> fail if the memory is fragmented. Linux kernel currently does on-demand
> compaction as we request more hugepages, but this style of compaction
> incurs very high latency. Experiments with one-time full memory
> compaction (followed by hugepage allocations) show that kernel is able
> to restore a highly fragmented memory state to a fairly compacted memory
> state within <1 sec for a 32G system. Such data suggests that a more
> proactive compaction can help us allocate a large fraction of memory as
> hugepages keeping allocation latencies low.
>
> ...
>

All looks straightforward to me and easy to disable if it goes wrong.

All the hard-coded magic numbers are a worry, but such is life.

One teeny complaint:

>
> ...
>
> @@ -2650,12 +2801,34 @@ static int kcompactd(void *p)
>  		unsigned long pflags;
>  
>  		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
> -		wait_event_freezable(pgdat->kcompactd_wait,
> -				kcompactd_work_requested(pgdat));
> +		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> +			kcompactd_work_requested(pgdat),
> +			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
> +
> +			psi_memstall_enter(&pflags);
> +			kcompactd_do_work(pgdat);
> +			psi_memstall_leave(&pflags);
> +			continue;
> +		}
>  
> -		psi_memstall_enter(&pflags);
> -		kcompactd_do_work(pgdat);
> -		psi_memstall_leave(&pflags);
> +		/* kcompactd wait timeout */
> +		if (should_proactive_compact_node(pgdat)) {
> +			unsigned int prev_score, score;

Everywhere else, scores have type `int'.  Here they are unsigned.  How come?

Would it be better to make these unsigned throughout?  I don't think a
score can ever be negative?

> +			if (proactive_defer) {
> +				proactive_defer--;
> +				continue;
> +			}
> +			prev_score = fragmentation_score_node(pgdat);
> +			proactive_compact_node(pgdat);
> +			score = fragmentation_score_node(pgdat);
> +			/*
> +			 * Defer proactive compaction if the fragmentation
> +			 * score did not go down i.e. no progress made.
> +			 */
> +			proactive_defer = score < prev_score ?
> +					0 : 1 << COMPACT_MAX_DEFER_SHIFT;
> +		}
>  	}