linux-kernel - Re: [PATCH resend] mm: compaction: optimize proactive compaction deferrals

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Date:   Wed, 21 Jul 2021 13:18:06 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Charan Teja Reddy <charante@...eaurora.org>
Cc:     vbabka@...e.cz, rientjes@...gle.com, nigupta@...dia.com,
        khalid.aziz@...cle.com, vinmenon@...eaurora.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] mm: compaction: optimize proactive compaction
 deferrals

On Wed, 21 Jul 2021 17:43:19 +0530 Charan Teja Reddy <charante@...eaurora.org> wrote:

> Vlastimil Babka figured out that when fragmentation score didn't go down
> across the proactive compaction i.e. when no progress is made, next wake
> up for proactive compaction is deferred for 1 <<
> COMPACT_MAX_DEFER_SHIFT, i.e. 64 times, with each wakeup interval of
> HPAGE_FRAG_CHECK_INTERVAL_MSEC(=500). In each of this wakeup, it just
> decrement 'proactive_defer' counter and goes sleep i.e. it is getting
> woken to just decrement a counter. The same deferral time can also
> achieved by simply doing the HPAGE_FRAG_CHECK_INTERVAL_MSEC <<
> COMPACT_MAX_DEFER_SHIFT thus unnecessary wakeup of kcompact thread is
> avoided thus also removes the need of 'proactive_defer' thread counter.
> 
> @@ -2902,23 +2903,30 @@ static int kcompactd(void *p)
>  
>  		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
>  		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> -			kcompactd_work_requested(pgdat),
> -			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
> +			kcompactd_work_requested(pgdat), timeout)) {
>  
>  			psi_memstall_enter(&pflags);
>  			kcompactd_do_work(pgdat);
>  			psi_memstall_leave(&pflags);
> +			/*
> +			 * Reset the timeout value. The defer timeout by
> +			 * proactive compaction can effectively lost
> +			 * here but that is fine as the condition of the
> +			 * zone changed substantionally and carrying on
> +			 * with the previous defer is not useful.
> +			 */
> +			timeout = default_timeout;
>  			continue;

I find this comment hard to follow.  Is this better?

--- a/mm/compaction.c~mm-compaction-optimize-proactive-compaction-deferrals-fix
+++ a/mm/compaction.c
@@ -2909,11 +2909,11 @@ static int kcompactd(void *p)
 			kcompactd_do_work(pgdat);
 			psi_memstall_leave(&pflags);
 			/*
-			 * Reset the timeout value. The defer timeout by
-			 * proactive compaction can effectively lost
-			 * here but that is fine as the condition of the
-			 * zone changed substantionally and carrying on
-			 * with the previous defer is not useful.
+			 * Reset the timeout value. The defer timeout from
+			 * proactive compaction is lost here but that is fine
+			 * as the condition of the zone changing substantionally
+			 * then carrying on with the previous defer interval is
+			 * not useful.
 			 */
 			timeout = default_timeout;
 			continue;
_