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Date:   Thu, 25 May 2023 15:37:43 +0200
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Subject: Re: [PATCH 3/4] mm: compaction: Update pageblock skip when first
 migration candidate is not at the start
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To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        Michal Hocko <mhocko@kernel.org>,
        Pedro Falcato <pedro.falcato@gmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Chuyi Zhou <zhouchuyi@bytedance.com>,
        Linux-MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20230515113344.6869-1-mgorman@techsingularity.net>
 <20230515113344.6869-4-mgorman@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20230515113344.6869-4-mgorman@techsingularity.net>
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On 5/15/23 13:33, Mel Gorman wrote:
> isolate_migratepages_block should mark a pageblock as skip if scanning
> started on an aligned pageblock boundary but it only updates the skip
> flag if the first migration candidate is also aligned. Tracing during
> a compaction stress load (mmtests: workload-usemem-stress-numa-compact)
> that many pageblocks are not marked skip causing excessive scanning of
> blocks that had been recently checked. Update pageblock skip based on
> "valid_page" which is set if scanning started on a pageblock boundary.
> 
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>

I wonder if this has an unintended side-effect that if we resume
isolate_migratepages_block() of a partially compacted pageblock to finish
it, test_and_set_skip() will now tell us to abort, because we already set
the skip bit in the previous call. This would include the
cc->finish_pageblock rescan cases.

So unless I miss something that already prevents that, I agree we should not
tie setting the skip bit to pageblock_aligned(pfn), but maybe if we are not
pageblock aligned, we should ignore the already-set skip bit, as it was most
likely being set by us in the previous iteration and should not prevent us
from finishing the pageblock?

> ---
>  mm/compaction.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index accc6568091a..d7be990b1d60 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -392,18 +392,14 @@ void reset_isolation_suitable(pg_data_t *pgdat)
>   * Sets the pageblock skip bit if it was clear. Note that this is a hint as
>   * locks are not required for read/writers. Returns true if it was already set.
>   */
> -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
> -							unsigned long pfn)
> +static bool test_and_set_skip(struct compact_control *cc, struct page *page)
>  {
>  	bool skip;
>  
> -	/* Do no update if skip hint is being ignored */
> +	/* Do not update if skip hint is being ignored */
>  	if (cc->ignore_skip_hint)
>  		return false;
>  
> -	if (!pageblock_aligned(pfn))
> -		return false;
> -
>  	skip = get_pageblock_skip(page);
>  	if (!skip && !cc->no_set_skip_hint)
>  		set_pageblock_skip(page);
> @@ -470,8 +466,7 @@ static void update_cached_migrate(struct compact_control *cc, unsigned long pfn)
>  {
>  }
>  
> -static bool test_and_set_skip(struct compact_control *cc, struct page *page,
> -							unsigned long pfn)
> +static bool test_and_set_skip(struct compact_control *cc, struct page *page)
>  {
>  	return false;
>  }
> @@ -1075,9 +1070,9 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
>  			lruvec_memcg_debug(lruvec, page_folio(page));
>  
>  			/* Try get exclusive access under lock */
> -			if (!skip_updated) {
> +			if (!skip_updated && valid_page) {
>  				skip_updated = true;
> -				if (test_and_set_skip(cc, page, low_pfn))
> +				if (test_and_set_skip(cc, valid_page))
>  					goto isolate_abort;
>  			}
>