Re: [PATCH 3/3] migrate_pages: try migrate in batch asynchronously firstly

[Hugh Dickins]

On Fri, 24 Feb 2023, Huang Ying wrote:

> When we have locked more than one folios, we cannot wait the lock or
> bit (e.g., page lock, buffer head lock, writeback bit) synchronously.
> Otherwise deadlock may be triggered.  This make it hard to batch the
> synchronous migration directly.
> 
> This patch re-enables batching synchronous migration via trying to
> migrate in batch asynchronously firstly.  And any folios that are
> failed to be migrated asynchronously will be migrated synchronously
> one by one.
> 
> Test shows that this can restore the TLB flushing batching performance
> for synchronous migration effectively.
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>

I'm not sure whether my 48 hours on two machines counts for a
Tested-by: Hugh Dickins <hughd@xxxxxxxxxx>
or not; but it certainly looks like you've fixed my deadlock.

> Cc: "Xu, Pengfei" <pengfei.xu@xxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Stefan Roesch <shr@xxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Xin Hao <xhao@xxxxxxxxxxxxxxxxx>
> Cc: Zi Yan <ziy@xxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> ---
>  mm/migrate.c | 65 ++++++++++++++++++++++++++++++++++++++++++++--------
>  1 file changed, 55 insertions(+), 10 deletions(-)

I was initially disappointed, that this was more complicated than I had
thought it should be; but came to understand why.  My "change the mode
to MIGRATE_ASYNC after the first" model would have condemned most of the
MIGRATE_SYNC batch of pages to be handled as lightly as MIGRATE_ASYNC:
not good enough, you're right be trying harder here.

> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 91198b487e49..c17ce5ee8d92 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1843,6 +1843,51 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
>  	return rc;
>  }
>  
> +static int migrate_pages_sync(struct list_head *from, new_page_t get_new_page,
> +		free_page_t put_new_page, unsigned long private,
> +		enum migrate_mode mode, int reason, struct list_head *ret_folios,
> +		struct list_head *split_folios, struct migrate_pages_stats *stats)
> +{
> +	int rc, nr_failed = 0;
> +	LIST_HEAD(folios);
> +	struct migrate_pages_stats astats;
> +
> +	memset(&astats, 0, sizeof(astats));
> +	/* Try to migrate in batch with MIGRATE_ASYNC mode firstly */
> +	rc = migrate_pages_batch(from, get_new_page, put_new_page, private, MIGRATE_ASYNC,
> +				 reason, &folios, split_folios, &astats,
> +				 NR_MAX_MIGRATE_PAGES_RETRY);

I wonder if that and below would better be NR_MAX_MIGRATE_PAGES_RETRY / 2.

Though I've never got down to adjusting that number (and it's not a job
to be done in this set of patches), those 10 retries sometimes terrify
me, from a latency point of view.  They can have such different weights:
in the unmapped case, 10 retries is okay; but when a pinned page is mapped
into 1000 processes, the thought of all that unmapping and TLB flushing
and remapping is terrifying.

Since you're retrying below, halve both numbers of retries for now?

> +	stats->nr_succeeded += astats.nr_succeeded;
> +	stats->nr_thp_succeeded += astats.nr_thp_succeeded;
> +	stats->nr_thp_split += astats.nr_thp_split;
> +	if (rc < 0) {
> +		stats->nr_failed_pages += astats.nr_failed_pages;
> +		stats->nr_thp_failed += astats.nr_thp_failed;
> +		list_splice_tail(&folios, ret_folios);
> +		return rc;
> +	}
> +	stats->nr_thp_failed += astats.nr_thp_split;
> +	nr_failed += astats.nr_thp_split;
> +	/*
> +	 * Fall back to migrate all failed folios one by one synchronously. All
> +	 * failed folios except split THPs will be retried, so their failure
> +	 * isn't counted
> +	 */
> +	list_splice_tail_init(&folios, from);
> +	while (!list_empty(from)) {
> +		list_move(from->next, &folios);
> +		rc = migrate_pages_batch(&folios, get_new_page, put_new_page,
> +					 private, mode, reason, ret_folios,
> +					 split_folios, stats, NR_MAX_MIGRATE_PAGES_RETRY);

NR_MAX_MIGRATE_PAGES_RETRY / 2 ?

> +		list_splice_tail_init(&folios, ret_folios);
> +		if (rc < 0)
> +			return rc;
> +		nr_failed += rc;
> +	}
> +
> +	return nr_failed;
> +}
> +
>  /*
>   * migrate_pages - migrate the folios specified in a list, to the free folios
>   *		   supplied as the target for the page migration
> @@ -1874,7 +1919,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		enum migrate_mode mode, int reason, unsigned int *ret_succeeded)
>  {
>  	int rc, rc_gather;
> -	int nr_pages, batch;
> +	int nr_pages;
>  	struct folio *folio, *folio2;
>  	LIST_HEAD(folios);
>  	LIST_HEAD(ret_folios);
> @@ -1890,10 +1935,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  	if (rc_gather < 0)
>  		goto out;
>  
> -	if (mode == MIGRATE_ASYNC)
> -		batch = NR_MAX_BATCHED_MIGRATION;
> -	else
> -		batch = 1;
>  again:
>  	nr_pages = 0;
>  	list_for_each_entry_safe(folio, folio2, from, lru) {
> @@ -1904,16 +1945,20 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
>  		}
>  
>  		nr_pages += folio_nr_pages(folio);
> -		if (nr_pages >= batch)
> +		if (nr_pages >= NR_MAX_BATCHED_MIGRATION)
>  			break;
>  	}
> -	if (nr_pages >= batch)
> +	if (nr_pages >= NR_MAX_BATCHED_MIGRATION)
>  		list_cut_before(&folios, from, &folio2->lru);
>  	else
>  		list_splice_init(from, &folios);
> -	rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
> -				 mode, reason, &ret_folios, &split_folios, &stats,
> -				 NR_MAX_MIGRATE_PAGES_RETRY);
> +	if (mode == MIGRATE_ASYNC)
> +		rc = migrate_pages_batch(&folios, get_new_page, put_new_page, private,
> +					 mode, reason, &ret_folios, &split_folios, &stats,
> +					 NR_MAX_MIGRATE_PAGES_RETRY);
> +	else
> +		rc = migrate_pages_sync(&folios, get_new_page, put_new_page, private,
> +					mode, reason, &ret_folios, &split_folios, &stats);
>  	list_splice_tail_init(&folios, &ret_folios);
>  	if (rc < 0) {
>  		rc_gather = rc;
> -- 
> 2.39.1