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

[Huang, Ying]

Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> writes:

> On 2/24/2023 10:11 PM, 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>
>> 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(-)
>> 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);
>> +	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);
>> +		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);
>
> For split folios, it seems also reasonable to use migrate_pages_sync()
> instead of always using fixed MIGRATE_ASYNC mode?

For split folios, we only try to migrate them with minimal effort.
Previously, we decrease the retry number from 10 to 1.  Now, I think
that it's reasonable to change the migration mode to MIGRATE_ASYNC to
reduce latency.  They have been counted as failure anyway.

>>   	list_splice_tail_init(&folios, &ret_folios);
>>   	if (rc < 0) {
>>   		rc_gather = rc;

Best Regards,
Huang, Ying