Re: [PATCH] mm:vmscan: fix shrink sc->nr counter values issue

[zhiguojiang <justinjiang@vivo.com>]

在 2023/11/29 23:17, Matthew Wilcox 写道:
> On Wed, Nov 29, 2023 at 09:01:26PM +0800, Zhiguo Jiang wrote:
>> It is needed to ensure sc->nr.unqueued_dirty > 0, which can avoid to
>> set PGDAT_DIRTY flag when sc->nr.unqueued_dirty and sc->nr.file_taken
>> are both zero at the same time.
> Have you observed this happening, or is this from code review?

Found in code review. The other sc->nr parameters are also judged whether they themselves are zero first in shrink_node.

>
>> It can't be guaranteed for the PGDAT_WRITEBACK flag that only pages
>> marked for immediate reclaim are on evictable LRUs in other following
>> shrink processes of the same kswapd shrink recycling. So when both a
>> small amount of pages marked for immediate reclaim and a large amount
>> of pages marked for non-immediate reclaim are on evictable LRUs at the
>> same time, if it's only determined that there is at least a page marked
>> for immediate reclaim on evictable LRUs, kswapd shrink is throttled to
>> sleep, which will increase kswapd process consumption.
>>
>> It can be fixed to throttle kswapd shrink when sc->nr.immediate is equal
>> to sc->nr.file_taken.
> So you're fixing two distinct things in the same patch?
It can be understood as two issues, and I will submit them separately.
>
>> +++ b/mm/vmscan.c
>> @@ -5915,17 +5915,17 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>>   			set_bit(PGDAT_WRITEBACK, &pgdat->flags);
>>   
>>   		/* Allow kswapd to start writing pages during reclaim.*/
>> -		if (sc->nr.unqueued_dirty == sc->nr.file_taken)
>> +		if (sc->nr.unqueued_dirty && sc->nr.unqueued_dirty == sc->nr.file_taken)
>>   			set_bit(PGDAT_DIRTY, &pgdat->flags);
>>   
>>   		/*
>> -		 * If kswapd scans pages marked for immediate
>> +		 * If kswapd scans massive pages marked for immediate
> I don't understand why you've added the word "massive".  Do you mean
> that the pages are large, or that kswapd has scanned a lot of pages?
The added "massive" means that there are a large number of pages marked 
for immediate reclaim on evictable LRUs.

The added "massive" is relative to the situation that there is only a 
small amount of pages marked for immediate reclaim or even only one page 
marked for immediate reclaim on the evictable LRUs for throttle kswapd, 
and I think this situation don't need to throttle, because there may be 
other types of pages on evictable LRUs.
>
>>   		 * reclaim and under writeback (nr_immediate), it
>>   		 * implies that pages are cycling through the LRU
>>   		 * faster than they are written so forcibly stall
>>   		 * until some pages complete writeback.
>>   		 */
>> -		if (sc->nr.immediate)
>> +		if (sc->nr.immediate && sc->nr.immediate == sc->nr.file_taken)
>>   			reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
>>   	}