linux-kernel - Re: [PATCH 2/7] mm, vmscan: add active list aging tracepoint

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Date:   Wed, 4 Jan 2017 14:34:22 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        Minchan Kim <minchan@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/7] mm, vmscan: add active list aging tracepoint

On 01/04/2017 02:16 PM, Michal Hocko wrote:
> On Wed 04-01-17 13:52:24, Vlastimil Babka wrote:
>> On 01/04/2017 11:19 AM, Michal Hocko wrote:
>>> From: Michal Hocko <mhocko@...e.com>
>>>
>>> Our reclaim process has several tracepoints to tell us more about how
>>> things are progressing. We are, however, missing a tracepoint to track
>>> active list aging. Introduce mm_vmscan_lru_shrink_active which reports
>>> the number of
>>> 	- nr_scanned, nr_taken pages to tell us the LRU isolation
>>> 	  effectiveness.
>>
>> Well, this point is no longer true, is it...
> 
> ups, leftover
> 	- nr_take - the number of isolated pages

nr_taken

> 
>>> 	- nr_referenced pages which tells us that we are hitting referenced
>>> 	  pages which are deactivated. If this is a large part of the
>>> 	  reported nr_deactivated pages then we might be hitting into
>>> 	  the active list too early because they might be still part of
>>> 	  the working set. This might help to debug performance issues.
>>> 	- nr_activated pages which tells us how many pages are kept on the
>>
>> "nr_activated" is slightly misleading? They remain active, they are not
>> being activated (that's why the pgactivate vmstat is also not increased
>> on them, right?). I guess rename to "nr_active" ? Or something like
>> "nr_remain_active" although that's longer.
> 
> will go with nr_active

OK.

> 
>>
>> [...]
>>
>>> @@ -1857,6 +1859,7 @@ static void move_active_pages_to_lru(struct lruvec *lruvec,
>>>  	unsigned long pgmoved = 0;
>>>  	struct page *page;
>>>  	int nr_pages;
>>> +	int nr_moved = 0;
>>>  
>>>  	while (!list_empty(list)) {
>>>  		page = lru_to_page(list);
>>> @@ -1882,11 +1885,15 @@ static void move_active_pages_to_lru(struct lruvec *lruvec,
>>>  				spin_lock_irq(&pgdat->lru_lock);
>>>  			} else
>>>  				list_add(&page->lru, pages_to_free);
>>> +		} else {
>>> +			nr_moved += nr_pages;
>>>  		}
>>>  	}
>>>  
>>>  	if (!is_active_lru(lru))
>>>  		__count_vm_events(PGDEACTIVATE, pgmoved);
>>
>> So we now have pgmoved and nr_moved. One is used for vmstat, other for
>> tracepoint, and the only difference is that vmstat includes pages where
>> we raced with page being unmapped from all pte's (IIUC?) and thus
>> removed from lru, which should be rather rare? I guess those are being
>> counted into vmstat only due to how the code evolved from using pagevec.
>> If we don't consider them in the tracepoint, then I'd suggest we don't
>> count them into vmstat either, and simplify this.
> 
> OK, but I would prefer to have this in a separate patch, OK?

Sure, thanks!