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Subject: Re: [PATCH] mm, page_alloc: reset the zone->watermark_boost early
From:   Charan Teja Kalla <charante@codeaurora.org>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        vinmenon@codeaurora.org
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Date:   Wed, 13 May 2020 15:16:53 +0530
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On 5/12/2020 7:01 PM, Charan Teja Kalla wrote:
> 
> Thank you Andrew for the reply.
> 
> On 5/12/2020 1:41 AM, Andrew Morton wrote:
>> On Mon, 11 May 2020 19:10:08 +0530 Charan Teja Reddy <charante@codeaurora.org> wrote:
>>
>>> Updating the zone watermarks by any means, like extra_free_kbytes,
>>> min_free_kbytes, water_mark_scale_factor e.t.c, when watermark_boost is
>>> set will result into the higher low and high watermarks than the user
>>> asks. This can be avoided by resetting the zone->watermark_boost to zero
>>> early.
>>
>> Does this solve some problem which has been observed in testing?

Sorry that I misunderstood your question. Yes it has solved problem of higher
water marks seen in the zone than what I set through min_free_kbytes.

Below are the steps I pursued to reproduce the problem
1) My system setup of Android kernel running on snapdragon hardware have the 
   below settings as default:
   #cat /proc/sys/vm/min_free_kbytes = 5162
   #cat /proc/zoneinfo | grep -e boost -e low -e "high " -e min -e Node
	Node 0, zone   Normal
        	min      797
	        low      8340
        	high     8539
        	boost    0 // This is the extra print I have added to check the boosting
2) Now I just try to change the zone watermark when the ->watermark_boost
   is greater than zero. I just write the same value of min_free_kbytes in 
   which case we should have seen the watermarks same as default(I mean of step 1)

   #echo 5162 > /proc/sys/vm/min_free_kbytes

   But I have seen very high values of watermarks in the system,
  # cat /proc/zoneinfo | grep -e boost -e low -e "high " -e min -e Node
	Node 0, zone   Normal
      	  min      797
      	  low      21148
      	  high     21347
      	  boost   0

So, yes, this problem is got fixed with the changes made in this patch.

> 
> Sorry, what are those issues observed in testing? It would be helpful
> If you post them here. 
> 
>>
>>> ...
>>>
>>> --- a/mm/page_alloc.c
>>> +++ b/mm/page_alloc.c
>>> @@ -7746,9 +7746,9 @@ static void __setup_per_zone_wmarks(void)
>>>  			    mult_frac(zone_managed_pages(zone),
>>>  				      watermark_scale_factor, 10000));
>>>  
>>> +		zone->watermark_boost = 0;
>>>  		zone->_watermark[WMARK_LOW]  = min_wmark_pages(zone) + tmp;
>>>  		zone->_watermark[WMARK_HIGH] = min_wmark_pages(zone) + tmp * 2;
>>> -		zone->watermark_boost = 0;
>>>  
>>>  		spin_unlock_irqrestore(&zone->lock, flags);
>>>  	}
>>
>> This could only be a problem if code is accessing these things without
>> holding zone->lock.  Is that ever the case?
>>
> 
> This is a problem even when accessing these things with zone->lock
> held because we are directly using the macro min_wmark_pages(zone)
> which leads to the issue. Pasting macro here for reference.
> 
> #define min_wmark_pages(z) (z->_watermark[WMARK_MIN] + z->watermark_boost)
> 
> Steps that lead to the issue is like below:
> 1) On the extfrag event, we try to boost the watermark by storing the
>    value in ->watermark_boost.
> 
> 2) User changes the value of extra|min_free_kbytes or watermark_scale_factor.
>   
>    In __setup_perzone_wmarks, we directly store the user asked
>    watermarks in the zones structure. In this step, the value
>    is always offsets by ->watermark_boost as we use the min_wmark_pages() macro.
> 
> 3) Later, when kswapd woke up, it resets the zone's watermark_boost to zero. 
> 
> Step 2 from the above is what resulting into the issue.
> 

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