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@xxxxxxxxxxxxxx> 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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