Re: [PATCH] mm, page_alloc: skip ->waternark_boost for atomic order-0 allocations

[Charan Teja Kalla]

Thanks Mel for feedback.

On 6/9/2020 5:58 PM, Mel Gorman wrote:
> On Tue, May 19, 2020 at 03:28:04PM +0530, Charan Teja Reddy wrote:
>> When boosting is enabled, it is observed that rate of atomic order-0
>> allocation failures are high due to the fact that free levels in the
>> system are checked with ->watermark_boost offset. This is not a problem
>> for sleepable allocations but for atomic allocations which looks like
>> regression.
>>
> 
> Are high-order allocations in general of interest to this platform? If
> not then a potential option is to simply disable boosting. The patch is
> still relevant but it's worth thinking about.
> 

Yes we do care till order-3.

>> This problem is seen frequently on system setup of Android kernel
>> running on Snapdragon hardware with 4GB RAM size. When no extfrag event
>> occurred in the system, ->watermark_boost factor is zero, thus the
>> watermark configurations in the system are:
>>    _watermark = (
>>           [WMARK_MIN] = 1272, --> ~5MB
>>           [WMARK_LOW] = 9067, --> ~36MB
>>           [WMARK_HIGH] = 9385), --> ~38MB
>>    watermark_boost = 0
>>
>> After launching some memory hungry applications in Android which can
>> cause extfrag events in the system to an extent that ->watermark_boost
>> can be set to max i.e. default boost factor makes it to 150% of high
>> watermark.
>>    _watermark = (
>>           [WMARK_MIN] = 1272, --> ~5MB
>>           [WMARK_LOW] = 9067, --> ~36MB
>>           [WMARK_HIGH] = 9385), --> ~38MB
>>    watermark_boost = 14077, -->~57MB
>>
>> With default system configuration, for an atomic order-0 allocation to
>> succeed, having free memory of ~2MB will suffice. But boosting makes
>> the min_wmark to ~61MB thus for an atomic order-0 allocation to be
>> successful system should have minimum of ~23MB of free memory(from
>> calculations of zone_watermark_ok(), min = 3/4(min/2)). But failures are
>> observed despite system is having ~20MB of free memory. In the testing,
>> this is reproducible as early as first 300secs since boot and with
>> furtherlowram configurations(<2GB) it is observed as early as first
>> 150secs since boot.
>>
>> These failures can be avoided by excluding the ->watermark_boost in
>> watermark caluculations for atomic order-0 allocations.
>>
>> Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
>> ---
>>  mm/page_alloc.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index d001d61..5193d7e 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3709,6 +3709,18 @@ static bool zone_allows_reclaim(struct zone *local_zone, struct zone *zone)
>>  		}
>>  
>>  		mark = wmark_pages(zone, alloc_flags & ALLOC_WMARK_MASK);
>> +		/*
>> +		 * Allow GFP_ATOMIC order-0 allocations to exclude the
>> +		 * zone->watermark_boost in its watermark calculations.
>> +		 * We rely on the ALLOC_ flags set for GFP_ATOMIC
>> +		 * requests in gfp_to_alloc_flags() for this. Reason not to
>> +		 * use the GFP_ATOMIC directly is that we want to fall back
>> +		 * to slow path thus wake up kswapd.
>> +		 */
> 
> The comment is a bit difficult to parse. Maybe this.
> 
> 	/*
> 	 * Ignore watermark boosting for GFP_ATOMIC order-0 allocations
> 	 * when checking the min watermark. The min watermark is the
> 	 * point where boosting is ignored so that kswapd is woken up
> 	 * when below the low watermark.
> 	 */
> 
> I left out the ALLOC_ part for reasons that are explained blow.
> 
>> +		if (unlikely(!order && !(alloc_flags & ALLOC_WMARK_MASK) &&
>> +		     (alloc_flags & (ALLOC_HARDER | ALLOC_HIGH)))) {
>> +			mark = zone->_watermark[WMARK_MIN];
>> +		}
> 
> The second check is a bit more obscure than it needs to be and depends
> on WMARK_MIN == 0. That will probably be true forever but it's not
> obvious at a glance. I suggest something like
> ((alloc_flags & ALLOC_WMARK_MASK) == WMARK_MIN).
> 
> For detecting atomic alloctions, you rely on the either ALLOC_HARDER or
> ALLOC_HIGH being set. ALLOC_HIGH can be set for non-atomic allocations
> and ALLOC_HARDER can be set for RT tasks. You probably should just test
> the gfp_mask because as it stands non-atomic allocations can ignore the
> boost too.
> 
> Finally, the patch puts an unlikely check into a relatively fast path even
> though watermarks may be fine with or without boosting.  Instead you could
> put the checks in zone_watermark_fast() if and only if the watermarks
> failed the first time. If the checks pass, the watermarks get checked
> a second time. This will be fractionally slower for requests failing
> watermark checks but there is no penalty for most allocation requests.
> It would need the gfp_mask to be passed into zone_watermark_fast but
> as it's an inlined function, there should be no cost to passing in the
> arguement i.e. do something like this at the end of zone_watermark_fast
> 
> 	if (__zone_watermark_ok(z, order, mark, classzone_idx, alloc_flags, free_pages))
> 		return true;
> 
> 	/* Ignore watermark boosting for .... */
> 	if (unlikely(!order .....) {
> 		mark = ...
> 		return __zone_watermark_ok(...);
> 	}
> 
> 	return false;
> 

Incorporated these suggestions at:
https://lore.kernel.org/patchwork/patch/1254998/. Can you please help in
reviewing?

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