Re: [PATCH 02/11] memcg: Reclaim when more than one page needed.

[Glauber Costa <glommer-bzQdu9zFT3WakBO8gow8eQ@public.gmane.org>]

>
>> + * retries
>> + */
>> +#define NR_PAGES_TO_RETRY 2
>> +
>
> Should be 1 << PAGE_ALLOC_COSTLY_ORDER?  Where does this number come from?
> The changelog doesn't specify.

Hocko complained about that, and I changed. Where the number comes from, 
is stated in the comments: it is a number small enough to have high 
changes of had been freed by the previous reclaim, and yet around the 
number of pages of a kernel allocation.

Of course there are allocations for nr_pages > 2. But 2 will already 
service the stack most of the time, and most of the slab caches.

>>   static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>> -				unsigned int nr_pages, bool oom_check)
>> +				unsigned int nr_pages, unsigned int min_pages,
>> +				bool oom_check)
>>   {
>>   	unsigned long csize = nr_pages * PAGE_SIZE;
>>   	struct mem_cgroup *mem_over_limit;
>> @@ -2182,18 +2190,18 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>   	} else
>>   		mem_over_limit = mem_cgroup_from_res_counter(fail_res, res);
>>   	/*
>> -	 * nr_pages can be either a huge page (HPAGE_PMD_NR), a batch
>> -	 * of regular pages (CHARGE_BATCH), or a single regular page (1).
>> -	 *
>>   	 * Never reclaim on behalf of optional batching, retry with a
>>   	 * single page instead.
>>   	 */
>> -	if (nr_pages == CHARGE_BATCH)
>> +	if (nr_pages > min_pages)
>>   		return CHARGE_RETRY;
>>
>>   	if (!(gfp_mask & __GFP_WAIT))
>>   		return CHARGE_WOULDBLOCK;
>>
>> +	if (gfp_mask & __GFP_NORETRY)
>> +		return CHARGE_NOMEM;
>> +
>>   	ret = mem_cgroup_reclaim(mem_over_limit, gfp_mask, flags);
>>   	if (mem_cgroup_margin(mem_over_limit) >= nr_pages)
>>   		return CHARGE_RETRY;
>> @@ -2206,7 +2214,7 @@ static int mem_cgroup_do_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>   	 * unlikely to succeed so close to the limit, and we fall back
>>   	 * to regular pages anyway in case of failure.
>>   	 */
>> -	if (nr_pages == 1 && ret)
>> +	if (nr_pages <= NR_PAGES_TO_RETRY && ret)
>>   		return CHARGE_RETRY;
>>
>>   	/*
>> @@ -2341,7 +2349,8 @@ again:
>>   			nr_oom_retries = MEM_CGROUP_RECLAIM_RETRIES;
>>   		}
>>
>> -		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, oom_check);
>> +		ret = mem_cgroup_do_charge(memcg, gfp_mask, batch, nr_pages,
>> +		    oom_check);
>>   		switch (ret) {
>>   		case CHARGE_OK:
>>   			break;