Re: [PATCH v8] mm: Proactive compaction

[Nitin Gupta <ngupta@nitingupta.dev>]

On 6/17/20 1:53 PM, Andrew Morton wrote:
> On Tue, 16 Jun 2020 13:45:27 -0700 Nitin Gupta <nigupta@nvidia.com> wrote:
> 
>> For some applications, we need to allocate almost all memory as
>> hugepages. However, on a running system, higher-order allocations can
>> fail if the memory is fragmented. Linux kernel currently does on-demand
>> compaction as we request more hugepages, but this style of compaction
>> incurs very high latency. Experiments with one-time full memory
>> compaction (followed by hugepage allocations) show that kernel is able
>> to restore a highly fragmented memory state to a fairly compacted memory
>> state within <1 sec for a 32G system. Such data suggests that a more
>> proactive compaction can help us allocate a large fraction of memory as
>> hugepages keeping allocation latencies low.
>>
>> ...
>>
> 
> All looks straightforward to me and easy to disable if it goes wrong.
> 
> All the hard-coded magic numbers are a worry, but such is life.
> 
> One teeny complaint:
> 
>>
>> ...
>>
>> @@ -2650,12 +2801,34 @@ static int kcompactd(void *p)
>>   		unsigned long pflags;
>>   
>>   		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
>> -		wait_event_freezable(pgdat->kcompactd_wait,
>> -				kcompactd_work_requested(pgdat));
>> +		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
>> +			kcompactd_work_requested(pgdat),
>> +			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
>> +
>> +			psi_memstall_enter(&pflags);
>> +			kcompactd_do_work(pgdat);
>> +			psi_memstall_leave(&pflags);
>> +			continue;
>> +		}
>>   
>> -		psi_memstall_enter(&pflags);
>> -		kcompactd_do_work(pgdat);
>> -		psi_memstall_leave(&pflags);
>> +		/* kcompactd wait timeout */
>> +		if (should_proactive_compact_node(pgdat)) {
>> +			unsigned int prev_score, score;
> 
> Everywhere else, scores have type `int'.  Here they are unsigned.  How come?
> 
> Would it be better to make these unsigned throughout?  I don't think a
> score can ever be negative?
> 

The score is always in [0, 100], so yes, it should be unsigned.
I will send another patch which fixes this.

Thanks,
Nitin