linux-kernel - Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events properly

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Date:   Tue, 2 Feb 2021 18:19:46 +0530
From:   Charan Teja Kalla <charante@...eaurora.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        vinmenon@...eaurora.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: page_alloc: update the COMPACT[STALL|FAIL] events
 properly

Thanks David for the review!!

On 2/2/2021 2:54 AM, David Rientjes wrote:
> On Mon, 1 Feb 2021, Charan Teja Reddy wrote:
> 
>> By defination, COMPACT[STALL|FAIL] events needs to be counted when there
> 
> s/defination/definition/\

Done.

> 
>> is 'At least in one zone compaction wasn't deferred or skipped from the
>> direct compaction'. And when compaction is skipped or deferred,
>> COMPACT_SKIPPED will be returned but it will still go and update these
>> compaction events which is wrong in the sense that COMPACT[STALL|FAIL]
>> is counted without even trying the compaction.
>>
>> Correct this by skipping the counting of these events when
>> COMPACT_SKIPPED is returned for compaction. This indirectly also avoid
>> the unnecessary try into the get_page_from_freelist() when compaction is
>> not even tried.
>>
>> Signed-off-by: Charan Teja Reddy <charante@...eaurora.org>
>> ---
>>  mm/page_alloc.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 519a60d..531f244 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -4152,6 +4152,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>>  	memalloc_noreclaim_restore(noreclaim_flag);
>>  	psi_memstall_leave(&pflags);
>>  
>> +	if (*compact_result == COMPACT_SKIPPED)
>> +		return NULL;
>>  	/*
>>  	 * At least in one zone compaction wasn't deferred or skipped, so let's
>>  	 * count a compaction stall
> 
> This makes sense, I wonder if it would also be useful to check that 
> page == NULL, either in try_to_compact_pages() or here for 
> COMPACT_SKIPPED?

In the code, when COMPACT_SKIPPED is being returned, the page will
always be NULL. So, I'm not sure how much useful it is for the page ==
NULL check here. Or I failed to understand your point here?

As, till now, COMPACTFAIL also presents page allocation failures because
of the direct compaction is skipped, creating a separate COMPACTSKIPFAIL
event which indicates that 'failed to get the free page as direct
compaction is skipped' is useful?
> 

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