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Subject: Re: [PATCH resend] mm: compaction: optimize proactive compaction
 deferrals
To:     Andrew Morton <akpm@linux-foundation.org>,
        Charan Teja Reddy <charante@codeaurora.org>
Cc:     rientjes@google.com, nigupta@nvidia.com, khalid.aziz@oracle.com,
        vinmenon@codeaurora.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <1626869599-25412-1-git-send-email-charante@codeaurora.org>
 <20210721131806.5898dab3e329940fd8bd2db6@linux-foundation.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <a693d1b0-e249-cca5-c910-5fbfd8f1a11c@suse.cz>
Date:   Wed, 21 Jul 2021 23:29:02 +0200
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On 7/21/21 10:18 PM, Andrew Morton wrote:
> On Wed, 21 Jul 2021 17:43:19 +0530 Charan Teja Reddy <charante@codeaurora.org> wrote:
> 
>> Vlastimil Babka figured out that when fragmentation score didn't go down
>> across the proactive compaction i.e. when no progress is made, next wake
>> up for proactive compaction is deferred for 1 <<
>> COMPACT_MAX_DEFER_SHIFT, i.e. 64 times, with each wakeup interval of
>> HPAGE_FRAG_CHECK_INTERVAL_MSEC(=500). In each of this wakeup, it just
>> decrement 'proactive_defer' counter and goes sleep i.e. it is getting
>> woken to just decrement a counter. The same deferral time can also
>> achieved by simply doing the HPAGE_FRAG_CHECK_INTERVAL_MSEC <<
>> COMPACT_MAX_DEFER_SHIFT thus unnecessary wakeup of kcompact thread is
>> avoided thus also removes the need of 'proactive_defer' thread counter.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

>>
>> @@ -2902,23 +2903,30 @@ static int kcompactd(void *p)
>>  
>>  		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
>>  		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
>> -			kcompactd_work_requested(pgdat),
>> -			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {
>> +			kcompactd_work_requested(pgdat), timeout)) {
>>  
>>  			psi_memstall_enter(&pflags);
>>  			kcompactd_do_work(pgdat);
>>  			psi_memstall_leave(&pflags);
>> +			/*
>> +			 * Reset the timeout value. The defer timeout by
>> +			 * proactive compaction can effectively lost
>> +			 * here but that is fine as the condition of the
>> +			 * zone changed substantionally and carrying on
>> +			 * with the previous defer is not useful.
>> +			 */
>> +			timeout = default_timeout;
>>  			continue;
> 
> I find this comment hard to follow.  Is this better?

Yes, thanks.

> --- a/mm/compaction.c~mm-compaction-optimize-proactive-compaction-deferrals-fix
> +++ a/mm/compaction.c
> @@ -2909,11 +2909,11 @@ static int kcompactd(void *p)
>  			kcompactd_do_work(pgdat);
>  			psi_memstall_leave(&pflags);
>  			/*
> -			 * Reset the timeout value. The defer timeout by
> -			 * proactive compaction can effectively lost
> -			 * here but that is fine as the condition of the
> -			 * zone changed substantionally and carrying on
> -			 * with the previous defer is not useful.
> +			 * Reset the timeout value. The defer timeout from
> +			 * proactive compaction is lost here but that is fine
> +			 * as the condition of the zone changing substantionally
> +			 * then carrying on with the previous defer interval is
> +			 * not useful.
>  			 */
>  			timeout = default_timeout;
>  			continue;
> _
>