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Subject: Re: [PATCH V5] mm: compaction: support triggering of proactive
 compaction by user
To:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        dave.hansen@linux.intel.com, mgorman@techsingularity.net,
        nigupta@nvidia.com, corbet@lwn.net, rppt@kernel.org,
        khalid.aziz@oracle.com, rientjes@google.com
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        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        vinmenon@codeaurora.org
References: <1627653207-12317-1-git-send-email-charante@codeaurora.org>
 <8fe4ba65-28e1-02d8-cf4d-74aaa76fe9df@suse.cz>
From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <690ffed8-9c2a-1a9e-e592-a103b09e05a7@codeaurora.org>
Date:   Fri, 30 Jul 2021 20:16:09 +0530
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Thanks Vlastimil!!

On 7/30/2021 7:36 PM, Vlastimil Babka wrote:
>> The proactive compaction[1] gets triggered for every 500msec and run
>> compaction on the node for COMPACTION_HPAGE_ORDER (usually order-9)
>> pages based on the value set to sysctl.compaction_proactiveness.
>> Triggering the compaction for every 500msec in search of
>> COMPACTION_HPAGE_ORDER pages is not needed for all applications,
>> especially on the embedded system usecases which may have few MB's of
>> RAM. Enabling the proactive compaction in its state will endup in
>> running almost always on such systems.
>>
>> Other side, proactive compaction can still be very much useful for
>> getting a set of higher order pages in some controllable
>> manner(controlled by using the sysctl.compaction_proactiveness). So, on
>> systems where enabling the proactive compaction always may proove not
>> required, can trigger the same from user space on write to its sysctl
>> interface. As an example, say app launcher decide to launch the memory
>> heavy application which can be launched fast if it gets more higher
>> order pages thus launcher can prepare the system in advance by
>> triggering the proactive compaction from userspace.
>>
>> This triggering of proactive compaction is done on a write to
>> sysctl.compaction_proactiveness by user.
>>
>> [1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=facdaa917c4d5a376d09d25865f5a863f906234a
>>
>> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Thanks for the tag here.

> 
>> @@ -2895,9 +2920,16 @@ static int kcompactd(void *p)
>>  	while (!kthread_should_stop()) {
>>  		unsigned long pflags;
>>  
>> +		/*
>> +		 * Avoid the unnecessary wakeup for proactive compaction
>> +		 * when it is disabled.
>> +		 */
>> +		if (!sysctl_compaction_proactiveness)
>> +			timeout = MAX_SCHEDULE_TIMEOUT;
> Does this part actually logically belong more to your previous patch that
> optimized the deferred timeouts?

IMO, it won't fit there. Reason is that when user writes
sysctl_compaction_proactiveness = 0, it will goes to sleep with
MAX_SCHEDULE_TIMEOUT. Say now user writes non-zero value to
sysctl_compaction_proactiveness then no condition is there to wake it up
for proactive compaction, means, it will still be in sleep with
MAX_SCHEDULE_TIMEOUT.

Thus this logic is put in this patch, where, proactive compaction work
will be scheduled immediately on switch of proactiveness value from zero
to a non-zero.

> 

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