linux-kernel - Re: [PATCH V5] mm: compaction: support triggering of proactive compaction by user

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Date:   Fri, 30 Jul 2021 16:47:44 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Charan Teja Kalla <charante@...eaurora.org>,
        akpm@...ux-foundation.org, mcgrof@...nel.org,
        keescook@...omium.org, yzaikin@...gle.com,
        dave.hansen@...ux.intel.com, mgorman@...hsingularity.net,
        nigupta@...dia.com, corbet@....net, rppt@...nel.org,
        khalid.aziz@...cle.com, rientjes@...gle.com
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        vinmenon@...eaurora.org
Subject: Re: [PATCH V5] mm: compaction: support triggering of proactive
 compaction by user

On 7/30/21 4:46 PM, Charan Teja Kalla wrote:
> 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@...eaurora.org>
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 
> Thanks for the tag here.

Np.

>> 
>>> @@ -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.

Good point!

> 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.

Agreed. Thanks!

>> 
>