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

[Vlastimil Babka]

On 7/30/21 3:53 PM, Charan Teja Reddy 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@xxxxxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> @@ -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?

>  		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
>  		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
> -			kcompactd_work_requested(pgdat), timeout)) {
> +			kcompactd_work_requested(pgdat), timeout) &&
> +			!pgdat->proactive_compact_trigger) {
>  
>  			psi_memstall_enter(&pflags);
>  			kcompactd_do_work(pgdat);
> @@ -2932,6 +2964,8 @@ static int kcompactd(void *p)
>  				timeout =
>  				   default_timeout << COMPACT_MAX_DEFER_SHIFT;
>  		}
> +		if (unlikely(pgdat->proactive_compact_trigger))
> +			pgdat->proactive_compact_trigger = false;
>  	}
>  
>  	return 0;
>