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SubjectRe: [PATCH v3 1/2] mm: compaction: support triggering of proactive compaction by user
Thanks Vlastimil !!

On 6/17/2021 8:07 PM, Vlastimil Babka wrote:
> On 6/17/21 9:30 AM, Charan Teja Kalla wrote:
>> Thanks Vlastimil for your inputs!!
>> On 6/16/2021 5:29 PM, Vlastimil Babka wrote:
>>>> This triggering of proactive compaction is done on a write to
>>>> sysctl.compaction_proactiveness by user.
>>>> [1]
>>>> Signed-off-by: Charan Teja Reddy <>
>>>> ---
>>>> changes in V2:
>>> You forgot to also summarize the changes. Please do in next version.
>> I think we can get rid off 'proactive_defer' thread variable with the
>> timeout approach you suggested. But it is still requires to have one
>> additional variable 'proactive_compact_trigger', which main purpose is
>> to decide if the kcompactd wakeup is for proactive compaction or not.
>> Please see below code:
>> if (wait_event_freezable_timeout() && !proactive_compact_trigger) {
>> // do the non-proactive work
>> continue
>> }
>> // do the proactive work
>> .................
>> Thus I feel that on writing new proactiveness, it is required to do
>> wakeup_kcomppactd() + set a flag that this wakeup is for proactive work.
>> Am I failed to get your point here?
> The check whether to do non-proactive work is already guarded by
> kcompactd_work_requested(), which looks at pgdat->kcompactd_max_order and this
> is set by wakeup_kcompactd().
> So with a plain wakeup where we don't set pgdat->kcompactd_max_order will make
> it consider proactive work instead and we don't need another trigger variable

The wait_event/freezable_timeout() documentation says that:
* Returns:
* 0 if the @condition evaluated to %false after the @timeout elapsed,
* 1 if the @condition evaluated to %true after the @timeout elapsed,
* or the remaining jiffies (at least 1) if the @condition evaluated
* to %true before the @timeout elapsed.

which means the condition must be evaluated to true or timeout should be
elapsed for the function wait_event_freezable_timeout() to return.

Please check the macro implementation of __wait_event, where it will be
in for(;;) till the condition is evaluated to true or timeout happens.
#define __wait_event_freezable_timeout(wq_head, condition, timeout)

___wait_event(wq_head, ___wait_cond_timeout(condition),


__ret = freezable_schedule_timeout(__ret))

Thus the plain wakeup of kcompactd don't do the proactive compact work.
And so we should identify its wakeup for proactive work with a separate

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