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Subject: Re: [PATCH v3 1/2] mm: compaction: support triggering of proactive
 compaction by user
To:     Vlastimil Babka <vbabka@suse.cz>, akpm@linux-foundation.org,
        nigupta@nvidia.com, hannes@cmpxchg.org, corbet@lwn.net,
        mcgrof@kernel.org, keescook@chromium.org, yzaikin@google.com,
        aarcange@redhat.com, cl@linux.com, xi.fengfei@h3c.com,
        mchehab+huawei@kernel.org, andrew.a.klychkov@gmail.com,
        dave.hansen@linux.intel.com, bhe@redhat.com,
        iamjoonsoo.kim@lge.com, mateusznosek0@gmail.com, sh_def@163.com,
        vinmenon@codeaurora.org
Cc:     linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org
References: <cover.1622454385.git.charante@codeaurora.org>
 <7db6a29a64b29d56cde46c713204428a4b95f0ab.1622454385.git.charante@codeaurora.org>
 <88abfdb6-2c13-b5a6-5b46-742d12d1c910@suse.cz>
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 <0d516cfa-f41c-5ccc-26aa-67871f23dcd3@suse.cz>
From:   Charan Teja Kalla <charante@codeaurora.org>
Message-ID: <8d91a81b-09f3-e814-c9ce-16ff246ed359@codeaurora.org>
Date:   Thu, 17 Jun 2021 21:35:52 +0530
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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]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=facdaa917c4d5a376d09d25865f5a863f906234a
>>>>
>>>> Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>
>>>> ---
>>>> 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
> AFAICS.

The wait_event/freezable_timeout() documentation says that:
 * Returns:
 * 0 if the @condition evaluated to %false after the @timeout elapsed,
			or
 * 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),

                      TASK_INTERRUPTIBLE, 0, timeout,

                      __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
flag.
> 

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