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Subject: Re: [PATCH v3 1/2] mm: compaction: support triggering of proactive
 compaction by user
To:     Charan Teja Kalla <charante@codeaurora.org>,
        akpm@linux-foundation.org, nigupta@nvidia.com, hannes@cmpxchg.org,
        corbet@lwn.net, mcgrof@kernel.org, keescook@chromium.org,
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        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>
 <0ca491e8-6d3a-6537-dfa0-ece5f3bb6a1e@codeaurora.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <0d516cfa-f41c-5ccc-26aa-67871f23dcd3@suse.cz>
Date:   Thu, 17 Jun 2021 16:37:20 +0200
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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.