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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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        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>
 <8d91a81b-09f3-e814-c9ce-16ff246ed359@codeaurora.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <546f7169-5a9e-fe16-bd83-b0f7a338c3cf@suse.cz>
Date:   Thu, 17 Jun 2021 18:17:25 +0200
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On 6/17/21 6:05 PM, Charan Teja Kalla wrote:
> 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.

OK, you're right, I forgot that the macro has the for loop to guard against
spurious wakeups.