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From:   SeongJae Park <sj@kernel.org>
To:     Xin Hao <xhao@linux.alibaba.com>
Cc:     sjpark@amazon.de, akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] mm/damon/core: Optimize kdamod.%d thread creation code
Date:   Tue, 19 Oct 2021 10:03:36 +0000
Message-Id: <20211019100336.5861-1-sj@kernel.org>
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Hello Xin,

On Sun, 17 Oct 2021 00:59:14 +0800 Xin Hao <xhao@linux.alibaba.com> wrote:

> When the ctx->adaptive_targets list is empty,
> i did some test on monitor_on interface like this.
> 
> echo on > /sys/kernel/debug/damon/monitor_on
> [  851.988307] damon: kdamond (5390) starts
> 
> Though the ctx->adaptive_targets list is empty, but the
> kthread_run still be called, and the kdamond.x thread still
> be created, this is meaningless.

Good finding, thank you!

> 
> So there adds a judgment. only if the ctx->adaptive_targets
> list is not empty, and ctx->kdamond pointer is NULL, then call
> the __damon_start function.

I think we cannot believe user-space users, but kernel-space API users.  How
about adding the check in 'dbgfs_monitor_on_write()' rather than
'__damon_start()'?

Also, apparently this patch is based on latest 'damon/next' branch of my tree.
Could you please rebase this on latest -mm tree?


Thanks,
SJ

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