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Date:   Thu, 2 Apr 2020 11:18:52 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Chris Down <chris@chrisdown.name>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Tejun Heo <tj@kernel.org>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com
Subject: Re: [PATCH] mm, memcg: Bypass high reclaim iteration for cgroup
 hierarchy root
Message-ID: <20200402151852.GD2089@cmpxchg.org>
References: <20200312164137.GA1753625@chrisdown.name>
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On Thu, Mar 12, 2020 at 04:41:37PM +0000, Chris Down wrote:
> The root of the hierarchy cannot have high set, so we will never reclaim
> based on it. This makes that clearer and avoids another entry.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: linux-mm@kvack.org
> Cc: cgroups@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: kernel-team@fb.com

This makes sense, memory.high doesn't exist on the root.

And the mem_cgroup_is_root() check, a simple pointer comparison, is
cheaper than reading the page_counter atomic and memcg->high (which we
already know to be PAGE_COUNTER_MAX).

Acked-by: Johannes Weiner <hannes@cmpxchg.org>