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Date:   Tue, 8 Jun 2021 17:12:37 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Tejun Heo <tj@kernel.org>, <linux-fsdevel@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Jan Kara <jack@suse.cz>, Dennis Zhou <dennis@kernel.org>,
        Dave Chinner <dchinner@redhat.com>, <cgroups@vger.kernel.org>
Subject: Re: [PATCH v9 8/8] writeback, cgroup: release dying cgwbs by
 switching attached inodes
Message-Id: <20210608171237.be2f4223de89458841c10fd4@linux-foundation.org>
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        <20210608230225.2078447-9-guro@fb.com>
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On Tue, 8 Jun 2021 16:02:25 -0700 Roman Gushchin <guro@fb.com> wrote:

> Asynchronously try to release dying cgwbs by switching attached inodes
> to the nearest living ancestor wb. It helps to get rid of per-cgroup
> writeback structures themselves and of pinned memory and block cgroups,
> which are significantly larger structures (mostly due to large per-cpu
> statistics data). This prevents memory waste and helps to avoid
> different scalability problems caused by large piles of dying cgroups.
> 
> Reuse the existing mechanism of inode switching used for foreign inode
> detection. To speed things up batch up to 115 inode switching in a
> single operation (the maximum number is selected so that the resulting
> struct inode_switch_wbs_context can fit into 1024 bytes). Because
> every switching consists of two steps divided by an RCU grace period,
> it would be too slow without batching. Please note that the whole
> batch counts as a single operation (when increasing/decreasing
> isw_nr_in_flight). This allows to keep umounting working (flush the
> switching queue), however prevents cleanups from consuming the whole
> switching quota and effectively blocking the frn switching.
> 
> A cgwb cleanup operation can fail due to different reasons (e.g. not
> enough memory, the cgwb has an in-flight/pending io, an attached inode
> in a wrong state, etc). In this case the next scheduled cleanup will
> make a new attempt. An attempt is made each time a new cgwb is offlined
> (in other words a memcg and/or a blkcg is deleted by a user). In the
> future an additional attempt scheduled by a timer can be implemented.
> 
> ...
>
> +/*
> + * Maximum inodes per isw.  A specific value has been chosen to make
> + * struct inode_switch_wbs_context fit into 1024 bytes kmalloc.
> + */
> +#define WB_MAX_INODES_PER_ISW	115

Can't we do 1024/sizeof(struct inode_switch_wbs_context)?