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Date:   Tue, 8 Jun 2021 10:54:58 +0200
From:   Jan Kara <jack@suse.cz>
To:     Roman Gushchin <guro@fb.com>
Cc:     Jan Kara <jack@suse.cz>, 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>,
        Dennis Zhou <dennis@kernel.org>,
        Dave Chinner <dchinner@redhat.com>, cgroups@vger.kernel.org
Subject: Re: [PATCH v8 8/8] writeback, cgroup: release dying cgwbs by
 switching attached inodes
Message-ID: <20210608085458.GC5562@quack2.suse.cz>
References: <20210608013123.1088882-1-guro@fb.com>
 <20210608013123.1088882-9-guro@fb.com>
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On Mon 07-06-21 18:31:23, Roman Gushchin 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.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> Acked-by: Tejun Heo <tj@kernel.org>
> Acked-by: Dennis Zhou <dennis@kernel.org>

The patch looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Just one codingstyle nit below.

> +		if (!wb_tryget(wb))
> +			continue;
> +
> +		spin_unlock_irq(&cgwb_lock);
> +		while ((cleanup_offline_cgwb(wb)))
			^^ too many parentheses here...


> +			cond_resched();
> +		spin_lock_irq(&cgwb_lock);

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR