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Date:	Tue, 25 Sep 2012 15:57:51 +0800
From:	Zhi Yong Wu <zwu.kernel@...il.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org,
	linuxram@...ux.vnet.ibm.com, viro@...iv.linux.org.uk,
	cmm@...ibm.com, tytso@....edu, marco.stornelli@...il.com,
	stroetmann@...olinux.com, diegocg@...il.com, chris@...muel.org,
	Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
Subject: Re: [RFC v2 01/10] vfs: introduce private rb structures

On Tue, Sep 25, 2012 at 3:37 PM, Dave Chinner <david@...morbit.com> wrote:
> On Sun, Sep 23, 2012 at 08:56:26PM +0800, zwu.kernel@...il.com wrote:
>> From: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
>>
>>   One root structure hot_info is defined, is hooked
>> up in super_block, and will be used to hold rb trees
>> root, hash list root and some other information, etc.
>>   Adds hot_inode_tree struct to keep track of
>> frequently accessed files, and be keyed by {inode, offset}.
>> Trees contain hot_inode_items representing those files
>> and ranges.
>>   Having these trees means that vfs can quickly determine the
>> temperature of some data by doing some calculations on the
>> hot_freq_data struct that hangs off of the tree item.
>>   Define two items hot_inode_item and hot_range_item,
>> one of them represents one tracked file
>> to keep track of its access frequency and the tree of
>> ranges in this file, while the latter represents
>> a file range of one inode.
>>   Each of the two structures contains a hot_freq_data
>> struct with its frequency of access metrics (number of
>> {reads, writes}, last {read,write} time, frequency of
>> {reads,writes}).
>>   Also, each hot_inode_item contains one hot_range_tree
>> struct which is keyed by {inode, offset, length}
>> and used to keep track of all the ranges in this file.
>>
>> Signed-off-by: Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
>
> Just a coupl eof minor formatting things first up - I'll have more
> comments as I get deeper into the series.
All comments are very reasonable, and will be applied.
thanks for your review.

>
> ....
>> +/*
>> + * Initialize the inode tree. Should be called for each new inode
>> + * access or other user of the hot_inode interface.
>> + */
>> +static void hot_rb_inode_tree_init(struct hot_inode_tree *tree)
>
> The names of these are a bit clunky. You probably don't need the
> "_rb_" in the function name. i.e. hot_inode_tree_init() is
> sufficient, and if we every want to change in the tree type we don't
> have to rename every single function...
>
> .....
>> +/*
>> + * Initialize a new hot_inode_item structure. The new structure is
>> + * returned with a reference count of one and needs to be
>> + * freed using free_inode_item()
>> + */
>> +void hot_rb_inode_item_init(void *_item)
>> +{
>
> The usual naming convention for slab initialiser functions is to use
> a suffix of "_once" to indicate it is only ever called once per
> slab object instantiation, not every time the object is allocated
> fom the slab. See, for example, inode_init_once() and
> inode_init_always().
>
> so, that would make this function hot_inode_item_init_once().
>
> ....
>> +/* init hot_inode_item and hot_range_item kmem cache */
>> +static int __init hot_rb_item_cache_init(void)
>> +{
>> +     hot_inode_item_cache = kmem_cache_create("hot_inode_item",
>> +                     sizeof(struct hot_inode_item), 0,
>> +                     SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD,
>> +                     hot_rb_inode_item_init);
>> +     if (!hot_inode_item_cache)
>> +             goto inode_err;
>> +
>> +     hot_range_item_cache = kmem_cache_create("hot_range_item",
>> +                                     sizeof(struct hot_range_item), 0,
>> +                                     SLAB_RECLAIM_ACCOUNT | SLAB_MEM_SPREAD,
>> +                                     hot_rb_range_item_init);
>> +     if (!hot_range_item_cache)
>> +             goto range_err;
>> +
>> +     return 0;
>> +
>> +range_err:
>> +     kmem_cache_destroy(hot_inode_item_cache);
>> +inode_err:
>> +     return -ENOMEM;
>> +}
>> +
>> +/*
>> + * Initialize kmem cache for hot_inode_item
>> + * and hot_range_item
>> + */
>> +void __init hot_track_cache_init(void)
>> +{
>> +     if (hot_rb_item_cache_init())
>> +             return;
>
> No real need to have a hot_rb_item_cache_init() function here - just
> open code it all in the hot_track_cache_init() function.
>
>> +}
>> diff --git a/fs/hot_tracking.h b/fs/hot_tracking.h
>> new file mode 100644
>> index 0000000..269b67a
>> --- /dev/null
>> +++ b/fs/hot_tracking.h
>> @@ -0,0 +1,27 @@
>> +/*
>> + * fs/hot_tracking.h
>> + *
>> + * Copyright (C) 2012 IBM Corp. All rights reserved.
>> + * Written by Zhi Yong Wu <wuzhy@...ux.vnet.ibm.com>
>> + *            Ben Chociej <bchociej@...il.com>
>> + *
>> + * This program is free software; you can redistribute it and/or
>> + * modify it under the terms of the GNU General Public
>> + * License v2 as published by the Free Software Foundation.
>> + */
>> +
>> +#ifndef __HOT_TRACKING__
>> +#define __HOT_TRACKING__
>> +
>> +#include <linux/rbtree.h>
>> +#include <linux/hot_tracking.h>
>> +
>> +/* values for hot_freq_data flags */
>> +/* freq data struct is for an inode */
>> +#define FREQ_DATA_TYPE_INODE (1 << 0)
>> +/* freq data struct is for a range */
>> +#define FREQ_DATA_TYPE_RANGE (1 << 1)
>
> The comments are redundant - the name of the object documents it's
> use sufficiently.  ie.
>
> /* values for hot_freq_data flags */
> #define FREQ_DATA_TYPE_INODE (1 << 0)
> #define FREQ_DATA_TYPE_RANGE (1 << 1)
>
> is just fine by itself.
>
> ....
>> +/* A frequency data struct holds values that are used to
>> + * determine temperature of files and file ranges. These structs
>> + * are members of hot_inode_item and hot_range_item
>> + */
>
> /*
>  * This is a
>  * multiline comment. ;)
>  */
>
>> +struct hot_freq_data {
>> +     struct timespec last_read_time;
>> +     struct timespec last_write_time;
>> +     u32 nr_reads;
>> +     u32 nr_writes;
>> +     u64 avg_delta_reads;
>> +     u64 avg_delta_writes;
>> +     u8 flags;
>> +     u32 last_temperature;
>
> may as well make the flags a u32 - the compiler will ues that much
> space anyway as it aligned the u32 last_temperature variable after
> it.
>
>> +};
>> +
>> +/* An item representing an inode and its access frequency */
>> +struct hot_inode_item {
>> +     /* node for hot_inode_tree rb_tree */
>> +     struct rb_node rb_node;
>> +     /* tree of ranges in this inode */
>> +     struct hot_range_tree hot_range_tree;
>> +     /* frequency data for this inode */
>> +     struct hot_freq_data hot_freq_data;
>> +     /* inode number, copied from inode */
>> +     unsigned long i_ino;
>> +     /* used to check for errors in ref counting */
>> +     u8 in_tree;
>> +     /* protects hot_freq_data, i_no, in_tree */
>> +     spinlock_t lock;
>> +     /* prevents kfree */
>> +     struct kref refs;
>
> It's hard to see the code in the commentsi, and some of comments are
> redundant.. It's easier to read if you do this:
>
> struct hot_inode_item {
>         struct rb_node node;                    /* hot_inode_tree index */
>         struct hot_range_tree hot_range_tree;   /* tree of ranges */
>         struct hot_freq_data hot_freq_data;     /* frequency data */
>         unsigned long i_ino;                    /* inode number from inode */
>         u8 in_tree;                             /* ref counting check */
>         spinlock_t lock;                        /* protects object data */
>         struct kref refs;                       /* prevents kfree */
> }
>
> Also:
>         - i_ino really needs to be a 64 bit quantity as some
>           filesystems can use 64 bit inode numbers even on 32
>           bit systems (e.g. XFS).
>         - in_tree can be u32 or a flags field if it is boolean. if
>           it is just debug, then maybe it can be removed whenteh
>           code is ready for commit.
>
>> +};
>> +
>> +/*
>> + * An item representing a range inside of an inode whose frequency
>> + * is being tracked
>> + */
>> +struct hot_range_item {
>> +     /* node for hot_range_tree rb_tree */
>> +     struct rb_node rb_node;
>> +     /* frequency data for this range */
>> +     struct hot_freq_data hot_freq_data;
>> +     /* the hot_inode_item associated with this hot_range_item */
>> +     struct hot_inode_item *hot_inode;
>> +     /* starting offset of this range */
>> +     u64 start;
>> +     /* length of this range */
>> +     u64 len;
>
> What units?
>         u64 start;      /* start offset in bytes */
>         u64 len         /* length in bytes */
>
>> +     /* used to check for errors in ref counting */
>> +     u8 in_tree;
>> +     /* protects hot_freq_data, start, len, and in_tree */
>> +     spinlock_t lock;
>> +     /* prevents kfree */
>> +     struct kref refs;
>> +};
>> +
>> +struct hot_info {
>> +     /* red-black tree that keeps track of fs-wide hot data */
>> +     struct hot_inode_tree hot_inode_tree;
>> +};
>
> The comment is redundant...
>
> Cheers,
>
> Dave.
>
> --
> Dave Chinner
> david@...morbit.com



-- 
Regards,

Zhi Yong Wu
--
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