Date:   Fri, 4 Nov 2022 16:31:02 +0700
From:   Bagas Sanjaya <bagasdotme@...il.com>
To:     Joe Stringer <joe@...valent.com>, bpf@...r.kernel.org
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        ast@...nel.org, corbet@....net, martin.lau@...ux.dev
Subject: Re: [PATCH bpf-next v2] docs/bpf: Add LRU internals description and
 graph

On 11/4/22 03:50, Joe Stringer wrote:
> +An LRU hashmap type consists of two properties: Firstly, it is a hash map and
> +hence is indexable by key for constant time lookups. Secondly, when at map
> +capacity, map updates will trigger eviction of old entries based on the age of
> +the elements in a set of lists. Each of these properties may be either global
> +or per-CPU, depending on the map type and flags used to create the map:
> +
> +.. flat-table:: Comparison of map properties by map type (x-axis) and flags
> +   (y-axis)
> +
> +   * -
> +     - ``BPF_MAP_TYPE_LRU_HASH``
> +     - ``BPF_MAP_TYPE_LRU_PERCPU_HASH``
> +
> +   * - ``BPF_NO_COMMON_LRU``
> +     - Per-CPU LRU, global map
> +     - Per-CPU LRU, per-cpu map
> +
> +   * - ``!BPF_NO_COMMON_LRU``
> +     - Global LRU, global map
> +     - Global LRU, per-cpu map
> +

Shouldn't the table be written in reST table syntax instead?

> +The commit message for LRU map support provides a general overview of the
> +underlying LRU algorithm used for entry eviction when the table is full:
> +
> +::
> +
> +    commit 3a08c2fd763450a927d1130de078d6f9e74944fb
> +    Author: Martin KaFai Lau <kafai@...com>
> +    Date:   Fri Nov 11 10:55:06 2016 -0800
> +
> +        bpf: LRU List
> +
> +        Introduce bpf_lru_list which will provide LRU capability to
> +        the bpf_htab in the later patch.
> +
> +        * General Thoughts:
> +        1. Target use case.  Read is more often than update.
> +           (i.e. bpf_lookup_elem() is more often than bpf_update_elem()).
> +           If bpf_prog does a bpf_lookup_elem() first and then an in-place
> +           update, it still counts as a read operation to the LRU list concern.
> +        2. It may be useful to think of it as a LRU cache
> +        3. Optimize the read case
> +           3.1 No lock in read case
> +           3.2 The LRU maintenance is only done during bpf_update_elem()
> +        4. If there is a percpu LRU list, it will lose the system-wise LRU
> +           property.  A completely isolated percpu LRU list has the best
> +           performance but the memory utilization is not ideal considering
> +           the work load may be imbalance.
> +        5. Hence, this patch starts the LRU implementation with a global LRU
> +           list with batched operations before accessing the global LRU list.
> +           As a LRU cache, #read >> #update/#insert operations, it will work well.
> +        6. There is a local list (for each cpu) which is named
> +           'struct bpf_lru_locallist'.  This local list is not used to sort
> +           the LRU property.  Instead, the local list is to batch enough
> +           operations before acquiring the lock of the global LRU list.  More
> +           details on this later.
> +        7. In the later patch, it allows a percpu LRU list by specifying a
> +           map-attribute for scalability reason and for use cases that need to
> +           prepare for the worst (and pathological) case like DoS attack.
> +           The percpu LRU list is completely isolated from each other and the
> +           LRU nodes (including free nodes) cannot be moved across the list.  The
> +           following description is for the global LRU list but mostly applicable
> +           to the percpu LRU list also.
> +
> +        * Global LRU List:
> +        1. It has three sub-lists: active-list, inactive-list and free-list.
> +        2. The two list idea, active and inactive, is borrowed from the
> +           page cache.
> +        3. All nodes are pre-allocated and all sit at the free-list (of the
> +           global LRU list) at the beginning.  The pre-allocation reasoning
> +           is similar to the existing BPF_MAP_TYPE_HASH.  However,
> +           opting-out prealloc (BPF_F_NO_PREALLOC) is not supported in
> +           the LRU map.
> +
> +        * Active/Inactive List (of the global LRU list):
> +        1. The active list, as its name says it, maintains the active set of
> +           the nodes.  We can think of it as the working set or more frequently
> +           accessed nodes.  The access frequency is approximated by a ref-bit.
> +           The ref-bit is set during the bpf_lookup_elem().
> +        2. The inactive list, as its name also says it, maintains a less
> +           active set of nodes.  They are the candidates to be removed
> +           from the bpf_htab when we are running out of free nodes.
> +        3. The ordering of these two lists is acting as a rough clock.
> +           The tail of the inactive list is the older nodes and
> +           should be released first if the bpf_htab needs free element.
> +
> +        * Rotating the Active/Inactive List (of the global LRU list):
> +        1. It is the basic operation to maintain the LRU property of
> +           the global list.
> +        2. The active list is only rotated when the inactive list is running
> +           low.  This idea is similar to the current page cache.
> +           Inactive running low is currently defined as
> +           "# of inactive < # of active".
> +        3. The active list rotation always starts from the tail.  It moves
> +           node without ref-bit set to the head of the inactive list.
> +           It moves node with ref-bit set back to the head of the active
> +           list and then clears its ref-bit.
> +        4. The inactive rotation is pretty simply.
> +           It walks the inactive list and moves the nodes back to the head of
> +           active list if its ref-bit is set. The ref-bit is cleared after moving
> +           to the active list.
> +           If the node does not have ref-bit set, it just leave it as it is
> +           because it is already in the inactive list.
> +
> +        * Shrinking the Inactive List (of the global LRU list):
> +        1. Shrinking is the operation to get free nodes when the bpf_htab is
> +           full.
> +        2. It usually only shrinks the inactive list to get free nodes.
> +        3. During shrinking, it will walk the inactive list from the tail,
> +           delete the nodes without ref-bit set from bpf_htab.
> +        4. If no free node found after step (3), it will forcefully get
> +           one node from the tail of inactive or active list.  Forcefully is
> +           in the sense that it ignores the ref-bit.
> +
> +        * Local List:
> +        1. Each CPU has a 'struct bpf_lru_locallist'.  The purpose is to
> +           batch enough operations before acquiring the lock of the
> +           global LRU.
> +        2. A local list has two sub-lists, free-list and pending-list.
> +        3. During bpf_update_elem(), it will try to get from the free-list
> +           of (the current CPU local list).
> +        4. If the local free-list is empty, it will acquire from the
> +           global LRU list.  The global LRU list can either satisfy it
> +           by its global free-list or by shrinking the global inactive
> +           list.  Since we have acquired the global LRU list lock,
> +           it will try to get at most LOCAL_FREE_TARGET elements
> +           to the local free list.
> +        5. When a new element is added to the bpf_htab, it will
> +           first sit at the pending-list (of the local list) first.
> +           The pending-list will be flushed to the global LRU list
> +           when it needs to acquire free nodes from the global list
> +           next time.
> +
> +        * Lock Consideration:
> +        The LRU list has a lock (lru_lock).  Each bucket of htab has a
> +        lock (buck_lock).  If both locks need to be acquired together,
> +        the lock order is always lru_lock -> buck_lock and this only
> +        happens in the bpf_lru_list.c logic.
> +
> +        In hashtab.c, both locks are not acquired together (i.e. one
> +        lock is always released first before acquiring another lock).
> +
> +        Signed-off-by: Martin KaFai Lau <kafai@...com>
> +        Acked-by: Alexei Starovoitov <ast@...nel.org>
> +        Signed-off-by: David S. Miller <davem@...emloft.net>
> +

What about just writing the pointer ("See commit 3a08c2fd7634 ("bpf: LRU List")")
instead?

> +Notably, there are various steps that the update algorithm attempts in order to
> +enforce the LRU property which have increasing impacts on other CPUs involved
> +in the operations:
> +
> +- Attempt to use CPU-local state to batch operations
> +- Attempt to fetch free nodes from global lists
> +- Attempt to pull any node from a global list and remove it from the hashmap
> +- Attempt to pull any node from any CPU's list and remove it from the hashmap
> +

Better say "... other CPUs involved in the following operation attempts:"

> +Even if an LRU node may be acquired, maps of type ``BPF_MAP_TYPE_LRU_HASH``
> +may fail to insert the entry into the map if other CPUs are heavily contending
> +on the global hashmap lock.
> +
> +This algorithm is described visually in the following diagram:
> +
> +.. kernel-figure::  map_lru_hash_update.dot
> +   :alt:    Diagram outlining the LRU eviction steps taken during map update
> +
> +   LRU hash eviction during map update for ``BPF_MAP_TYPE_LRU_HASH`` and
> +   variants
> +
<snipped>...
> +
> +The dot file source for the above diagram is uses internal kernel function
> +names for the node names in order to make the corresponding logic easier to
> +find. See ``Documentation/bpf/map_lru_hash_update.dot`` for more details.

Since it references the same figure, just say "See the figure above for more
details".

Otherwise LGTM, thanks.

-- 
An old man doll... just what I always wanted! - Clara

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