From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 58D77C433EF for ; Mon, 18 Jul 2022 17:28:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235471AbiGRR2t (ORCPT ); Mon, 18 Jul 2022 13:28:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:47198 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233707AbiGRR2s (ORCPT ); Mon, 18 Jul 2022 13:28:48 -0400 Received: from mail-pj1-x1049.google.com (mail-pj1-x1049.google.com [IPv6:2607:f8b0:4864:20::1049]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id EE12B2CCAA for ; Mon, 18 Jul 2022 10:28:45 -0700 (PDT) Received: by mail-pj1-x1049.google.com with SMTP id q5-20020a17090a7a8500b001f0253f5aa3so7436454pjf.4 for ; Mon, 18 Jul 2022 10:28:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=date:in-reply-to:message-id:mime-version:references:subject:from:to :cc; bh=E9tNfVFMzHorelHMKNIX5eaOvDPoVcvcoBrPPHzeNe0=; b=dHNSKTMOmQK0wNLziMMI9cfBU5PiOV78Yzg6ACt3nLsUvYkSHk7P+20BV56C2AnIYi 1/Z8vt0HtSxt5DGbbtBMyOaP6E99WlaAuZVgHT2tNiz3zUArxBnp2QydhlezPz5Sa5wg aFZkmuYh3aE1Gj43QTJRDM4IR4eVZTqPn05KqDxIbwY1xVj+5oLYjQSi4h0uR3miXPBe 1wtr0CzO1ljkTyv+k4/My4LsE79BOvuNBfLPiL5jeIZiExPA5g059PCpNcf2IUOdpWG+ cD8jlGhfKvirgpJRIRkqw1lB4o+GkcfsQ16rfp0iaMOWNvNR5rvflQrnKB3iHw96LWcc 7f6Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc; bh=E9tNfVFMzHorelHMKNIX5eaOvDPoVcvcoBrPPHzeNe0=; b=ZoWUyTfD72gc3kZ5keE40f23xtpNhT4CEqwDPLd4r1q9HnF+kWbmTMDD9k5w636AMM jXu1lLXtlHpQUhnyUGFQz7YzN9htTnI8FwiRWJr1xrXDt+bT7GxCGlf/9J2TT5ipuZz6 mn7NSb4rQrqBaEk3deUxhcyBMHaXGntHrROXBfKDrfVoEExtHcYWyRFbTJQFkcN5XIqu otYYxBn12VnfB98ZPqHbyay+gJN5KWEwUtv55zDzfthS82FNHyuku5SFYMqIa3kE73I/ zQKS//ReXx6knAi5piPO/5uM/z3DwwYkEzj7P2E9ML2Yn/X2V74LCKbMKqCNi7ESkIU5 Dc1g== X-Gm-Message-State: AJIora8Un71n4OnVI8Q50TsDTC7DqMC6QSGimg0sC+pXbGYVMgfFV4CI 15x32M/6actbd7cqYZPF3JWeyXA= X-Google-Smtp-Source: AGRyM1ue+yJ+o+7jMt38bcvsdo6TUdmnFT8H3rOb4uGrBCpJ/uae2kJEAd3+o99qmwAUamADshpeZBU= X-Received: from sdf.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5935]) (user=sdf job=sendgmr) by 2002:a17:90b:4a83:b0:1ef:de4c:65f0 with SMTP id lp3-20020a17090b4a8300b001efde4c65f0mr39946627pjb.149.1658165325313; Mon, 18 Jul 2022 10:28:45 -0700 (PDT) Date: Mon, 18 Jul 2022 10:28:43 -0700 In-Reply-To: <20220718125847.1390-1-donald.hunter@gmail.com> Message-Id: Mime-Version: 1.0 References: <20220718125847.1390-1-donald.hunter@gmail.com> Subject: Re: [PATCH v3] bpf, docs: document BPF_MAP_TYPE_HASH and variants From: sdf@google.com To: Donald Hunter Cc: bpf@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet , yhs@fb.com Content-Type: text/plain; charset="UTF-8"; format=flowed; delsp=yes Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On 07/18, Donald Hunter wrote: > Add documentation for BPF_MAP_TYPE_HASH including kernel version > introduced, usage and examples. Document BPF_MAP_TYPE_PERCPU_HASH, > BPF_MAP_TYPE_LRU_HASH and BPF_MAP_TYPE_LRU_PERCPU_HASH variations. > Note that this file is included in the BPF documentation by the glob in > Documentation/bpf/maps.rst > v3: > Fix typos reported by Stanislav Fomichev and Yonghong Song. > Add note about iteration and deletion as requested by Yonghong Song. > v2: > Describe memory allocation semantics as suggested by Stanislav Fomichev. > Fix u64 typo reported by Stanislav Fomichev. > Cut down usage examples to only show usage in context. > Updated patch description to follow style recommendation, reported by > Bagas Sanjaya. > Signed-off-by: Donald Hunter Reviewed-by: Stanislav Fomichev In the future, can you try to add bpf/bpf-next subtree into the subj? This email should have been [PATCH bpf-next v3]. > --- > Documentation/bpf/map_hash.rst | 186 +++++++++++++++++++++++++++++++++ > 1 file changed, 186 insertions(+) > create mode 100644 Documentation/bpf/map_hash.rst > diff --git a/Documentation/bpf/map_hash.rst > b/Documentation/bpf/map_hash.rst > new file mode 100644 > index 000000000000..b1d6562f36ae > --- /dev/null > +++ b/Documentation/bpf/map_hash.rst > @@ -0,0 +1,186 @@ > +.. SPDX-License-Identifier: GPL-2.0-only > +.. Copyright (C) 2022 Red Hat, Inc. > + > +=============================================== > +BPF_MAP_TYPE_HASH, with PERCPU and LRU Variants > +=============================================== > + > +.. note:: > + - ``BPF_MAP_TYPE_HASH`` was introduced in kernel version 3.19 > + - ``BPF_MAP_TYPE_PERCPU_HASH`` was introduced in version 4.6 > + - Both ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` > + were introduced in version 4.10 > + > +``BPF_MAP_TYPE_HASH`` and ``BPF_MAP_TYPE_PERCPU_HASH`` provide general > +purpose hash map storage. Both the key and the value can be structs, > +allowing for composite keys and values. > + > +The kernel is responsible for allocating and freeing key/value pairs, up > +to the max_entries limit that you specify. Hash maps use pre-allocation > +of hash table elements by default. The ``BPF_F_NO_PREALLOC`` flag can be > +used to disable pre-allocation when it is too memory expensive. > + > +``BPF_MAP_TYPE_PERCPU_HASH`` provides a separate value slot per > +CPU. The per-cpu values are stored internally in an array. > + > +The ``BPF_MAP_TYPE_LRU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` > +variants add LRU semantics to their respective hash tables. An LRU hash > +will automatically evict the least recently used entries when the hash > +table reaches capacity. An LRU hash maintains an internal LRU list that > +is used to select elements for eviction. This internal LRU list is > +shared across CPUs but it is possible to request a per CPU LRU list with > +the ``BPF_F_NO_COMMON_LRU`` flag when calling ``bpf_map_create``. > + > +Usage > +===== > + > +.. c:function:: > + long bpf_map_update_elem(struct bpf_map *map, const void *key, const > void *value, u64 flags) > + > +Hash entries can be added or updated using the ``bpf_map_update_elem()`` > +helper. This helper replaces existing elements atomically. The ``flags`` > +parameter can be used to control the update behaviour: > + > +- ``BPF_ANY`` will create a new element or update an existing element > +- ``BPF_NOTEXIST`` will create a new element only if one did not already > + exist > +- ``BPF_EXIST`` will update an existing element > + > +``bpf_map_update_elem()`` returns 0 on success, or negative error in > +case of failure. > + > +.. c:function:: > + void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) > + > +Hash entries can be retrieved using the ``bpf_map_lookup_elem()`` > +helper. This helper returns a pointer to the value associated with > +``key``, or ``NULL`` if no entry was found. > + > +.. c:function:: > + long bpf_map_delete_elem(struct bpf_map *map, const void *key) > + > +Hash entries can be deleted using the ``bpf_map_delete_elem()`` > +helper. This helper will return 0 on success, or negative error in case > +of failure. > + > +Per CPU Hashes > +-------------- > + > +For ``BPF_MAP_TYPE_PERCPU_HASH`` and ``BPF_MAP_TYPE_LRU_PERCPU_HASH`` > +the ``bpf_map_update_elem()`` and ``bpf_map_lookup_elem()`` helpers > +automatically access the hash slot for the current CPU. > + > +.. c:function:: > + void *bpf_map_lookup_percpu_elem(struct bpf_map *map, const void > *key, u32 cpu) > + > +The ``bpf_map_lookup_percpu_elem()`` helper can be used to lookup the > +value in the hash slot for a specific CPU. Returns value associated with > +``key`` on ``cpu`` , or ``NULL`` if no entry was found or ``cpu`` is > +invalid. > + > +Concurrency > +----------- > + > +Values stored in ``BPF_MAP_TYPE_HASH`` can be accessed concurrently by > +programs running on different CPUs. Since Kernel version 5.1, the BPF > +infrastructure provides ``struct bpf_spin_lock`` to synchronise access. > +See ``tools/testing/selftests/bpf/progs/test_spin_lock.c``. > + > +Userspace > +--------- > + > +.. c:function:: > + int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key) > + > +In userspace, it is possible to iterate through the keys of a hash using > +the ``bpf_map_get_next_key()`` function. The first key can be fetched by > +calling ``bpf_map_get_next_key()`` with ``cur_key`` set to > +``NULL``. Subsequent calls will fetch the next key that follows the > +current key. ``bpf_map_get_next_key()`` returns 0 on success, -ENOENT if > +cur_key is the last key in the hash, or negative error in case of > +failure. > + > +Note that if ``cur_key`` gets deleted then ``bpf_map_get_next_key()`` > +will instead return the *first* key in the hash table which is > +undesirable. It is recommended to use batched lookup if there is going > +to be key deletion intermixed with ``bpf_map_get_next_key()``. > + > +Examples > +======== > + > +Please see the ``tools/testing/selftests/bpf`` directory for functional > +examples. The code snippets below demonstrates API usage. > + > +This example shows how to declare an LRU Hash with a struct key and a > +struct value. > + > +.. code-block:: c > + > + #include > + #include > + > + struct key { > + __u32 srcip; > + }; > + > + struct value { > + __u64 packets; > + __u64 bytes; > + }; > + > + struct { > + __uint(type, BPF_MAP_TYPE_LRU_HASH); > + __uint(max_entries, 32); > + __type(key, struct key); > + __type(value, struct value); > + } packet_stats SEC(".maps"); > + > +This example shows how to create or update hash values using atomic > +instructions: > + > +.. code-block:: c > + > + static inline void (__u32 srcip, int bytes) > + { > + struct key key = { > + .srcip = srcip > + }; > + struct value *value = bpf_map_lookup_elem(&packet_stats, > &key); > + if (value) { > + __sync_fetch_and_add(&value->packets, 1); > + __sync_fetch_and_add(&value->bytes, bytes); > + } else { > + struct value newval = { 1, bytes }; > + bpf_map_update_elem(&packet_stats, &key, &newval, > BPF_NOEXIST); > + } > + } > + > +Userspace walking the map elements from the map declared above: > + > +.. code-block:: c > + > + #include > + #include > + > + static void walk_hash_elements(int map_fd) > + { > + struct key *cur_key = NULL; > + struct key next_key; > + int next; > + do { > + // error checking omitted > + next = bpf_map_get_next_key(stats_fd, cur_key, > &next_key); > + if (next == -ENOENT) > + break; > + > + struct in_addr src_addr = { > + .s_addr = next_key.srcip > + }; > + struct value value; > + int ret = bpf_map_lookup_elem(stats_fd, &next_key, > &value); > + > + // Use key and value here > + > + cur_key = &next_key; > + } while (next == 0); > + } > -- > 2.35.1