From mboxrd@z Thu Jan 1 00:00:00 1970 From: Quentin Monnet Subject: [RFC bpf-next v2 7/8] bpf: add documentation for eBPF helpers (51-57) Date: Wed, 11 Apr 2018 16:44:35 +0100 Message-ID: <7e388b10-ccea-a2b0-e776-5420c8e7f521@netronome.com> References: <20180410144157.4831-1-quentin.monnet@netronome.com> <20180410144157.4831-8-quentin.monnet@netronome.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: Content-Language: en-US Sender: netdev-owner@vger.kernel.org To: Yonghong Song , daniel@iogearbox.net, ast@kernel.org Cc: netdev@vger.kernel.org, oss-drivers@netronome.com, linux-doc@vger.kernel.org, linux-man@vger.kernel.org, Lawrence Brakmo , Josef Bacik , Andrey Ignatov List-Id: linux-man@vger.kernel.org 2018-04-10 09:58 UTC-0700 ~ Yonghong Song > On 4/10/18 7:41 AM, Quentin Monnet wrote: >> Add documentation for eBPF helper functions to bpf.h user header file. >> This documentation can be parsed with the Python script provided in >> another commit of the patch series, in order to provide a RST document >> that can later be converted into a man page. >> >> The objective is to make the documentation easily understandable and >> accessible to all eBPF developers, including beginners. >> >> This patch contains descriptions for the following helper functions: >> >> Helpers from Lawrence: >> - bpf_setsockopt() >> - bpf_getsockopt() >> - bpf_sock_ops_cb_flags_set() >> >> Helpers from Yonghong: >> - bpf_perf_event_read_value() >> - bpf_perf_prog_read_value() >> >> Helper from Josef: >> - bpf_override_return() >> >> Helper from Andrey: >> - bpf_bind() >> >> Cc: Lawrence Brakmo >> Cc: Yonghong Song >> Cc: Josef Bacik >> Cc: Andrey Ignatov >> Signed-off-by: Quentin Monnet >> --- >>   include/uapi/linux/bpf.h | 184 >> +++++++++++++++++++++++++++++++++++++++++++++++ >>   1 file changed, 184 insertions(+) >> >> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h >> index 15d9ccafebbe..7343af4196c8 100644 >> --- a/include/uapi/linux/bpf.h >> +++ b/include/uapi/linux/bpf.h [...] >> @@ -1255,6 +1277,168 @@ union bpf_attr { >>    *         performed again. >>    *     Return >>    *         0 on success, or a negative error in case of failure. >> + * >> + * int bpf_perf_event_read_value(struct bpf_map *map, u64 flags, >> struct bpf_perf_event_value *buf, u32 buf_size) >> + *     Description >> + *         Read the value of a perf event counter, and store it into >> *buf* >> + *         of size *buf_size*. This helper relies on a *map* of type >> + *         **BPF_MAP_TYPE_PERF_EVENT_ARRAY**. The nature of the perf >> + *         event counter is selected at the creation of the *map*. The > > The nature of the perf event counter is selected when *map* is updated > with perf_event fd's. > Thanks, I will fix it. >> + *         *map* is an array whose size is the number of available CPU >> + *         cores, and each cell contains a value relative to one >> core. The > > It is confusing to mix core/cpu here. Maybe just use perf_event > convention, always using cpu? > Right, I'll remove occurrences of "core". >> + *         value to retrieve is indicated by *flags*, that contains the >> + *         index of the core to look up, masked with >> + *         **BPF_F_INDEX_MASK**. Alternatively, *flags* can be set to >> + *         **BPF_F_CURRENT_CPU** to indicate that the value for the >> + *         current CPU core should be retrieved. >> + * >> + *         This helper behaves in a way close to >> + *         **bpf_perf_event_read**\ () helper, save that instead of >> + *         just returning the value observed, it fills the *buf* >> + *         structure. This allows for additional data to be >> retrieved: in >> + *         particular, the enabled and running times (in *buf*\ >> + *         **->enabled** and *buf*\ **->running**, respectively) are >> + *         copied. >> + * >> + *         These values are interesting, because hardware PMU >> (Performance >> + *         Monitoring Unit) counters are limited resources. When >> there are >> + *         more PMU based perf events opened than available counters, >> + *         kernel will multiplex these events so each event gets certain >> + *         percentage (but not all) of the PMU time. In case that >> + *         multiplexing happens, the number of samples or counter value >> + *         will not reflect the case compared to when no multiplexing >> + *         occurs. This makes comparison between different runs >> difficult. >> + *         Typically, the counter value should be normalized before >> + *         comparing to other experiments. The usual normalization is >> done >> + *         as follows. >> + * >> + *         :: >> + * >> + *             normalized_counter = counter * t_enabled / t_running >> + * >> + *         Where t_enabled is the time enabled for event and >> t_running is >> + *         the time running for event since last normalization. The >> + *         enabled and running times are accumulated since the perf >> event >> + *         open. To achieve scaling factor between two invocations of an >> + *         eBPF program, users can can use CPU id as the key (which is >> + *         typical for perf array usage model) to remember the previous >> + *         value and do the calculation inside the eBPF program. >> + *     Return >> + *         0 on success, or a negative error in case of failure. >> + * [...] Thanks Yonghong for the review! I have a favor to ask of you. I got a bounce for Kaixu Xia's email address, and I don't know what alternative email address I could use. I CC-ed to have a review for helper bpf_perf_event_read() (in patch 6 of this series), which is rather close to bpf_perf_event_read_value(). Would you mind having a look at that one too, please? The description is not long. Quentin