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https://github.com/eunomia-bpf/bpf-developer-tutorial.git
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121 lines
3.2 KiB
Markdown
121 lines
3.2 KiB
Markdown
## eBPF 入门实践教程：编写 eBPF 程序 Biolatency: 统计系统中发生的I/O事件
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### 背景
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Biolatency 可以统计在该工具运行后系统中发生的I/O事件个数，并且计算I/O事件在不同时间段内的分布情况，以
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直方图的形式展现给用户。
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### 实现原理
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Biolatency 主要通过 tracepoint 实现，其在 block_rq_insert, block_rq_issue,
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block_rq_complete 挂载点下设置了处理函数。在 block_rq_insert 和 block_rq_issue 挂载点下，
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Biolatency 会将IO操作发生时的request queue和时间计入map中。
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```c
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int trace_rq_start(struct request *rq, int issue)
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{
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if (issue && targ_queued && BPF_CORE_READ(rq->q, elevator))
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return 0;
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u64 ts = bpf_ktime_get_ns();
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if (filter_dev) {
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struct gendisk *disk = get_disk(rq);
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u32 dev;
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dev = disk ? MKDEV(BPF_CORE_READ(disk, major),
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BPF_CORE_READ(disk, first_minor)) : 0;
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if (targ_dev != dev)
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return 0;
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}
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bpf_map_update_elem(&start, &rq, &ts, 0);
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return 0;
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}
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SEC("tp_btf/block_rq_insert")
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int block_rq_insert(u64 *ctx)
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{
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if (filter_cg && !bpf_current_task_under_cgroup(&cgroup_map, 0))
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return 0;
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if (LINUX_KERNEL_VERSION < KERNEL_VERSION(5, 11, 0))
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return trace_rq_start((void *)ctx[1], false);
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else
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return trace_rq_start((void *)ctx[0], false);
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}
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SEC("tp_btf/block_rq_issue")
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int block_rq_issue(u64 *ctx)
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{
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if (filter_cg && !bpf_current_task_under_cgroup(&cgroup_map, 0))
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return 0;
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if (LINUX_KERNEL_VERSION < KERNEL_VERSION(5, 11, 0))
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return trace_rq_start((void *)ctx[1], true);
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else
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return trace_rq_start((void *)ctx[0], true);
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}
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```
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在block_rq_complete 挂载点下，Biolatency 会根据 request queue 从map中读取
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上一次操作发生的时间，然后计算与当前时间的差值来判断其在直方图中存在的区域，将该区域内的IO操作
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计数加一。
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```c
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SEC("tp_btf/block_rq_complete")
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int BPF_PROG(block_rq_complete, struct request *rq, int error,
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unsigned int nr_bytes)
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{
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if (filter_cg && !bpf_current_task_under_cgroup(&cgroup_map, 0))
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return 0;
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u64 slot, *tsp, ts = bpf_ktime_get_ns();
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struct hist_key hkey = {};
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struct hist *histp;
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s64 delta;
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tsp = bpf_map_lookup_elem(&start, &rq);
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if (!tsp)
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return 0;
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delta = (s64)(ts - *tsp);
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if (delta < 0)
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goto cleanup;
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if (targ_per_disk) {
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struct gendisk *disk = get_disk(rq);
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hkey.dev = disk ? MKDEV(BPF_CORE_READ(disk, major),
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BPF_CORE_READ(disk, first_minor)) : 0;
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}
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if (targ_per_flag)
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hkey.cmd_flags = rq->cmd_flags;
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histp = bpf_map_lookup_elem(&hists, &hkey);
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if (!histp) {
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bpf_map_update_elem(&hists, &hkey, &initial_hist, 0);
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histp = bpf_map_lookup_elem(&hists, &hkey);
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if (!histp)
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goto cleanup;
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}
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if (targ_ms)
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delta /= 1000000U;
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else
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delta /= 1000U;
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slot = log2l(delta);
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if (slot >= MAX_SLOTS)
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slot = MAX_SLOTS - 1;
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__sync_fetch_and_add(&histp->slots[slot], 1);
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cleanup:
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bpf_map_delete_elem(&start, &rq);
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return 0;
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}
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```
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当用户中止程序时，用户态程序会读取直方图map中的数据，并打印呈现。
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### Eunomia中使用方式
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### 总结
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Biolatency 通过 tracepoint 挂载点实现了对IO事件个数的统计，并且能以直方图的
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形式进行展现，可以方便开发者了解系统I/O事件情况。 |