|author||David S. Miller <email@example.com>||2016-02-20 00:21:44 -0500|
|committer||David S. Miller <firstname.lastname@example.org>||2016-02-20 00:21:44 -0500|
|parent||net: use skb_postpush_rcsum instead of own implementations (diff)|
|parent||samples/bpf: offwaketime example (diff)|
Merge branch 'bpf-get-stackid'
Alexei Starovoitov says: ==================== bpf_get_stackid() and stack_trace map This patch set introduces new map type to store stack traces and corresponding bpf_get_stackid() helper. BPF programs already can walk the stack via unrolled loop of bpf_probe_read()s which is ok for simple analysis, but it's not efficient and limited to <30 frames after that the programs don't fit into MAX_BPF_STACK. With bpf_get_stackid() helper the programs can collect up to PERF_MAX_STACK_DEPTH both user and kernel frames. Using stack traces as a key in a map turned out to be very useful for generating flame graphs, off-cpu graphs, waker and chain graphs. Patch 3 is a simplified version of 'offwaketime' tool which is described in detail here: http://brendangregg.com/blog/2016-02-01/linux-wakeup-offwake-profiling.html Earlier version of this patch were using save_stack_trace() helper, but 'unreliable' frames add to much noise and two equiavlent stack traces produce different 'stackid's. Using lockdep style of storing frames with MAX_STACK_TRACE_ENTRIES is great for lockdep, but not acceptable for bpf, since the stack_trace map needs to be freed when user Ctrl-C the tool. The ftrace style with per_cpu(struct ftrace_stack) is great, but it's tightly coupled with ftrace ring buffer and has the same 'unreliable' noise. perf_event's perf_callchain() mechanism is also very efficient and it only needed minor generalization which is done in patch 1 to be used by bpf stack_trace maps. Peter, please take a look at patch 1. If you're ok with it, I'd like to take the whole set via net-next. Patch 1 - generalization of perf_callchain() Patch 2 - stack_trace map done as lock-less hashtable without link list to avoid spinlock on insertion which is critical path when bpf_get_stackid() helper is called for every task switch event Patch 3 - offwaketime example After the patch the 'perf report' for artificial 'sched_bench' benchmark that doing pthread_cond_wait/signal and 'offwaketime' example is running in the background: 16.35% swapper [kernel.vmlinux] [k] intel_idle 2.18% sched_bench [kernel.vmlinux] [k] __switch_to 2.18% sched_bench libpthread-2.12.so [.] pthread_cond_signal@@GLIBC_2.3.2 1.72% sched_bench libpthread-2.12.so [.] pthread_mutex_unlock 1.53% sched_bench [kernel.vmlinux] [k] bpf_get_stackid 1.44% sched_bench [kernel.vmlinux] [k] entry_SYSCALL_64 1.39% sched_bench [kernel.vmlinux] [k] __call_rcu.constprop.73 1.13% sched_bench libpthread-2.12.so [.] pthread_mutex_lock 1.07% sched_bench libpthread-2.12.so [.] pthread_cond_wait@@GLIBC_2.3.2 1.07% sched_bench [kernel.vmlinux] [k] hash_futex 1.05% sched_bench [kernel.vmlinux] [k] do_futex 1.05% sched_bench [kernel.vmlinux] [k] get_futex_key_refs.isra.13 The hotest part of bpf_get_stackid() is inlined jhash2, so we may consider using some faster hash in the future, but it's good enough for now. ==================== Signed-off-by: David S. Miller <email@example.com>
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