[RFC] Improve startup time #502
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Allowing to use the newly added program attach types (kprobe w/ opts, raw tp w/ opts and kprobe multi). Signed-off-by: Antoine Tenart <[email protected]>
The kernel symbol address is used as the primary probe configuration key. It could be retrieved dynamically in k(ret)probes but not in raw_tracepoints and was there set as rodata. In an effort to move away from probe-specific rodata, set the symbol address as the BPF cookie when attaching all probes and use it to get the symbol address. No functional change intended. Signed-off-by: Antoine Tenart <[email protected]>
Instead of attaching all probes one by one, add them to the builders (transferring ownership, which is more logical) and only then attach all probes at once. Attaching probes added to a builder does not consume the builder, it can be reused for later probe(s). This does not change the current behavior but opens the door for optimizations. Signed-off-by: Antoine Tenart <[email protected]>
Signed-off-by: Antoine Tenart <[email protected]>
Instead of attaching kprobes one by one, use kprobe_multi to attach them all at once, speeding up the attaching time a lot when using lots of kprobes. Signed-off-by: Antoine Tenart <[email protected]>
We can get the program once and use it for all attachs. Signed-off-by: Antoine Tenart <[email protected]>
…me nargs Raw tracepoints are special as the verifier checks at load time no out of bound argument is accessed, which prevents us from loading a program once and reusing it for all raw tracepoint probes. However since we moved the symbol address to the BPF cookie the only probe-specific variable in our tracepoints is the number of arguments. This one has to be rodata for the verifier to work, but nothing prevents us from sharing the same program for tracepoints have the same number of arguments. Do this. This speed up loading time a lot when using tracepoints, e.g. the generic profile startup time is divided by two as we have the following and as loading a raw tracepoint program always take quite some time: nargs 1: [ "tp:net:napi_gro_receive_entry", "tp:net:napi_gro_frags_entry", "tp:net:netif_rx", "tp:net:net_dev_queue", "tp:net:netif_receive_skb"] nargs 2: ["tp:net:net_dev_start_xmit"] nargs 4: ["tp:skb:kfree_skb"] Signed-off-by: Antoine Tenart <[email protected]>
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Targeted probes use a single use builder. But when they have no hook attached, they can instead use shared builders. This speeds up attaching time for those (e.g. skb tracking probes). Signed-off-by: Antoine Tenart <[email protected]>
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I just realized support for getting the BPF cookie in raw tracepoints was only added a year ago[1]. For those kernels we could not use the cookie in raw tracepoints and keep the legacy one program per probe logic. [1] 68ca5d4eebb8 ("bpf: support BPF cookie in raw tracepoint (raw_tp, tp_btf) programs") |
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As of now Retis can take quite some time to handle various things before starting a collection. Investigating where time is spent boils down to three main points:
/proc/kallsyms; not much we can do here.Combining this and #496 and using the
genericprofile, we can go from ~6.5s to ~2.5s starting time. In addition to this using kprobe multi helps when using lots of kprobes, especially at exit time (1 link vs 1 per probe). The raw tracepoints benefit from an optimization too, which helps a lot when using a lot of them — this was a major pain point in the past and one of the important reasons Retis could be slow to start.Note that the last commit, grouping targeted probes with no hooks, might not show a lot of improvement atm but could be in the future. Nevertheless it is more logical, future proof and the changes are really not invasive.
This is based on multiple
libbpf-rschanges, which have been merged upstream already but not released yet. This is the only reason this is marked as an RFC.Some extra details in the commit logs.