netdev - Re: [PATCH bpf-next v2 0/3] bpf: implement bpf_send

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Date:   Wed, 22 May 2019 16:43:09 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Stanislav Fomichev <sdf@...ichev.me>, Yonghong Song <yhs@...com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH bpf-next v2 0/3] bpf: implement bpf_send_signal() helper

On 5/22/19 9:38 AM, Stanislav Fomichev wrote:
> On 05/21, Yonghong Song wrote:
>> This patch tries to solve the following specific use case.
>>
>> Currently, bpf program can already collect stack traces
>> through kernel function get_perf_callchain()
>> when certain events happens (e.g., cache miss counter or
>> cpu clock counter overflows). But such stack traces are
>> not enough for jitted programs, e.g., hhvm (jited php).
>> To get real stack trace, jit engine internal data structures
>> need to be traversed in order to get the real user functions.
>>
>> bpf program itself may not be the best place to traverse
>> the jit engine as the traversing logic could be complex and
>> it is not a stable interface either.
>>
>> Instead, hhvm implements a signal handler,
>> e.g. for SIGALARM, and a set of program locations which
>> it can dump stack traces. When it receives a signal, it will
>> dump the stack in next such program location.
>>
> 
> [..]
>> This patch implements bpf_send_signal() helper to send
>> a signal to hhvm in real time, resulting in intended stack traces.
> Series looks good. One minor nit/question: maybe rename bpf_send_signal
> to something like bpf_send_signal_to_current/bpf_current_send_signal/etc?
> bpf_send_signal is too generic now that you send the signal
> to the current process..

bpf_send_signal_to_current was Yonghong's original name
and I asked him to rename it to bpf_send_signal :)
I don't see bpf sending signals to arbitrary processes
in the near future.