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I wanted to try splice syscall, which trace-cmd uses to zero copy raw files of off ftrace.

Here is the partial list of ftrace raw files splice fails to read:

/sys/kernel/debug/tracing/per_cpu/cpo0/trace_pipe_raw

/sys/kernel/debug/tracing/per_cpu/cpo0/snapshot_raw

/sys/kernel/debug/tracing/per_cpu/cpo1/trace_pipe_raw

/sys/kernel/debug/tracing/per_cpu/cpo1/snapshot_raw

And here are some other files (that splice handles just fine):

/sys/kernel/debug/tracing/per_cpu/cpo0/trace_pipe

/sys/kernel/debug/tracing/per_cpu/cpo0/snapshot

/sys/kernel/debug/tracing/per_cpu/cpo1/trace_pipe

/sys/kernel/debug/tracing/per_cpu/cpo1/snapshot

What works:

  • using the read() system call works great to read from raw ftrace files.
  • using the cat() system call will display raw ftrace files.
  • using the trace-cmd tool which is a CLI front end for ftrace.

This is my code: 

static void unit_test_x(void)
{  
    int buffer_pipe[2];
    pipe(buffer_pipe);

    std::string source_path = "/sys/kernel/debug/tracing/per_cpu/cpu1/trace_pipe_raw";
    int trace_fd = open(source_path.c_str(), O_RDONLY);

    std::string destination_path = "foo";
    int dest_fd = open (destination_path.c_str(), O_WRONLY | O_CREAT | O_TRUNC | O_LARGEFILE, 0644);

    int actually_read = splice(trace_fd,
                               NULL,
                               buffer_pipe[1],
                               NULL,
                               1000,
                               SPLICE_F_MORE | SPLICE_F_MOVE );

    if (0 > actually_read )
    {   
        printf("Oh dear, something went wrong %s\n", s  trerror(errno));
        throw std::runtime_error("writing from source to pipe failed");
    }

    actually_read = splice(buffer_pipe[0],
                           NULL,
                           dest_fd,
                           NULL,
                           actually_read,
                           SPLICE_F_MORE | SPLICE_F_MOVE);

}

note:

All accsess to /sys/kernel/debug/tracing is done with sudo

p3t3
  • 95
  • 8

1 Answers1

1

After a lot of troubleshooting I have solved the problem. Splice won't read less then a page size(4096 bytes) from a "raw" file which is a stream of binary data. Providing less then a page size will result in "Invalid argument" errno.

p3t3
  • 95
  • 8