tools api io: Move filling the io buffer to its own function

In general a read fills 4kb so filling the buffer is a 1 in 4096
operation, move it out of the io__get_char function to avoid some
checking overhead and to better hint the function is good to inline.

For perf's IO intensive internal (non-rigorous) benchmarks there's a
small improvement to kallsyms-parsing with a default build.

Before:
```
$ perf bench internals all
Computing performance of single threaded perf event synthesis by
synthesizing events on the perf process itself:
  Average synthesis took: 146.322 usec (+- 0.305 usec)
  Average num. events: 61.000 (+- 0.000)
  Average time per event 2.399 usec
  Average data synthesis took: 145.056 usec (+- 0.155 usec)
  Average num. events: 329.000 (+- 0.000)
  Average time per event 0.441 usec

  Average kallsyms__parse took: 162.313 ms (+- 0.599 ms)
...
Computing performance of sysfs PMU event scan for 100 times
  Average core PMU scanning took: 53.720 usec (+- 7.823 usec)
  Average PMU scanning took: 375.145 usec (+- 23.974 usec)
```
After:
```
$ perf bench internals all
Computing performance of single threaded perf event synthesis by
synthesizing events on the perf process itself:
  Average synthesis took: 127.829 usec (+- 0.079 usec)
  Average num. events: 61.000 (+- 0.000)
  Average time per event 2.096 usec
  Average data synthesis took: 133.652 usec (+- 0.101 usec)
  Average num. events: 327.000 (+- 0.000)
  Average time per event 0.409 usec

  Average kallsyms__parse took: 150.415 ms (+- 0.313 ms)
...
Computing performance of sysfs PMU event scan for 100 times
  Average core PMU scanning took: 47.790 usec (+- 1.178 usec)
  Average PMU scanning took: 376.945 usec (+- 23.683 usec)
```

Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240519181716.4088459-1-irogers@google.com

diff --git a/tools/lib/api/io.h b/tools/lib/api/io.h
index 84adf81020185171b0d839eb639ba523d4d75355..d3eb04d1bc8970924cb42cb091c5501a66ced336 100644 (file)
--- a/tools/lib/api/io.h
+++ b/tools/lib/api/io.h
@@ -43,48 +43,55 @@ static inline void io__init(struct io *io, int fd,
        io->eof = false;
 }
 
-/* Reads one character from the "io" file with similar semantics to fgetc. */
-static inline int io__get_char(struct io *io)
+/* Read from fd filling the buffer. Called when io->data == io->end. */
+static inline int io__fill_buffer(struct io *io)
 {
-       char *ptr = io->data;
+       ssize_t n;
 
        if (io->eof)
                return -1;
 
-       if (ptr == io->end) {
-               ssize_t n;
-
-               if (io->timeout_ms != 0) {
-                       struct pollfd pfds[] = {
-                               {
-                                       .fd = io->fd,
-                                       .events = POLLIN,
-                               },
-                       };
-
-                       n = poll(pfds, 1, io->timeout_ms);
-                       if (n == 0)
-                               errno = ETIMEDOUT;
-                       if (n > 0 && !(pfds[0].revents & POLLIN)) {
-                               errno = EIO;
-                               n = -1;
-                       }
-                       if (n <= 0) {
-                               io->eof = true;
-                               return -1;
-                       }
+       if (io->timeout_ms != 0) {
+               struct pollfd pfds[] = {
+                       {
+                               .fd = io->fd,
+                               .events = POLLIN,
+                       },
+               };
+
+               n = poll(pfds, 1, io->timeout_ms);
+               if (n == 0)
+                       errno = ETIMEDOUT;
+               if (n > 0 && !(pfds[0].revents & POLLIN)) {
+                       errno = EIO;
+                       n = -1;
                }
-               n = read(io->fd, io->buf, io->buf_len);
-
                if (n <= 0) {
                        io->eof = true;
                        return -1;
                }
-               ptr = &io->buf[0];
-               io->end = &io->buf[n];
        }
-       io->data = ptr + 1;
-       return *ptr;
+       n = read(io->fd, io->buf, io->buf_len);
+
+       if (n <= 0) {
+               io->eof = true;
+               return -1;
+       }
+       io->data = &io->buf[0];
+       io->end = &io->buf[n];
+       return 0;
+}
+
+/* Reads one character from the "io" file with similar semantics to fgetc. */
+static inline int io__get_char(struct io *io)
+{
+       if (io->data == io->end) {
+               int ret = io__fill_buffer(io);
+
+               if (ret)
+                       return ret;
+       }
+       return *io->data++;
 }
 
 /* Read a hexadecimal value with no 0x prefix into the out argument hex. If the