Re: [PATCH v2 4/5] tools api: add a lightweight buffered reading api

Ian Rogers
2020-04-06T16:16:08Z

On Mon, Apr 6, 2020 at 7:09 AM Arnaldo Carvalho de Melo
<arnaldo.melo@gmail.com> wrote:
>
> Em Sat, Apr 04, 2020 at 12:06:45PM +0900, Namhyung Kim escreveu:
> > Hello,
> >
> > On Fri, Apr 3, 2020 at 12:44 AM Ian Rogers <irogers@google.com> wrote:
> > >
> > > The synthesize benchmark shows the majority of execution time going to
> > > fgets and sscanf, necessary to parse /proc/pid/maps. Add a new buffered
> > > reading library that will be used to replace these calls in a follow-up
> > > CL.
>
> waiting for some conclusion to this thread,
>
> - Arnaldo

Thanks, sorry I was busy at the weekend. I agree with Namhyung's
comments, nice catch! Fwiw, it comes from my refactoring this api out
of a specific /proc/pid/maps reader. I'll work to address the issue
and ideally stick some tests of the corner cases somewhere - any
suggestions? This doesn't feel like a perf test, nor is it a kernel
side test.

Thanks,
Ian

> > > Signed-off-by: Ian Rogers <irogers@google.com>
> > > ---
> > > tools/lib/api/io.h | 107 +++++++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 107 insertions(+)
> > > create mode 100644 tools/lib/api/io.h
> > >
> > > diff --git a/tools/lib/api/io.h b/tools/lib/api/io.h
> > > new file mode 100644
> > > index 000000000000..5aa5b0e26a7a
> > > --- /dev/null
> > > +++ b/tools/lib/api/io.h
> > > @@ -0,0 +1,107 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +/*
> > > + * Lightweight buffered reading library.
> > > + *
> > > + * Copyright 2019 Google LLC.
> > > + */
> > > +#ifndef __API_IO__
> > > +#define __API_IO__
> > > +
> > > +struct io {
> > > + /* File descriptor being read/ */
> > > + int fd;
> > > + /* Size of the read buffer. */
> > > + unsigned int buf_len;
> > > + /* Pointer to storage for buffering read. */
> > > + char *buf;
> > > + /* End of the storage. */
> > > + char *end;
> > > + /* Currently accessed data pointer. */
> > > + char *data;
> > > + /* Set true on when the end of file on read error. */
> > > + bool eof;
> > > +};
> > > +
> > > +static inline void io__init(struct io *io, int fd,
> > > + char *buf, unsigned int buf_len)
> > > +{
> > > + io->fd = fd;
> > > + io->buf_len = buf_len;
> > > + io->buf = buf;
> > > + io->end = buf;
> > > + io->data = buf;
> > > + io->eof = false;
> > > +}
> > > +
> > > +/* Reads one character from the "io" file with similar semantics to fgetc. */
> > > +static inline int io__get_char(struct io *io)
> > > +{
> > > + char *ptr = io->data;
> > > +
> > > + if (ptr == io->end) {
> > > + ssize_t 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;
> > > +}
> > > +
> > > +/* Read a hexadecimal value with no 0x prefix into the out argument hex.
> > > + * Returns -1 on error or if nothing is read, otherwise returns the character
> > > + * after the hexadecimal value.
> > > + */
> > > +static inline int io__get_hex(struct io *io, __u64 *hex)
> > > +{
> > > + bool first_read = true;
> > > +
> > > + *hex = 0;
> > > + while (true) {
> > > + char ch = io__get_char(io);
> > > +
> >
> > Maybe you can add this
> >
> > if (io->eof)
> > return 0;
> >
> > Please see below
> >
> >
> > > + if (ch < 0)
> > > + return ch;
> > > + if (ch >= '0' && ch <= '9')
> > > + *hex = (*hex << 4) | (ch - '0');
> > > + else if (ch >= 'a' && ch <= 'f')
> > > + *hex = (*hex << 4) | (ch - 'a' + 10);
> > > + else if (ch >= 'A' && ch <= 'F')
> > > + *hex = (*hex << 4) | (ch - 'A' + 10);
> > > + else if (first_read)
> > > + return -1;
> > > + else
> > > + return ch;
> > > + first_read = false;
> > > + }
> > > +}
> >
> > What if a file contains hex digits at the end (without trailing spaces)?
> > I guess it'd see EOF and return -1, right?
> >
> > And it'd better to be clear when it sees a big hex numbers -
> > it could have a comment that it'd simply discard upper bits
> > or return an error.
> >
> > > +
> > > +/* Read a decimal value into the out argument dec.
> > > + * Returns -1 on error or if nothing is read, otherwise returns the character
> > > + * after the decimal value.
> > > + */
> > > +static inline int io__get_dec(struct io *io, __u64 *dec)
> > > +{
> > > + bool first_read = true;
> > > +
> > > + *dec = 0;
> > > + while (true) {
> > > + char ch = io__get_char(io);
> > > +
> > > + if (ch < 0)
> > > + return ch;
> > > + if (ch >= '0' && ch <= '9')
> > > + *dec = (*dec * 10) + ch - '0';
> > > + else if (first_read)
> > > + return -1;
> > > + else
> > > + return ch;
> > > + first_read = false;
> > > + }
> > > +}
> >
> > Ditto.
> >
> > Thanks
> > Namhyung
> >
> >
> > > +
> > > +#endif /* __API_IO__ */
> > > --
> > > 2.26.0.rc2.310.g2932bb562d-goog
> > >
>
> --
>
> - Arnaldo