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

[Arnaldo Carvalho de Melo]

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@xxxxxxxxxx> 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

> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  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