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

Arnaldo Carvalho de Melo
2020-04-07T12:32:22Z

Em Mon, Apr 06, 2020 at 09:15:51AM -0700, Ian Rogers escreveu:
> 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:
> > > 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,

> 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

It may not be uniquely useful for perf, but I'd start by adding an entry
to 'perf test' anyway, as its used by perf, after all what we want is
that code gets tested regularly, adding it to 'perf test' achieves that.

Thanks,

- Arnaldo

> 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

--

- Arnaldo