Re: [PATCH v3 7/7] kunit: Don't waste first attempt to format string in kunit_log_append()

[David Gow]

On Wed, 9 Aug 2023 at 23:54, Richard Fitzgerald
<rf@xxxxxxxxxxxxxxxxxxxxx> wrote:
>
> It's wasteful to call vsnprintf() only to figure out the length of the
> string. The string might fit in the available buffer space but we have to
> vsnprintf() again to do that.
>
> Instead, attempt to format the string to the available buffer at the same
> time as finding the string length. Only if the string didn't fit the
> available space is it necessary to extend the log and format the string
> again to a new fragment buffer.
>
> Signed-off-by: Richard Fitzgerald <rf@xxxxxxxxxxxxxxxxxxxxx>
> ---

This looks good.

The only case I'm not totally convinced about is the last one, where
the message doesn't fit in the current log fragment, but is not a
whole fragment itself. I'm not sure if the added efficiency of not
doing a second vsprintf() is worth the memory fragmentation of always
starting a new fragment: the worst case where a log message is just
over half the length of frag->buf would result in every message being
in its own fragment (which would not necessarily have a consistent
size), and memory use would be ~doubled.

But I could accept it either way: until there's a real-world example
which strongly benefits from either implementation, let's go with
whatever we have working.

Reviewed-by: David Gow <davidgow@xxxxxxxxxx>

Cheers,
-- David



>  lib/kunit/test.c | 33 +++++++++++++++++----------------
>  1 file changed, 17 insertions(+), 16 deletions(-)
>
> diff --git a/lib/kunit/test.c b/lib/kunit/test.c
> index 28d0048d6171..230ec5e9824f 100644
> --- a/lib/kunit/test.c
> +++ b/lib/kunit/test.c
> @@ -196,11 +196,21 @@ void kunit_log_append(struct list_head *log, const char *fmt, ...)
>         if (!log)
>                 return;
>
> -       /* Evaluate length of line to add to log */
> +       frag = list_last_entry(log, struct kunit_log_frag, list);
> +       log_len = strlen(frag->buf);
> +       len_left = sizeof(frag->buf) - log_len - 1;
> +
> +       /* Attempt to print formatted line to current fragment */
>         va_start(args, fmt);
> -       len = vsnprintf(NULL, 0, fmt, args) + 1;
> +       len = vsnprintf(frag->buf + log_len, len_left, fmt, args) + 1;
>         va_end(args);
>
> +       if (len <= len_left)
> +               goto out_newline;
> +
> +       /* Abandon the truncated result of vsnprintf */
> +       frag->buf[log_len] = '\0';
> +
>         if (len > sizeof(frag->buf) - 1) {
>                 va_start(args, fmt);
>                 tmp = kvasprintf(GFP_KERNEL, fmt, args);
> @@ -212,24 +222,15 @@ void kunit_log_append(struct list_head *log, const char *fmt, ...)
>                 return;
>         }
>
> -       frag = list_last_entry(log, struct kunit_log_frag, list);
> -       log_len = strlen(frag->buf);
> -       len_left = sizeof(frag->buf) - log_len - 1;
> -
> -       if (len > len_left) {
> -               frag = kunit_log_extend(log);
> -               if (!frag)
> -                       return;
> -
> -               len_left = sizeof(frag->buf) - 1;
> -               log_len = 0;
> -       }
> +       frag = kunit_log_extend(log);
> +       if (!frag)
> +               return;
>
>         /* Print formatted line to the log */
>         va_start(args, fmt);
> -       vsnprintf(frag->buf + log_len, min(len, len_left), fmt, args);
> +       vsnprintf(frag->buf, sizeof(frag->buf) - 1, fmt, args);
>         va_end(args);
> -
> +out_newline:
>         /* Add newline to end of log if not already present. */
>         kunit_log_newline(frag);
>  }
> --
> 2.30.2
>

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