Re: [PATCH 04/11] log: Fix sub-second part in relative log time calculation

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jul 24, 2024 at 11:50:10PM +0200, Stefano Brivio wrote:
> For some reason, in commit 01efc71ddd25 ("log, conf: Add support for
> logging to file"), I added calculations for relative logging
> timestamps using the difference for the seconds part only, not for
> accounting for the fractional part.
> 
> Fix that by storing the initial timestamp, log_start, as a timespec
> struct, and by calculating differences of both seconds and nanoseconds
> from the starting time. Do this in a macro as we need the same
> calculation and format in a few places.
> 
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  log.c | 42 ++++++++++++++++++++++--------------------
>  1 file changed, 22 insertions(+), 20 deletions(-)
> 
> diff --git a/log.c b/log.c
> index 54483e7..f57a54f 100644
> --- a/log.c
> +++ b/log.c
> @@ -40,12 +40,21 @@ static size_t	log_written;		/* Currently used bytes in log file */
>  static size_t	log_cut_size;		/* Bytes to cut at start on rotation */
>  static char	log_header[BUFSIZ];	/* File header, written back on cuts */
>  
> -static time_t	log_start;		/* Start timestamp */
> +static struct timespec log_start;	/* Start timestamp */
>  
>  int		log_trace;		/* --trace mode enabled */
>  bool		log_conf_parsed;	/* Logging options already parsed */
>  bool		log_runtime;		/* Daemonised, or ready in foreground */
>  
> +/**
> + * logtime_fmt_and_arg() - Build format and arguments to print relative log time
> + * @x:		Current timestamp
> + */
> +#define logtime_fmt_and_arg(x)						\
> +	"%lli.%04lli",							\
> +	((long long int)(x)->tv_sec - log_start.tv_sec),		\
> +	(((long long int)(x)->tv_nsec - log_start.tv_nsec) / (100L * 1000))

This doesn't look right.  If x->tv_nsec - log_start.tv_nsec is
negative it will produce weird results.  Instead you need more complex
logic to carry a sufficient difference in the nsec over into the
seconds difference.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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