Re: [Xen-devel] [PATCH v5 3/4] xen: refactor debugtrace data

[Juergen Gross <jgross@xxxxxxxx>]

On 05.09.19 14:01, Jan Beulich wrote:
> On 05.09.2019 13:39, Juergen Gross wrote:
> > As a preparation for per-cpu buffers do a little refactoring of the
> > debugtrace data: put the needed buffer admin data into the buffer as
> > it will be needed for each buffer. In order not to limit buffer size
> > switch the related fields from unsigned int to unsigned long, as on
> > huge machines with RAM in the TB range it might be interesting to
> > support buffers >4GB.
> Just as a further remark in this regard:
>
> >   void debugtrace_printk(const char *fmt, ...)
> >   {
> >       static char buf[DEBUG_TRACE_ENTRY_SIZE];
> > -    static unsigned int count, last_count, last_prd;
> > +    static unsigned int count, last_count;
> How long do we think will it take until their wrapping will become
> an issue with such huge buffers?
Count wrapping will not result in any misbehavior of tracing. With
per-cpu buffers it might result in ambiguity regarding sorting the
entries, but I guess chances are rather low this being a real issue.

BTW: wrapping of count is not related to buffer size, but to the
amount of trace data written.


Juergen

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