[RFC PATCH 11/17] define function to print error messages to user log

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at librato.com):
> 
> 
> Serge E. Hallyn wrote:
> > Quoting Matt Helsley (matthltc at us.ibm.com):
> >>> @@ -401,6 +409,9 @@ char *ckpt_generate_fmt(struct ckpt_ctx *ctx, char *fmt)
> >>>  		case 'E':
> >>>  			len += sprintf(format+len, "[%s]", "err %d");
> >>>  			break;
> >>> +		case 'C': /* count of bytes read/written to checkpoint image */
> >>> +			len += sprintf(format+len, "[%s]", "pos %d");
> >>> +			break;
> >> Instead we could always output ckpt->total and then we wouldn't need %(C). I
> >> suspect it's such a useful piece of information that it'll be repeated
> >> in many/all format strings eventually.
> > 
> > Yes, likewise %(T).  If that's what we want to do.
> 
> I agree. For the cases when there is not task, can put "none"
> 
> > 
> > Should we discuss here what we want an entry to look like?  For both
> > ckpt_write_err (to the checkpoint image) and ckpt_error()?
> > 
> 
> Yes please !

Actually %T isn't the current task, right, so it shouldn't always be prepended?
It actually is only meaningful during checkpoint_task(), collect_objs(), and
__tree_count_tasks?

Ok, so how about:

	1. ckpt_write_err() always also calls ckpt_error() (which in turn calls
		ckpt_debug).  Avoid duplication which exists in several places
		right now.
	2. We always prepend:

		[current->pid]:[ctx->root_pid]:[ctx->active_pid]:[ctx->errno][ctx->total]

	The %(X) expansions if specified come whereever they are in the fmt
	string (which is what's happening now with my patchset).

Kind of long, but again this is for ckpt_error and ckpt_write_err, not for all
ckpt_debugs().

> >>>  		case 'O':
> >>>  			len += sprintf(format+len, "[%s]", "obj %d");
> >>>  			break;
> >>> @@ -435,6 +446,51 @@ char *ckpt_generate_fmt(struct ckpt_ctx *ctx, char *fmt)
> >>>  	return format;
> >>>  }
> >>>
> >>> +void ckpt_log_error(struct ckpt_ctx *ctx, char *fmt, ...)
> >>> +{
> >>> +	mm_segment_t fs;
> >>> +	struct file *file;
> >>> +	int count;
> >>> +	va_list ap, aq, az;
> >>> +	char *format;
> >>> +	char buf[200], *bufp = buf;
> >> I believe this buffer is too big for a kernel stack -- especially
> >> for ckpt_log_error() which might be invoked "deep" in
> >> the kernel stack.
> > 
> > 200 bytes?  Well, I guess I can try with 50 which still may often be
> > enough.
> 
> How about using a dedicated buffer on @ctx for that ?

I was going to do that originally, but then thought back to your
comments about parallel checkpoint, and didn't feel like also adding
a spinlock.

Thoughts?

-serge