WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

Re: [Xen-devel] unconditionally enable the trace buffer

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] unconditionally enable the trace buffer
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Fri, 28 Oct 2005 00:21:45 +0100
Cc: Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx>, Rob Gardner <rob.gardner@xxxxxx>
Delivery-date: Thu, 27 Oct 2005 23:20:07 +0000
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
In-reply-to: <43614FE6.105@xxxxxx>
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <A95E2296287EAD4EB592B5DEEFCE0E9D32E61D@xxxxxxxxxxxxxxxxxxxxxxxxxxx> <43614FE6.105@xxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: KMail/1.8.3
> >* ability to turn on/off via hypercall
>
> Not currently implemented, but would not be difficult to add.

Just as an aside I'm not sure this matters: From what Rob's told me, having 
the (inline) trace() calls in there produces the same overhead whether 
tracing is active or not.  I guess it makes sense; once you've incurred the 
overhead of having the function there and evaluating the "is tracing on" 
conditional, you might as well have stored a few values also ;-)

> >* ability to set the per CPU tracebuffer size when turning it on
>
> Partially; You can enable the trace buffer on the xen (boot) command
> line, and you can specify the trace buffer size there. You cannot change
> the size dynamically.

Aside #2: You can disable at boot time by setting size=0.

> >* ability for the user-space reader to explicitly block (select on fd)
> >on an eventchn notification that the buffer is e.g. half full. (reader
> >should write out all the pages that are full of trace events)
>
> Not done.

We can just block on /dev/evtchn, I guess.  Shouldn't be too hard; we just 
need to define a new "trace" virq.

> >* user space reader should log when it misses blocks of events
> >(overwrite last trace message in buffer with a special 'missed X events'
> >message)
>
> Not done, but my XenMon patch includes a change to the trace buffer code
> to help with this. I've added a "sequence number" to each trace record
> which can be used to detect when blocks of events have been missed.

Yep, sounds good to me.

Cheers,
Mark0

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel