linux-kernel - Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include namespaces related info

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Date:   Wed, 21 Dec 2016 14:24:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Hari Bathini <hbathini@...ux.vnet.ibm.com>
Cc:     ast@...com, lkml <linux-kernel@...r.kernel.org>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, mingo@...hat.com,
        daniel@...earbox.net, rostedt@...dmis.org,
        Ananth N Mavinakayanahalli <ananth@...ux.vnet.ibm.com>,
        ebiederm@...ssion.com, sargun@...gun.me,
        Aravinda Prasad <aravinda@...ux.vnet.ibm.com>,
        brendan.d.gregg@...il.com
Subject: Re: [PATCH v4 1/3] perf: add PERF_RECORD_NAMESPACES to include
 namespaces related info

On Wed, Dec 21, 2016 at 06:39:01PM +0530, Hari Bathini wrote:
> Hi Peter,

> >I don't see how the tool can parse old records (with NAMESPACES_MAX ==
> >7) if you set its NAMESPACES_MAX to say 10.
> >
> >Then it will expect the link_info array to be 10 entries and either read
> >past the end of the record (if !sample_all) or try and interpret
> >sample_id as link_info records.
> >
> 
> Right. There will be inconsistency with data the perf tool tries to read
> beyond
> what the kernel supports. IIUC, you mean, include nr_namespaces field in the
> record and warn the user if it doesn't match with the one perf-tool supports
> before proceeding..?

Yes, if you add a nr_namespaces field its always parsable. If an old
tool finds more namespace than it has 'names' for it can always display
the raw index number. If a new tool finds the array short, it will not
display the missing ones.