linux-kernel - Re: [PATCH] perf: fix topology test on systems with sparse CPUs

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Date:   Thu, 2 Feb 2017 12:29:56 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Jan Stancek <jstancek@...hat.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
        mhiramat@...nel.org, rui.teng@...ux.vnet.ibm.com,
        sukadev@...ux.vnet.ibm.com
Subject: Re: [PATCH] perf: fix topology test on systems with sparse CPUs

On Tue, Jan 31, 2017 at 05:03:51PM +0100, Jan Stancek wrote:

SNIP

> 
> There are 2 problems wrt. cpu_topology_map on systems with sparse CPUs:
> 
> 1. offline/absent CPUs will have their socket_id and core_id set to -1
>    which triggers:
>    "socket_id number is too big.You may need to upgrade the perf tool."
> 
> 2. size of cpu_topology_map (perf_env.cpu[]) is allocated based on
>    _SC_NPROCESSORS_CONF, but can be indexed with CPU ids going above.
>    Users of perf_env.cpu[] are using CPU id as index. This can lead
>    to read beyond what was allocated:
>    ==19991== Invalid read of size 4
>    ==19991==    at 0x490CEB: check_cpu_topology (topology.c:69)
>    ==19991==    by 0x490CEB: test_session_topology (topology.c:106)
>    ...
> 
> For example:
>   _SC_NPROCESSORS_CONF == 16
>   available: 2 nodes (0-1)
>   node 0 cpus: 0 6 8 10 16 22 24 26
>   node 0 size: 12004 MB
>   node 0 free: 9470 MB
>   node 1 cpus: 1 7 9 11 23 25 27
>   node 1 size: 12093 MB
>   node 1 free: 9406 MB
>   node distances:
>   node   0   1
>     0:  10  20
>     1:  20  10

so what's max_present_cpu in this example?

jirka