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

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Date:   Thu, 2 Feb 2017 12:29:13 +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:
> On 01/30/2017 07:49 PM, Jiri Olsa wrote:
> > so basically we're changing from avail to online cpus
> > 
> > have you checked all the users of this FEATURE
> > if such change is ok?
> 
> Jiri,
> 
> It wasn't OK as there are other users who index cpu_topology_map by CPU id.
> I decided to give the alternative a try (attached): keep cpu_topology_map
> indexed by CPU id, but extend it to fit max present CPU.

please send this next time as a standard patchset,
it's hard to discuss over attachments

SNIP

> When build_cpu_topo() encounters offline/absent CPUs,
> it fails to find any sysfs entries and returns failure.
> This leads to build_cpu_topology() and write_cpu_topology()
> failing as well.
> 
> Because HEADER_CPU_TOPOLOGY has not been written, read leaves
> cpu_topology_map NULL and we get NULL ptr deref at:
> 
>  ...
>   cmd_test
>    __cmd_test
>     test_and_print
>      run_test
>       test_session_topology
>        check_cpu_topology

So IIUIC that's the key issue here.. write_cpu_topology that fails
to write the TOPO data and following readers crashing on processing
uncomplete data? if thats the case write_cpu_topology needs to
be fixed, instead of doing workarounds

SNIP

>  	u32 nr, i;
>  	size_t sz;
>  	long ncpus;
> -	int ret = -1;
> +	int ret = 0;
> +	struct cpu_map *map;
>  
>  	ncpus = sysconf(_SC_NPROCESSORS_CONF);
>  	if (ncpus < 0)
> -		return NULL;
> +		goto out;

can just return NULL

> +
> +	/* build online CPU map */
> +	map = cpu_map__new(NULL);
> +	if (map == NULL) {
> +		pr_debug("failed to get system cpumap\n");
> +		goto out;
> +	}
>  
>  	nr = (u32)(ncpus & UINT_MAX);
>  
>  	sz = nr * sizeof(char *);
> -
>  	addr = calloc(1, sizeof(*tp) + 2 * sz);
>  	if (!addr)
> -		return NULL;
> +		goto out_free;
>  
>  	tp = addr;
>  	tp->cpu_nr = nr;
> @@ -530,14 +537,21 @@ static struct cpu_topo *build_cpu_topology(void)
>  	tp->thread_siblings = addr;
>  
>  	for (i = 0; i < nr; i++) {
> +		if (!cpu_map__has(map, i))
> +			continue;
> +

so this prevents build_cpu_topo to fail due to missing topology
info because cpu is offline.. can it fail for other reasons?


>  		ret = build_cpu_topo(tp, i);
>  		if (ret < 0)
>  			break;


SNIP