Re: [PATCH] sched/isolcpus: Show isolated cpu map

From: Kefeng Wang
Date: Mon Feb 13 2017 - 08:09:07 EST

Hi Peter


On 2017/2/13 20:06, Peter Zijlstra wrote:
> On Mon, Feb 13, 2017 at 05:43:59PM +0800, Kefeng Wang wrote:
>> The commit a6e4491c682a ("sched/isolcpus: Output warning when the
>> 'isolcpus=' kernel parameter is invalid") adds an error message
>> when specified cpu bigger than nr_cpu_ids, but nr_cpumask_bits in
>> cpulist_parse() could be nr_cpu_ids or NR_CPUS.
>> eg, NR_CPUS=64, nr_cpu_ids=8 in ARM64, cpulist_parse() won't return
>> -ERANGE if isolcpus=1-10;
> But why does cpulist_parse() use nr_cpumask_bits, that seems to be the
> problem, so why not look there?

Paste the Tejun's patch,

commit 4d59b6ccf000862beed6fc0765d3209f98a8d8a2
Author: Tejun Heo <tj@xxxxxxxxxx>
Date: Wed Feb 8 14:30:56 2017 -0800

cpumask: use nr_cpumask_bits for parsing functions

Commit 513e3d2d11c9 ("cpumask: always use nr_cpu_ids in formatting and
parsing functions") converted both cpumask printing and parsing
functions to use nr_cpu_ids instead of nr_cpumask_bits. While this was
okay for the printing functions as it just picked one of the two output
formats that we were alternating between depending on a kernel config,
doing the same for parsing wasn't okay.

nr_cpumask_bits can be either nr_cpu_ids or NR_CPUS. We can always use
nr_cpu_ids but that is a variable while NR_CPUS is a constant, so it can
be more efficient to use NR_CPUS when we can get away with it.
Converting the printing functions to nr_cpu_ids makes sense because it
affects how the masks get presented to userspace and doesn't break
anything; however, using nr_cpu_ids for parsing functions can
incorrectly leave the higher bits uninitialized while reading in these
masks from userland. As all testing and comparison functions use
nr_cpumask_bits which can be larger than nr_cpu_ids, the parsed cpumasks
can erroneously yield false negative results.

This made the taskstats interface incorrectly return -EINVAL even when
the inputs were correct.

Fix it by restoring the parse functions to use nr_cpumask_bits instead
of nr_cpu_ids.

Fixes: 513e3d2d11c9 ("cpumask: always use nr_cpu_ids in formatting and parsing functions")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reported-by: Martin Steigerwald <martin.steigerwald@xxxxxxxxx>
Debugged-by: Ben Hutchings <ben.hutchings@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [4.0+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>