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

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Date:   Mon, 13 Feb 2017 21:07:02 +0800
From:   Kefeng Wang <wangkefeng.wang@...wei.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Ingo Molnar <mingo@...nel.org>, <linux-kernel@...r.kernel.org>,
        <guohanjun@...wei.com>, Prarit Bhargava <prarit@...hat.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] sched/isolcpus: Show isolated cpu map

Hi Peter

+Tejun

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@...nel.org>
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.

    Link: http://lkml.kernel.org/r/20170206182442.GB31078@htj.duckdns.org
    Fixes: 513e3d2d11c9 ("cpumask: always use nr_cpu_ids in formatting and parsing functions")
    Signed-off-by: Tejun Heo <tj@...nel.org>
    Reported-by: Martin Steigerwald <martin.steigerwald@...mix.de>
    Debugged-by: Ben Hutchings <ben.hutchings@...ethink.co.uk>
    Cc: <stable@...r.kernel.org>        [4.0+]
    Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>