linux-kernel - Re: aarch64 5.15.68 regression in topology/thread

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Date:   Thu, 22 Sep 2022 13:18:12 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Petr Štetiar <ynezz@...e.cz>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, yury.norov@...il.com,
        rafael@...nel.org
Subject: Re: aarch64 5.15.68 regression in topology/thread_siblings (huge
 file size and no content)

On Thu, Sep 22, 2022 at 04:05:04PM +0200 Petr Štetiar wrote:
> Phil Auld <pauld@...hat.com> [2022-09-22 09:18:47]:
> 
> Hi,
> 
> > I've seen the size cause problems for tools. Are we sure that it's the empty file and not
> > the size causing issues?  Maybe something is treating that as signed again for a count of
> > -1 bytes (which seems like it would be a bug anyway)?
> 
>   root@...nWrt:/# strace cat /sys/devices/system/cpu/cpu1/topology/thread_siblings
>   ...snip...
>   openat(AT_FDCWD, "/sys/devices/system/cpu/cpu1/topology/thread_siblings", O_RDONLY|O_LARGEFILE) = 3
>   sendfile(1, 3, NULL, 16777216)          = 0
> 
>   root@...nWrt:/# strace md5sum /sys/devices/system/cpu/cpu1/topology/thread_sibli
>   ...snip...
>   openat(AT_FDCWD, "/sys/devices/system/cpu/cpu1/topology/thread_siblings", O_RDONLY|O_LARGEFILE) = 3
>   read(3, "", 4096)                       = 0
> 
>   root@...nWrt:/# strace head /sys/devices/system/cpu/cpu1/topology/thread_siblings
>   ...snip...
>   openat(AT_FDCWD, "/sys/devices/system/cpu/cpu1/topology/thread_siblings", O_RDONLY|O_LARGEFILE) = 3
>   read(3, "", 1024)                       = 0
>

I tried this with the latest upstream (which doesn't yet have the fix
for the size issue) and got the same results.

Then I applied the fix and the problem went away:

6.0.0-rc6.nr_cpus2+
# find /sys -name thread_siblings -exec cat \{\} \; 
2
1

Cheers,
Phil

> Cheers,
> 
> Petr
> 

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