Re: [PATCH -tip 0/4] do not make cputime scaling in kernel

[Frederic Weisbecker]

2013/4/4 Stanislaw Gruszka <sgruszka@xxxxxxxxxx>:
> On Thu, Apr 04, 2013 at 02:31:42PM +0200, Frederic Weisbecker wrote:
>> I don't know. I'm not convinced userland is the right place to perform
>> this kind of check. The kernel perhaps doesn't give guarantee about
>> utime/stime precision but now users may have got used to that scaled
>> behaviour. It's also a matter of security, a malicous app can hide
>> from the tick to make its activity less visible from tools like top.
>>
>> It's sortof an ABI breakage to remove such an implicit protection. And
>> fixing that from userspace with a lib or so won't change that fact.
>
> I think number of fields in /proc/PID/stat is not part of ABI. For
> example commit 5b172087f99189416d5f47fd7ab5e6fb762a9ba3 add various
> new fields at the end of the file. What is imported to keep unchanged
> ABI is not changing order or meaning of fields we already have.

Oh I wasn't considering the layout of the proc file but the semantic
change in its utime/stime fields.

> Regarding top, I added those additional fields to allow top to detect
> those malicious software. Patched top will work well with old and new
> (patched) kernel. Problem is old top with new kernel, but I believe
> users who care about security update they software regularly.

The usual rule is that but you can't remove a feature from the kernel
and tell userspace to fix it itself. That's basically an ABI breakage.
A semantic one. We do it sometimes for some cases. But the more we are
dealing with a central ABI component, the harder it is to change it.
And here it is quite a central component.

>
> Besides for most cases (not counting hostile software), those
> statistical stime/utime accounting give good approximation of CPU
> time utilizing by each process.

Yeah but still the users are expecting that result to be scaled now.

>
>> How about that 128bits based idea? I'm adding Paul Turner in Cc
>> because he seemed to agree with doing it using 128bits maths.
>
> For problem that I try to solve 128bits math is not necessary, assuming
> we can do multiplication in user space. Taking into account how easily
> things can be done in user space using floating point math, I prefer not
> to add complexity in kernel. This solution make kernel simpler and
> faster.

I'm always all for making the kernel simpler and faster, as long as we
don't break userspace.
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