Re: Loss of vDSO support on s390x

[Stefan Liebler <stli at linux dot ibm dot com>]

On 1/15/20 1:37 PM, Florian Weimer wrote:
> * Siddhesh Poyarekar:
>
> > On 15/01/20 3:20 pm, Florian Weimer wrote:
> > > time, gettimeofday, and clock_gettime always seem to make system calls
> > > now.
> > >
> > > Is this intentional?  I'm worried that this is a major performance
> > > regression. >>
> > +Adhemerval; this may be related to the vDSO patchset.
>
> The kernel I used disables the vDSO with an explicit loader invocation.
> I assume this was fixed with this commit:
>
> commit 404861e15b5fa7edbab22400f9174c1a21fde731
> Author: Heiko Carstens <heiko.carstens@de.ibm.com>
> Date:   Wed Aug 7 14:31:59 2019 +0200
>
>      s390/vdso: map vdso also for statically linked binaries
>      
>      s390 does not map the vdso for statically linked binaries, assuming
>      that this doesn't make sense. See commit fc5243d98ac2 ("[S390]
>      arch_setup_additional_pages arguments").
>      
>      However with glibc commit d665367f596d ("linux: Enable vDSO for static
>      linking as default (BZ#19767)") and commit 5e855c895401 ("s390: Enable
>      VDSO for static linking") the vdso is also used for statically linked
>      binaries - if the kernel would make it available.
>      
>      Therefore map the vdso always, just like all other architectures.
>
> If I fix this, by hardcoding the new loader, I only see a system call
> for the time emulation.  It looks like the vDSO does not have
> acceleration for CLOCK_REALTIME_COARSE, which is used by the time
> emulation.
According to <kernel>/arch/s390/kernel/vdso64/clock_gettime.S there is 
support for:
- CLOCK_MONOTONIC
- CLOCK_MONOTONIC_COARSE
- CLOCK_REALTIME_COARSE
- CLOCK_REALTIME
- CPUCLOCK_VIRT for this thread
(See 
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/arch/s390/kernel/vdso64/clock_gettime.S?id=2115fbf7210bd053ba55a95e7ebc366df41aa9cf)

> I still think this needs to be fixed.
>
> Thanks,
> Florian

Do you run on s390x (64bit) or s390 (31bit)?
Heiko has also removed the vdso for 31bit in linux 5.5:
"s390: remove compat vdso code"
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=2115fbf7210bd053ba55a95e7ebc366df41aa9cf

Please double check if the vdso is mapped and the address is passed via 
AT_SYSINFO_EHDR. I've just run a test program linked against current 
glibc and stepped to the vdso.

But on 31bit, the vdso is mapped, but I end in a syscall as my used 
kernel has support for __NR_clock_gettime64 but has no support for 
HAVE_CLOCK_GETTIME64_VSYSCALL (only for HAVE_CLOCK_GETTIME_VSYSCALL).

Bye,
Stefan