Re: Loss of vDSO support on s390x

[Florian Weimer <fweimer at redhat dot com>]

* 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.

I still think this needs to be fixed.

Thanks,
Florian