Re: [PATCH v6 0/3] Add support for the RAPL MSRs series

[Anthony Harivel]

Hi Igor,

Igor Mammedov, Oct 16, 2024 at 13:52:
> On Wed, 22 May 2024 17:34:49 +0200
> Anthony Harivel <aharivel@redhat.com> wrote:
>
>> Dear maintainers, 
>> 
>> First of all, thank you very much for your review of my patch 
>> [1].
>
> I've tried to play with this feature and have a few questions about it
>

Thanks for testing this new feature. 

>  1. trying to start with non accessible or not existent socket
>         -accel kvm,rapl=on,rapl-helper-socket=/tmp/socket 
>     I get:
>       qemu-system-x86_64: -accel kvm,rapl=on,rapl-helper-socket=/tmp/socks: 
> vmsr socket opening failed
>       qemu-system-x86_64: -accel kvm,rapl=on,rapl-helper-socket=/tmp/socks: 
> kvm : error RAPL feature requirement not met
>     * is it possible to report actual OS error that happened during 
> open/connect,
>       instead of unhelpful 'socket opening failed'?
>
>       What I see in vmsr_open_socket() error is ignored
>       and btw it's error leak as well
>

Shame you missed the 6 iterations of that patch that last for a year. 
I would have changed that directly !
Anyway I take note on that comment and will send a modification.

>     * 2nd line shouldn't be there if the 1st error already present.
>
>  2.  getting periodic error on console where QEMU has been starter
>       # ./qemu-vmsr-helper -k /tmp/sock
>      ./qemu-system-x86_64 -snapshot -m 4G -accel 
> kvm,rapl=on,rapl-helper-socket=/tmp/sock rhel90.img  -vnc :0 -cpu host
>      and let it run
>
>       it appears rdmsr works (well, it returns some values at least)
>       however there are recurring errors in qemu's stderr(or out)
>       
>       qemu-system-x86_64: Error opening /proc/2496093/task/2496109/stat
>       qemu-system-x86_64: Error opening /proc/2496093/task/2496095/stat
>
>       My guess it's some temporary threads, that come and go, but still
>       they shouldn't cause errors if it's normal operation.
>

There a patch in WIP that change this into a Tracepoint. Maybe you can 
SSH to the VM in meanwhile ?

>       Also on daemon side, I a few times got while guest was running:
>         qemu-vmsr-helper: Failed to open /proc at /proc/2496026/task/2496044
>         qemu-vmsr-helper: Requested TID not in peer PID: 2496026 2496044
>       though I can't reproduce it reliably

This could happen only when a vCPU thread ID has changed between the 
call of a rdmsr throught the socket and the hepler that read the msr.
No idea how a vCPU can change TID or shutdown that fast.

>
>  3. when starting daemon not as root, it starts 'fine' but later on complains
>       qemu-vmsr-helper: Failed to open MSR file at /dev/cpu/0/msr
>     perhaps it would be better to fail at start daemon if it doesn't have
>     access to necessary files.
>

Right taking a note on that as well.


>  4. in case #3, guest also fails to start with errors:
>       qemu-system-x86_64: -accel kvm,rapl=on,rapl-helper-socket=/tmp/sock: 
> can't read any virtual msr
>       qemu-system-x86_64: -accel kvm,rapl=on,rapl-helper-socket=/tmp/sock: 
> kvm : error RAPL feature requirement not met
>      again line #2 is not useful and probably not needed (maybe make it 
> tracepoint)
>      and #1 is unhelpful - it would be better if it directed user to check 
> qemu-vmsr-helper
>

I will try to see how to improve that part. 
Thanks for your valuable feedback.

>  5. does AMD have similar MSRs that we could use to make this feature 
> complete?
>

Yes but the address are completely different. However, this in my ToDo 
list. First I need way more feedback like yours to move on extending 
this feature.

>  6. What happens to power accounting if host constantly migrates
>     vcpus between sockets, are values we are getting still correct/meaningful?
>     Or do we need to pin vcpus to get 'accurate' values?
>

It's taken into account during the ratio calculation which socket the 
vCPU has just been scheduled. But yes the value are more 'accurate' when 
the vCPU is pinned.

>  7. do we have to have a dedicated thread for pooling data from daemon?
>
>     Can we fetch data from vcpu thread that have accessed msr
>     (with some caching and rate limiting access to the daemon)?
>

This feature is revolving around a thread. Please look at the 
documentation is not already done:

https://www.qemu.org/docs/master/specs/rapl-msr.html#high-level-implementation

If we only fetch from vCPU thread, we won't have the consumption of the 
non-vcpu thread. They are taken into account in the total.



Thanks again for your feedback. 

Anthony


>> In this version (v6), I have attempted to address all the problems 
>> addressed by Daniel and Paolo during the last review. 
>> 
>> However, two open questions remains unanswered that would require the 
>> attention of a x86 maintainers: 
>> 
>> 1)Should I move from -kvm to -cpu the rapl feature ? [2]
>> 
>> 2)Should I already rename to "rapl_vmsr_*" in order to anticipate the 
>>   futur TMPI architecture ? [end of 3] 
>> 
>> Thank you again for your continued guidance. 
>> 
>> v5 -> v6
>> --------
>> - Better error consistency in qio_channel_get_peerpid()
>> - Memory leak g_strdup_printf/g_build_filename corrected
>> - Renaming several struct with "vmsr_*" for better namespace
>> - Renamed several struct with "guest_*" for better comprehension
>> - Optimization suggerate from Daniel
>> - Crash problem solved [4]
>> 
>> v4 -> v5
>> --------
>> 
>> - correct qio_channel_get_peerpid: return pid = -1 in case of error
>> - Vmsr_helper: compile only for x86
>> - Vmsr_helper: use qio_channel_read/write_all
>> - Vmsr_helper: abandon user/group
>> - Vmsr_energy.c: correct all error_report
>> - Vmsr thread: compute default socket path only once
>> - Vmsr thread: open socket only once
>> - Pass relevant QEMU CI
>> 
>> v3 -> v4
>> --------
>> 
>> - Correct memory leaks with AddressSanitizer  
>> - Add sanity check for QEMU and qemu-vmsr-helper for checking if host is 
>>   INTEL and if RAPL is activated.
>> - Rename poor variables naming for easier comprehension
>> - Move code that checks Host before creating the VMSR thread
>> - Get rid of libnuma: create function that read sysfs for reading the 
>>   Host topology instead
>> 
>> v2 -> v3
>> --------
>> 
>> - Move all memory allocations from Clib to Glib
>> - Compile on *BSD (working on Linux only)
>> - No more limitation on the virtual package: each vCPU that belongs to 
>>   the same virtual package is giving the same results like expected on 
>>   a real CPU.
>>   This has been tested topology like:
>>      -smp 4,sockets=2
>>      -smp 16,sockets=4,cores=2,threads=2
>> 
>> v1 -> v2
>> --------
>> 
>> - To overcome the CVE-2020-8694 a socket communication is created
>>   to a priviliged helper
>> - Add the priviliged helper (qemu-vmsr-helper)
>> - Add SO_PEERCRED in qio channel socket
>> 
>> RFC -> v1
>> ---------
>> 
>> - Add vmsr_* in front of all vmsr specific function
>> - Change malloc()/calloc()... with all glib equivalent
>> - Pre-allocate all dynamic memories when possible
>> - Add a Documentation of implementation, limitation and usage
>> 
>> Best regards,
>> Anthony
>> 
>> [1]: https://mail.gnu.org/archive/html/qemu-devel/2024-04/msg01570.html
>> [2]: https://mail.gnu.org/archive/html/qemu-devel/2024-04/msg03947.html
>> [3]: https://mail.gnu.org/archive/html/qemu-devel/2024-04/msg02350.html
>> [4]: https://mail.gnu.org/archive/html/qemu-devel/2024-04/msg02481.html
>> 
>> Anthony Harivel (3):
>>   qio: add support for SO_PEERCRED for socket channel
>>   tools: build qemu-vmsr-helper
>>   Add support for RAPL MSRs in KVM/Qemu
>> 
>>  accel/kvm/kvm-all.c                      |  27 ++
>>  contrib/systemd/qemu-vmsr-helper.service |  15 +
>>  contrib/systemd/qemu-vmsr-helper.socket  |   9 +
>>  docs/specs/index.rst                     |   1 +
>>  docs/specs/rapl-msr.rst                  | 155 +++++++
>>  docs/tools/index.rst                     |   1 +
>>  docs/tools/qemu-vmsr-helper.rst          |  89 ++++
>>  include/io/channel.h                     |  21 +
>>  include/sysemu/kvm_int.h                 |  32 ++
>>  io/channel-socket.c                      |  28 ++
>>  io/channel.c                             |  13 +
>>  meson.build                              |   7 +
>>  target/i386/cpu.h                        |   8 +
>>  target/i386/kvm/kvm.c                    | 431 +++++++++++++++++-
>>  target/i386/kvm/meson.build              |   1 +
>>  target/i386/kvm/vmsr_energy.c            | 337 ++++++++++++++
>>  target/i386/kvm/vmsr_energy.h            |  99 +++++
>>  tools/i386/qemu-vmsr-helper.c            | 530 +++++++++++++++++++++++
>>  tools/i386/rapl-msr-index.h              |  28 ++
>>  19 files changed, 1831 insertions(+), 1 deletion(-)
>>  create mode 100644 contrib/systemd/qemu-vmsr-helper.service
>>  create mode 100644 contrib/systemd/qemu-vmsr-helper.socket
>>  create mode 100644 docs/specs/rapl-msr.rst
>>  create mode 100644 docs/tools/qemu-vmsr-helper.rst
>>  create mode 100644 target/i386/kvm/vmsr_energy.c
>>  create mode 100644 target/i386/kvm/vmsr_energy.h
>>  create mode 100644 tools/i386/qemu-vmsr-helper.c
>>  create mode 100644 tools/i386/rapl-msr-index.h
>>