Re: [PATCH 0/3] migration: Downtime tracepoints

[Joao Martins]

On 26/10/2023 16:53, Peter Xu wrote:
> This small series (actually only the last patch; first two are cleanups)
> wants to improve ability of QEMU downtime analysis similarly to what Joao
> used to propose here:
> 
>   https://lore.kernel.org/r/20230926161841.98464-1-joao.m.martins@oracle.com
> 
Thanks for following up on the idea; It's been hard to have enough bandwidth for
everything on the past set of weeks :(

> But with a few differences:
> 
>   - Nothing exported yet to qapi, all tracepoints so far
> 
>   - Instead of major checkpoints (stop, iterable, non-iterable, resume-rp),
>     finer granule by providing downtime measurements for each vmstate (I
>     made microsecond to be the unit to be accurate).  So far it seems
>     iterable / non-iterable is the core of the problem, and I want to nail
>     it to per-device.
> 
>   - Trace dest QEMU too
> 
> For the last bullet: consider the case where a device save() can be super
> fast, while load() can actually be super slow.  Both of them will
> contribute to the ultimate downtime, but not a simple summary: when src
> QEMU is save()ing on device1, dst QEMU can be load()ing on device2.  So
> they can run in parallel.  However the only way to figure all components of
> the downtime is to record both.
> 
> Please have a look, thanks.
>

I like your series, as it allows a user to pinpoint one particular bad device,
while covering the load side too. The checkpoints of migration on the other hand
were useful -- while also a bit ugly -- for the sort of big picture of how
downtime breaks down. Perhaps we could add that /also/ as tracepoitns without
specifically commiting to be exposed in QAPI.

More fundamentally, how can one capture the 'stop' part? There's also time spent
there like e.g. quiescing/stopping vhost-net workers, or suspending the VF
device. All likely as bad to those tracepoints pertaining device-state/ram
related stuff (iterable and non-iterable portions).


> Peter Xu (3):
>   migration: Set downtime_start even for postcopy
>   migration: Add migration_downtime_start|end() helpers
>   migration: Add per vmstate downtime tracepoints
> 
>  migration/migration.c  | 38 +++++++++++++++++++++-----------
>  migration/savevm.c     | 49 ++++++++++++++++++++++++++++++++++++++----
>  migration/trace-events |  2 ++
>  3 files changed, 72 insertions(+), 17 deletions(-)
>