Re: [pve-devel] [PATCH qemu] async snapshot: stop vCPU throttling after finishing

[Fiona Ebner <f.ebner@proxmox.com>]

Ping, as there are still new affected users showing up in the forum thread.

Am 30.10.24 um 10:52 schrieb Fiona Ebner:
> In the community forum, users reported issues about RCU stalls and
> sluggish VMs after taking a snapshot with RAM in Proxmox VE [0]. Mario
> was also experiencing similar issues from time to time and recently,
> obtained a GDB stacktrace. The stacktrace showed that, in his case,
> the vCPU threads were waiting in cpu_throttle_thread(). It is a good
> guess that the issues in the forum could also be because of that.
> 
> From searching in the source code, it seems that migration is the only
> user of the vCPU throttling functions in QEMU relevant for Proxmox VE
> (the only other place where it is used is the Cocoa UI). In
> particular, RAM migration will begin throttling vCPUs for
> auto-converge.
> 
> In migration_iteration_finish() there is an unconditional call to
> cpu_throttle_stop(), so do the same in the async snapshot code
> specific to Proxmox VE.
> 
> It's not clear why the issue began to surface more prominently only
> now, since the vCPU throttling was there since commit 070afca258
> ("migration: Dynamic cpu throttling for auto-converge") in QEMU
> v2.10.0. However, there were a lot of changes in the migration code
> between v8.1.5 and v9.0.2 and a few of them might have affected the
> likelihood of cpu_throttle_set() being called, for example, 4e1871c450
> ("migration: Don't serialize devices in qemu_savevm_state_iterate()")
> 
> [0]: https://forum.proxmox.com/threads/153483
> 
> Reported-by: Mario Loderer <m.loderer@proxmox.com>
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> Tested-by: Mario Loderer <m.loderer@proxmox.com>


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