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SubjectRe: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
On Fri, Jan 20, 2023 at 04:12:20PM -0600, Seth Forshee (DigitalOcean) wrote:
> We've fairly regularaly seen liveptches which cannot transition within kpatch's
> timeout period due to busy vhost worker kthreads. In looking for a solution the
> only answer I found was to call klp_update_patch_state() from a safe location.
> I tried adding this call to vhost_worker(), and it works, but this creates the
> potential for problems if a livepatch attempted to patch vhost_worker().
> Without a call to klp_update_patch_state() fully loaded vhost kthreads can
> never switch because vhost_worker() will always appear on the stack, but with
> the call these kthreads can switch but will still be running the old version of
> vhost_worker().
> To avoid this situation I've added a new function, klp_switch_current(), which
> switches the current task only if its stack does not include any function being
> patched. This allows kthreads to safely attempt switching themselves if a patch
> is pending. There is at least one downside, however. Since there's no way for
> the kthread to track whether it has already tried to switch for a pending patch
> it can end up calling klp_switch_current() repeatedly when it can never be
> safely switched.
> I don't know whether this is the right solution, and I'm happy to try out other
> suggestions. But in my testing these patches proved effective in consistently
> switching heavily loaded vhost kthreads almost immediately.
> To: Josh Poimboeuf <>
> To: Jiri Kosina <>
> To: Miroslav Benes <>
> To: Petr Mladek <>
> To: Joe Lawrence <>
> To: "Michael S. Tsirkin" <>
> To: Jason Wang <>
> Cc:
> Cc:
> Cc:
> Cc:
> Cc:
> Signed-off-by: Seth Forshee (DigitalOcean) <>

Don't know enough about live patching to judge this.

I'll let livepatch maintainers judge this, and merge through
the livepatch tree if appropriate. For that:
Acked-by: Michael S. Tsirkin <>
but pls underestand this is more a 'looks ok superficially and
I don't have better ideas' than 'I have reviewed this thoroughly'.

> ---
> Seth Forshee (DigitalOcean) (2):
> livepatch: add an interface for safely switching kthreads
> vhost: check for pending livepatches from vhost worker kthreads
> drivers/vhost/vhost.c | 4 ++++
> include/linux/livepatch.h | 2 ++
> kernel/livepatch/transition.c | 11 +++++++++++
> 3 files changed, 17 insertions(+)
> ---
> base-commit: 5dc4c995db9eb45f6373a956eb1f69460e69e6d4
> change-id: 20230120-vhost-klp-switching-ba9a3ae38b8a
> Best regards,
> --
> Seth Forshee (DigitalOcean) <>

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