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SubjectRe: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads
On Fri, Jan 27, 2023 at 01:09:02PM +0100, Petr Mladek wrote:
> There might actually be two possibilities why the transition fails
> too often:
>
> 1. The task might be in the running state most of the time. Therefore
> the backtrace is not reliable most of the time.
>
> In this case, some cooperation with the scheduler would really
> help. We would need to stop the task and check the stack
> when it is stopped. Something like the patch you proposed.

This is the situation we are encountering.

> 2. The task might be sleeping but almost always in a livepatched
> function. Therefore it could not be transitioned.
>
> It might be the case with vhost_worker(). The main loop is "tiny".
> The kthread probaly spends most of the time with processing
> a vhost_work. And if the "works" are livepatched...
>
> In this case, it would help to call klp_try_switch_task(current)
> in the main loop in vhost_worker(). It would always succeed
> when vhost_worker() is not livepatched on its own.
>
> Note that even this would not help with kPatch when a single
> vhost_work might need more than the 1 minute timout to get proceed.
>
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index f1b25ec581e0..06746095a724 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -9,6 +9,7 @@
> >
> > #include <linux/cpu.h>
> > #include <linux/stacktrace.h>
> > +#include <linux/stop_machine.h>
> > #include "core.h"
> > #include "patch.h"
> > #include "transition.h"
> > @@ -334,6 +335,16 @@ static bool klp_try_switch_task(struct task_struct *task)
> > return !ret;
> > }
> >
> > +static int __stop_try_switch(void *arg)
> > +{
> > + return klp_try_switch_task(arg) ? 0 : -EBUSY;
> > +}
> > +
> > +static bool klp_try_switch_task_harder(struct task_struct *task)
> > +{
> > + return !stop_one_cpu(task_cpu(task), __stop_try_switch, task);
> > +}
> > +
> > /*
> > * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
> > * Kthreads with TIF_PATCH_PENDING set are woken up.
>
> Nice. I am surprised that it can be implemented so easily.

Yes, that's a neat solution. I will give it a try.

AIUI this still doesn't help for architectures without a reliable
stacktrace though, right? So we probably should only try this for
architectures which do have relaible stacktraces.

Thanks,
Seth

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