Re: [PATCH 2/2] vhost: check for pending livepatches from vhost worker kthreads
From: Petr Mladek <pmladek@suse.com>
Date: 2023-01-24 14:18:23
Also in:
kvm, live-patching, lkml, virtualization
On Fri 2023-01-20 16:12:22, Seth Forshee (DigitalOcean) wrote:
Livepatch relies on stack checking of sleeping tasks to switch kthreads, so a busy kthread can block a livepatch transition indefinitely. We've seen this happen fairly often with busy vhost kthreads.
To be precise, it would be "indefinitely" only when the kthread never sleeps. But yes. I believe that the problem is real. It might be almost impossible to livepatch some busy kthreads, especially when they have a dedicated CPU.
quoted hunk ↗ jump to hunk
Add a check to call klp_switch_current() from vhost_worker() when a livepatch is pending. In testing this allowed vhost kthreads to switch immediately when they had previously blocked livepatch transitions for long periods of time. Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org> --- drivers/vhost/vhost.c | 4 ++++ 1 file changed, 4 insertions(+)diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index cbe72bfd2f1f..d8624f1f2d64 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c@@ -366,6 +367,9 @@ static int vhost_worker(void *data) if (need_resched()) schedule(); } + + if (unlikely(klp_patch_pending(current))) + klp_switch_current();
I suggest to use the following intead: if (unlikely(klp_patch_pending(current))) klp_update_patch_state(current); We already use this in do_idle(). The reason is basically the same. It is almost impossible to livepatch the idle task when a CPU is very idle. klp_update_patch_state(current) does not check the stack. It switches the task immediately. It should be safe because the kthread never leaves vhost_worker(). It means that the same kthread could never re-enter this function and use the new code. Best Regards, Petr