On Mon 2025-03-24 14:49:09, Peter Zijlstra wrote: > > With the goal of deprecating / removing VOLUNTARY preempt, live-patch > needs to stop relying on cond_resched() to make forward progress. > > Instead, rely on schedule() with TASK_FREEZABLE set. Just like > live-patching, the freezer needs to be able to stop tasks in a safe / > known state.
> Compile tested only. > > Signed-off-by: Peter Zijlstra (Intel) <[email protected]> > --- > include/linux/livepatch_sched.h | 15 +++++-------- > include/linux/sched.h | 6 ----- > kernel/livepatch/transition.c | 30 ++++++------------------- > kernel/sched/core.c | 50 > +++++++---------------------------------- > 4 files changed, 21 insertions(+), 80 deletions(-) > > diff --git a/include/linux/livepatch_sched.h b/include/linux/livepatch_sched.h > index 013794fb5da0..7e8171226dd7 100644 > --- a/include/linux/livepatch_sched.h > +++ b/include/linux/livepatch_sched.h > @@ -3,27 +3,24 @@ > #define _LINUX_LIVEPATCH_SCHED_H_ > > #include <linux/jump_label.h> > -#include <linux/static_call_types.h> > +#include <linux/sched.h> > + > > #ifdef CONFIG_LIVEPATCH > > void __klp_sched_try_switch(void); > > -#if !defined(CONFIG_PREEMPT_DYNAMIC) || > !defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL) > - > DECLARE_STATIC_KEY_FALSE(klp_sched_try_switch_key); > > -static __always_inline void klp_sched_try_switch(void) > +static __always_inline void klp_sched_try_switch(struct task_struct *curr) > { > - if (static_branch_unlikely(&klp_sched_try_switch_key)) > + if (static_branch_unlikely(&klp_sched_try_switch_key) && > + READ_ONCE(curr->__state) & TASK_FREEZABLE) > __klp_sched_try_switch(); > } Do we really need to check the TASK_FREEZABLE state, please? My understanding is that TASK_FREEZABLE is set when kernel kthreads go into a "freezable" sleep, e.g. wait_event_freezable(). But __klp_sched_try_switch() should be safe when the task is not running and the stack is reliable. IMHO, it should be safe anytime it is being scheduled out. Note that wait_event_freezable() is a good location. It is usually called in the main loop of the kthread where the stack is small. So that the chance that it is not running a livepatched function is higher than on another random schedulable location. But we actually wanted to have it in cond_resched() because it might take a long time to reach the main loop, and sleep there. Best Regards, Petr
