Re: [RFC PATCH 07/86] Revert "livepatch,sched: Add livepatch task switching to cond_resched()"

[Ankur Arora <ankur.a.arora@oracle.com>]

Steven Rostedt <rostedt@goodmis.org> writes:

> On Tue,  7 Nov 2023 13:56:53 -0800
> Ankur Arora <ankur.a.arora@oracle.com> wrote:
>
>> This reverts commit e3ff7c609f39671d1aaff4fb4a8594e14f3e03f8.
>>
>> Note that removing this commit reintroduces "live patches failing to
>> complete within a reasonable amount of time due to CPU-bound kthreads."
>>
>> Unfortunately this fix depends quite critically on PREEMPT_DYNAMIC and
>> existence of cond_resched() so this will need an alternate fix.
>>
>
> Then it would probably be a good idea to Cc the live patching maintainers!

Indeed. Could have sworn that I had. But clearly not.

Apologies and thanks for adding them.

>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>>  include/linux/livepatch.h       |   1 -
>>  include/linux/livepatch_sched.h |  29 ---------
>>  include/linux/sched.h           |  20 ++----
>>  kernel/livepatch/core.c         |   1 -
>>  kernel/livepatch/transition.c   | 107 +++++---------------------------
>>  kernel/sched/core.c             |  64 +++----------------
>>  6 files changed, 28 insertions(+), 194 deletions(-)
>>  delete mode 100644 include/linux/livepatch_sched.h
>>
>> diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
>> index 9b9b38e89563..293e29960c6e 100644
>> --- a/include/linux/livepatch.h
>> +++ b/include/linux/livepatch.h
>> @@ -13,7 +13,6 @@
>>  #include <linux/ftrace.h>
>>  #include <linux/completion.h>
>>  #include <linux/list.h>
>> -#include <linux/livepatch_sched.h>
>>
>>  #if IS_ENABLED(CONFIG_LIVEPATCH)
>>
>> diff --git a/include/linux/livepatch_sched.h b/include/linux/livepatch_sched.h
>> deleted file mode 100644
>> index 013794fb5da0..000000000000
>> --- a/include/linux/livepatch_sched.h
>> +++ /dev/null
>> @@ -1,29 +0,0 @@
>> -/* SPDX-License-Identifier: GPL-2.0-or-later */
>> -#ifndef _LINUX_LIVEPATCH_SCHED_H_
>> -#define _LINUX_LIVEPATCH_SCHED_H_
>> -
>> -#include <linux/jump_label.h>
>> -#include <linux/static_call_types.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)
>> -{
>> -	if (static_branch_unlikely(&klp_sched_try_switch_key))
>> -		__klp_sched_try_switch();
>> -}
>> -
>> -#endif /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
>> -
>> -#else /* !CONFIG_LIVEPATCH */
>> -static inline void klp_sched_try_switch(void) {}
>> -static inline void __klp_sched_try_switch(void) {}
>> -#endif /* CONFIG_LIVEPATCH */
>> -
>> -#endif /* _LINUX_LIVEPATCH_SCHED_H_ */
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index 5bdf80136e42..c5b0ef1ecfe4 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -36,7 +36,6 @@
>>  #include <linux/seqlock.h>
>>  #include <linux/kcsan.h>
>>  #include <linux/rv.h>
>> -#include <linux/livepatch_sched.h>
>>  #include <asm/kmap_size.h>
>>
>>  /* task_struct member predeclarations (sorted alphabetically): */
>> @@ -2087,9 +2086,6 @@ extern int __cond_resched(void);
>>
>>  #if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>>
>> -void sched_dynamic_klp_enable(void);
>> -void sched_dynamic_klp_disable(void);
>> -
>>  DECLARE_STATIC_CALL(cond_resched, __cond_resched);
>>
>>  static __always_inline int _cond_resched(void)
>> @@ -2098,7 +2094,6 @@ static __always_inline int _cond_resched(void)
>>  }
>>
>>  #elif defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
>> -
>>  extern int dynamic_cond_resched(void);
>>
>>  static __always_inline int _cond_resched(void)
>> @@ -2106,25 +2101,20 @@ static __always_inline int _cond_resched(void)
>>  	return dynamic_cond_resched();
>>  }
>>
>> -#else /* !CONFIG_PREEMPTION */
>> +#else
>>
>>  static inline int _cond_resched(void)
>>  {
>> -	klp_sched_try_switch();
>>  	return __cond_resched();
>>  }
>>
>> -#endif /* PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
>> +#endif /* CONFIG_PREEMPT_DYNAMIC */
>>
>> -#else /* CONFIG_PREEMPTION && !CONFIG_PREEMPT_DYNAMIC */
>> +#else
>>
>> -static inline int _cond_resched(void)
>> -{
>> -	klp_sched_try_switch();
>> -	return 0;
>> -}
>> +static inline int _cond_resched(void) { return 0; }
>>
>> -#endif /* !CONFIG_PREEMPTION || CONFIG_PREEMPT_DYNAMIC */
>> +#endif /* !defined(CONFIG_PREEMPTION) || defined(CONFIG_PREEMPT_DYNAMIC) */
>>
>>  #define cond_resched() ({			\
>>  	__might_resched(__FILE__, __LINE__, 0);	\
>> diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
>> index 61328328c474..fc851455740c 100644
>> --- a/kernel/livepatch/core.c
>> +++ b/kernel/livepatch/core.c
>> @@ -33,7 +33,6 @@
>>   *
>>   * - klp_ftrace_handler()
>>   * - klp_update_patch_state()
>> - * - __klp_sched_try_switch()
>>   */
>>  DEFINE_MUTEX(klp_mutex);
>>
>> diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
>> index e54c3d60a904..70bc38f27af7 100644
>> --- a/kernel/livepatch/transition.c
>> +++ b/kernel/livepatch/transition.c
>> @@ -9,7 +9,6 @@
>>
>>  #include <linux/cpu.h>
>>  #include <linux/stacktrace.h>
>> -#include <linux/static_call.h>
>>  #include "core.h"
>>  #include "patch.h"
>>  #include "transition.h"
>> @@ -27,25 +26,6 @@ static int klp_target_state = KLP_UNDEFINED;
>>
>>  static unsigned int klp_signals_cnt;
>>
>> -/*
>> - * When a livepatch is in progress, enable klp stack checking in
>> - * cond_resched().  This helps CPU-bound kthreads get patched.
>> - */
>> -#if defined(CONFIG_PREEMPT_DYNAMIC) && defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>> -
>> -#define klp_cond_resched_enable() sched_dynamic_klp_enable()
>> -#define klp_cond_resched_disable() sched_dynamic_klp_disable()
>> -
>> -#else /* !CONFIG_PREEMPT_DYNAMIC || !CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
>> -
>> -DEFINE_STATIC_KEY_FALSE(klp_sched_try_switch_key);
>> -EXPORT_SYMBOL(klp_sched_try_switch_key);
>> -
>> -#define klp_cond_resched_enable() static_branch_enable(&klp_sched_try_switch_key)
>> -#define klp_cond_resched_disable() static_branch_disable(&klp_sched_try_switch_key)
>> -
>> -#endif /* CONFIG_PREEMPT_DYNAMIC && CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
>> -
>>  /*
>>   * This work can be performed periodically to finish patching or unpatching any
>>   * "straggler" tasks which failed to transition in the first attempt.
>> @@ -194,8 +174,8 @@ void klp_update_patch_state(struct task_struct *task)
>>  	 * barrier (smp_rmb) for two cases:
>>  	 *
>>  	 * 1) Enforce the order of the TIF_PATCH_PENDING read and the
>> -	 *    klp_target_state read.  The corresponding write barriers are in
>> -	 *    klp_init_transition() and klp_reverse_transition().
>> +	 *    klp_target_state read.  The corresponding write barrier is in
>> +	 *    klp_init_transition().
>>  	 *
>>  	 * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read
>>  	 *    of func->transition, if klp_ftrace_handler() is called later on
>> @@ -363,44 +343,6 @@ static bool klp_try_switch_task(struct task_struct *task)
>>  	return !ret;
>>  }
>>
>> -void __klp_sched_try_switch(void)
>> -{
>> -	if (likely(!klp_patch_pending(current)))
>> -		return;
>> -
>> -	/*
>> -	 * This function is called from cond_resched() which is called in many
>> -	 * places throughout the kernel.  Using the klp_mutex here might
>> -	 * deadlock.
>> -	 *
>> -	 * Instead, disable preemption to prevent racing with other callers of
>> -	 * klp_try_switch_task().  Thanks to task_call_func() they won't be
>> -	 * able to switch this task while it's running.
>> -	 */
>> -	preempt_disable();
>> -
>> -	/*
>> -	 * Make sure current didn't get patched between the above check and
>> -	 * preempt_disable().
>> -	 */
>> -	if (unlikely(!klp_patch_pending(current)))
>> -		goto out;
>> -
>> -	/*
>> -	 * Enforce the order of the TIF_PATCH_PENDING read above and the
>> -	 * klp_target_state read in klp_try_switch_task().  The corresponding
>> -	 * write barriers are in klp_init_transition() and
>> -	 * klp_reverse_transition().
>> -	 */
>> -	smp_rmb();
>> -
>> -	klp_try_switch_task(current);
>> -
>> -out:
>> -	preempt_enable();
>> -}
>> -EXPORT_SYMBOL(__klp_sched_try_switch);
>> -
>>  /*
>>   * Sends a fake signal to all non-kthread tasks with TIF_PATCH_PENDING set.
>>   * Kthreads with TIF_PATCH_PENDING set are woken up.
>> @@ -507,8 +449,7 @@ void klp_try_complete_transition(void)
>>  		return;
>>  	}
>>
>> -	/* Done!  Now cleanup the data structures. */
>> -	klp_cond_resched_disable();
>> +	/* we're done, now cleanup the data structures */
>>  	patch = klp_transition_patch;
>>  	klp_complete_transition();
>>
>> @@ -560,8 +501,6 @@ void klp_start_transition(void)
>>  			set_tsk_thread_flag(task, TIF_PATCH_PENDING);
>>  	}
>>
>> -	klp_cond_resched_enable();
>> -
>>  	klp_signals_cnt = 0;
>>  }
>>
>> @@ -617,9 +556,8 @@ void klp_init_transition(struct klp_patch *patch, int state)
>>  	 * see a func in transition with a task->patch_state of KLP_UNDEFINED.
>>  	 *
>>  	 * Also enforce the order of the klp_target_state write and future
>> -	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() and
>> -	 * __klp_sched_try_switch() don't set a task->patch_state to
>> -	 * KLP_UNDEFINED.
>> +	 * TIF_PATCH_PENDING writes to ensure klp_update_patch_state() doesn't
>> +	 * set a task->patch_state to KLP_UNDEFINED.
>>  	 */
>>  	smp_wmb();
>>
>> @@ -655,10 +593,14 @@ void klp_reverse_transition(void)
>>  		 klp_target_state == KLP_PATCHED ? "patching to unpatching" :
>>  						   "unpatching to patching");
>>
>> +	klp_transition_patch->enabled = !klp_transition_patch->enabled;
>> +
>> +	klp_target_state = !klp_target_state;
>> +
>>  	/*
>>  	 * Clear all TIF_PATCH_PENDING flags to prevent races caused by
>> -	 * klp_update_patch_state() or __klp_sched_try_switch() running in
>> -	 * parallel with the reverse transition.
>> +	 * klp_update_patch_state() running in parallel with
>> +	 * klp_start_transition().
>>  	 */
>>  	read_lock(&tasklist_lock);
>>  	for_each_process_thread(g, task)
>> @@ -668,28 +610,9 @@ void klp_reverse_transition(void)
>>  	for_each_possible_cpu(cpu)
>>  		clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING);
>>
>> -	/*
>> -	 * Make sure all existing invocations of klp_update_patch_state() and
>> -	 * __klp_sched_try_switch() see the cleared TIF_PATCH_PENDING before
>> -	 * starting the reverse transition.
>> -	 */
>> +	/* Let any remaining calls to klp_update_patch_state() complete */
>>  	klp_synchronize_transition();
>>
>> -	/*
>> -	 * All patching has stopped, now re-initialize the global variables to
>> -	 * prepare for the reverse transition.
>> -	 */
>> -	klp_transition_patch->enabled = !klp_transition_patch->enabled;
>> -	klp_target_state = !klp_target_state;
>> -
>> -	/*
>> -	 * Enforce the order of the klp_target_state write and the
>> -	 * TIF_PATCH_PENDING writes in klp_start_transition() to ensure
>> -	 * klp_update_patch_state() and __klp_sched_try_switch() don't set
>> -	 * task->patch_state to the wrong value.
>> -	 */
>> -	smp_wmb();
>> -
>>  	klp_start_transition();
>>  }
>>
>> @@ -703,9 +626,9 @@ void klp_copy_process(struct task_struct *child)
>>  	 * the task flag up to date with the parent here.
>>  	 *
>>  	 * The operation is serialized against all klp_*_transition()
>> -	 * operations by the tasklist_lock. The only exceptions are
>> -	 * klp_update_patch_state(current) and __klp_sched_try_switch(), but we
>> -	 * cannot race with them because we are current.
>> +	 * operations by the tasklist_lock. The only exception is
>> +	 * klp_update_patch_state(current), but we cannot race with
>> +	 * that because we are current.
>>  	 */
>>  	if (test_tsk_thread_flag(current, TIF_PATCH_PENDING))
>>  		set_tsk_thread_flag(child, TIF_PATCH_PENDING);
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 0e8764d63041..b43fda3c5733 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -8597,7 +8597,6 @@ EXPORT_STATIC_CALL_TRAMP(might_resched);
>>  static DEFINE_STATIC_KEY_FALSE(sk_dynamic_cond_resched);
>>  int __sched dynamic_cond_resched(void)
>>  {
>> -	klp_sched_try_switch();
>>  	if (!static_branch_unlikely(&sk_dynamic_cond_resched))
>>  		return 0;
>>  	return __cond_resched();
>> @@ -8746,17 +8745,13 @@ int sched_dynamic_mode(const char *str)
>>  #error "Unsupported PREEMPT_DYNAMIC mechanism"
>>  #endif
>>
>> -DEFINE_MUTEX(sched_dynamic_mutex);
>> -static bool klp_override;
>> -
>> -static void __sched_dynamic_update(int mode)
>> +void sched_dynamic_update(int mode)
>>  {
>>  	/*
>>  	 * Avoid {NONE,VOLUNTARY} -> FULL transitions from ever ending up in
>>  	 * the ZERO state, which is invalid.
>>  	 */
>> -	if (!klp_override)
>> -		preempt_dynamic_enable(cond_resched);
>> +	preempt_dynamic_enable(cond_resched);
>>  	preempt_dynamic_enable(might_resched);
>>  	preempt_dynamic_enable(preempt_schedule);
>>  	preempt_dynamic_enable(preempt_schedule_notrace);
>> @@ -8764,79 +8759,36 @@ static void __sched_dynamic_update(int mode)
>>
>>  	switch (mode) {
>>  	case preempt_dynamic_none:
>> -		if (!klp_override)
>> -			preempt_dynamic_enable(cond_resched);
>> +		preempt_dynamic_enable(cond_resched);
>>  		preempt_dynamic_disable(might_resched);
>>  		preempt_dynamic_disable(preempt_schedule);
>>  		preempt_dynamic_disable(preempt_schedule_notrace);
>>  		preempt_dynamic_disable(irqentry_exit_cond_resched);
>> -		if (mode != preempt_dynamic_mode)
>> -			pr_info("Dynamic Preempt: none\n");
>> +		pr_info("Dynamic Preempt: none\n");
>>  		break;
>>
>>  	case preempt_dynamic_voluntary:
>> -		if (!klp_override)
>> -			preempt_dynamic_enable(cond_resched);
>> +		preempt_dynamic_enable(cond_resched);
>>  		preempt_dynamic_enable(might_resched);
>>  		preempt_dynamic_disable(preempt_schedule);
>>  		preempt_dynamic_disable(preempt_schedule_notrace);
>>  		preempt_dynamic_disable(irqentry_exit_cond_resched);
>> -		if (mode != preempt_dynamic_mode)
>> -			pr_info("Dynamic Preempt: voluntary\n");
>> +		pr_info("Dynamic Preempt: voluntary\n");
>>  		break;
>>
>>  	case preempt_dynamic_full:
>> -		if (!klp_override)
>> -			preempt_dynamic_disable(cond_resched);
>> +		preempt_dynamic_disable(cond_resched);
>>  		preempt_dynamic_disable(might_resched);
>>  		preempt_dynamic_enable(preempt_schedule);
>>  		preempt_dynamic_enable(preempt_schedule_notrace);
>>  		preempt_dynamic_enable(irqentry_exit_cond_resched);
>> -		if (mode != preempt_dynamic_mode)
>> -			pr_info("Dynamic Preempt: full\n");
>> +		pr_info("Dynamic Preempt: full\n");
>>  		break;
>>  	}
>>
>>  	preempt_dynamic_mode = mode;
>>  }
>>
>> -void sched_dynamic_update(int mode)
>> -{
>> -	mutex_lock(&sched_dynamic_mutex);
>> -	__sched_dynamic_update(mode);
>> -	mutex_unlock(&sched_dynamic_mutex);
>> -}
>> -
>> -#ifdef CONFIG_HAVE_PREEMPT_DYNAMIC_CALL
>> -
>> -static int klp_cond_resched(void)
>> -{
>> -	__klp_sched_try_switch();
>> -	return __cond_resched();
>> -}
>> -
>> -void sched_dynamic_klp_enable(void)
>> -{
>> -	mutex_lock(&sched_dynamic_mutex);
>> -
>> -	klp_override = true;
>> -	static_call_update(cond_resched, klp_cond_resched);
>> -
>> -	mutex_unlock(&sched_dynamic_mutex);
>> -}
>> -
>> -void sched_dynamic_klp_disable(void)
>> -{
>> -	mutex_lock(&sched_dynamic_mutex);
>> -
>> -	klp_override = false;
>> -	__sched_dynamic_update(preempt_dynamic_mode);
>> -
>> -	mutex_unlock(&sched_dynamic_mutex);
>> -}
>> -
>> -#endif /* CONFIG_HAVE_PREEMPT_DYNAMIC_CALL */
>> -
>>  static int __init setup_preempt_mode(char *str)
>>  {
>>  	int mode = sched_dynamic_mode(str);


--
ankur