Re: [patch v11 05/13] task isolation: sync vmstats on return to userspace

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 04, 2022 at 02:35:42PM -0300, Marcelo Tosatti wrote:
> The logic to disable vmstat worker thread, when entering
> nohz full, does not cover all scenarios. For example, it is possible
> for the following to happen:
> 
> 1) enter nohz_full, which calls refresh_cpu_vm_stats, syncing the stats.
> 2) app runs mlock, which increases counters for mlock'ed pages.
> 3) start -RT loop
> 
> Since refresh_cpu_vm_stats from nohz_full logic can happen _before_
> the mlock, vmstat shepherd can restart vmstat worker thread on
> the CPU in question.
> 
> To fix this, use the task isolation prctl interface to quiesce 
> deferred actions when returning to userspace.
> 
> This patch adds hooks to fork and exit code paths.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> ---
> v11: fold patch to add task_isol_exit hooks (Frederic)
>      Use _TIF_TASK_ISOL bit on thread flags (Frederic)
>      
> v6: modify exit_to_user_mode_loop to cover exceptions and interrupts
> v5: no changes
> v4: add oneshot mode support
> 
>  include/linux/task_isolation.h |   16 ++++++++++++++++
>  include/linux/vmstat.h         |    8 ++++++++
>  kernel/entry/common.c          |   15 +++++++++++----
>  kernel/task_isolation.c        |   21 +++++++++++++++++++++
>  mm/vmstat.c                    |   21 +++++++++++++++++++++
>  5 files changed, 77 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6/include/linux/task_isolation.h
> ===================================================================
> --- linux-2.6.orig/include/linux/task_isolation.h
> +++ linux-2.6/include/linux/task_isolation.h
> @@ -27,6 +27,13 @@ static inline void task_isol_free(struct
>  		__task_isol_free(tsk);
>  }
>  
> +void __task_isol_exit(struct task_struct *tsk);
> +static inline void task_isol_exit(struct task_struct *tsk)
> +{
> +	if (tsk->task_isol_info)
> +		__task_isol_exit(tsk);
> +}
> +
>  int prctl_task_isol_feat_get(unsigned long arg2, unsigned long arg3,
>  			     unsigned long arg4, unsigned long arg5);
>  int prctl_task_isol_cfg_get(unsigned long arg2, unsigned long arg3,
> @@ -40,12 +47,22 @@ int prctl_task_isol_activate_set(unsigne
>  
>  int __copy_task_isol(struct task_struct *tsk);
>  
> +void task_isol_exit_to_user_mode(void);
> +
>  #else
>  
> +static inline void task_isol_exit_to_user_mode(void)
> +{
> +}
> +
>  static inline void task_isol_free(struct task_struct *tsk)
>  {
>  }
>  
> +static inline void task_isol_exit(struct task_struct *tsk)
> +{
> +}
> +
>  static inline int prctl_task_isol_feat_get(unsigned long arg2,
>  					   unsigned long arg3,
>  					   unsigned long arg4,
> Index: linux-2.6/include/linux/vmstat.h
> ===================================================================
> --- linux-2.6.orig/include/linux/vmstat.h
> +++ linux-2.6/include/linux/vmstat.h
> @@ -21,6 +21,14 @@ int sysctl_vm_numa_stat_handler(struct c
>  		void *buffer, size_t *length, loff_t *ppos);
>  #endif
>  
> +#if defined(CONFIG_SMP) && defined(CONFIG_TASK_ISOLATION)
> +void sync_vmstat(void);
> +#else
> +static inline void sync_vmstat(void)
> +{
> +}
> +#endif
> +
>  struct reclaim_stat {
>  	unsigned nr_dirty;
>  	unsigned nr_unqueued_dirty;
> Index: linux-2.6/kernel/entry/common.c
> ===================================================================
> --- linux-2.6.orig/kernel/entry/common.c
> +++ linux-2.6/kernel/entry/common.c
> @@ -6,6 +6,7 @@
>  #include <linux/livepatch.h>
>  #include <linux/audit.h>
>  #include <linux/tick.h>
> +#include <linux/task_isolation.h>
>  
>  #include "common.h"
>  
> @@ -174,6 +175,9 @@ static unsigned long exit_to_user_mode_l
>  		if (ti_work & _TIF_NOTIFY_RESUME)
>  			tracehook_notify_resume(regs);
>  
> +		if (ti_work & _TIF_TASK_ISOL)
> +			task_isol_exit_to_user_mode();
> +
>  		/* Architecture specific TIF work */
>  		arch_exit_to_user_mode_work(regs, ti_work);
>  
> Index: linux-2.6/kernel/task_isolation.c
> ===================================================================
> --- linux-2.6.orig/kernel/task_isolation.c
> +++ linux-2.6/kernel/task_isolation.c
> @@ -18,6 +18,12 @@
>  #include <linux/sysfs.h>
>  #include <linux/init.h>
>  #include <linux/sched/task.h>
> +#include <linux/mm.h>
> +#include <linux/vmstat.h>
> +
> +void __task_isol_exit(struct task_struct *tsk)
> +{
> +}
>  
>  void __task_isol_free(struct task_struct *tsk)
>  {
> @@ -251,6 +257,9 @@ static int cfg_feat_quiesce_set(unsigned
>  	task_isol_info->quiesce_mask = i_qctrl->quiesce_mask;
>  	task_isol_info->oneshot_mask = i_qctrl->quiesce_oneshot_mask;
>  	task_isol_info->conf_mask |= ISOL_F_QUIESCE;
> +	if (task_isol_info->quiesce_mask & ISOL_F_QUIESCE_VMSTATS)
> +		set_thread_flag(TIF_TASK_ISOL);

Should you check if (i->active_mask == ISOL_F_QUIESCE) before setting the
flag?

> +
>  	ret = 0;
>  
>  out_free:
> @@ -303,6 +312,7 @@ int __copy_task_isol(struct task_struct
>  		new_info->active_mask = info->active_mask;
>  
>  	tsk->task_isol_info = new_info;
> +	set_ti_thread_flag(task_thread_info(tsk), TIF_TASK_ISOL);

Same here?

>  
>  	return 0;

Thanks.