Re: [patch v12 09/13] task isolation: add preempt notifier to sync per-CPU vmstat dirty info to thread info

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, Apr 27 2022 at 09:11, Thomas Gleixner wrote:
> On Tue, Mar 15 2022 at 12:31, Marcelo Tosatti wrote:
>> If a thread has task isolation activated, is preempted by thread B,
>> which marks vmstat information dirty, and is preempted back in,
>> one might return to userspace with vmstat dirty information on the 
>> CPU in question.
>>
>> To address this problem, add a preempt notifier that transfers vmstat dirty
>> information to TIF_TASK_ISOL thread flag.
>
> How does this compile with CONFIG_KVM=n?

Aside of that, the existance of this preempt notifier alone tells me
that this is either a design fail or has no design in the first place.

The state of vmstat does not matter at all at the point where a task is
scheduled in. It matters when an isolated task goes out to user space or
enters a VM.

We already have something similar in the exit to user path:

   tick_nohz_user_enter_prepare()

So you can do something like the below and have:

static inline void task_isol_exit_to_user_prepare(void)
{
        if (unlikely(current_needs_isol_exit_to_user())
        	__task_isol_exit_to_user_prepare();
}

where current_needs_isol_exit_to_user() is a simple check of either an
existing mechanism like

         task->syscall_work & SYSCALL_WORK_TASK_ISOL_EXIT

or of some new task isolation specific member of task_struct which is
placed so it is cache hot at that point:

        task->isol_work & SYSCALL_TASK_ISOL_EXIT

which is going to be almost zero overhead for any non isolated task.

It's trivial enough to encode the real stuff into task->isol_work and
I'm pretty sure, that a 32bit member is sufficient for that. There is
absolutely no need for a potential 64x64 bit feature matrix.

Thanks,

        tglx
---
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -142,6 +142,12 @@ void noinstr exit_to_user_mode(void)
 /* Workaround to allow gradual conversion of architecture code */
 void __weak arch_do_signal_or_restart(struct pt_regs *regs) { }
 
+static void exit_to_user_update_work(void)
+{
+	tick_nohz_user_enter_prepare();
+	task_isol_exit_to_user_prepare();
+}
+
 static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
 					    unsigned long ti_work)
 {
@@ -178,8 +184,7 @@ static unsigned long exit_to_user_mode_l
 		 */
 		local_irq_disable_exit_to_user();
 
-		/* Check if any of the above work has queued a deferred wakeup */
-		tick_nohz_user_enter_prepare();
+		exit_to_user_update_work();
 
 		ti_work = read_thread_flags();
 	}
@@ -194,8 +199,7 @@ static void exit_to_user_mode_prepare(st
 
 	lockdep_assert_irqs_disabled();
 
-	/* Flush pending rcuog wakeup before the last need_resched() check */
-	tick_nohz_user_enter_prepare();
+	exit_to_user_update_work();
 
 	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
 		ti_work = exit_to_user_mode_loop(regs, ti_work);