Re: [PATCH] context_tracking: Add comments on interface and internals

[Frederic Weisbecker]

2012/12/13 Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>:
> On Thu, 13 Dec 2012 21:57:05 +0100
> Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
>
>> This subsystem lacks many explanations on its purpose and
>> design. Add these missing comments.
>
> Thanks, it helps.
>
>> --- a/kernel/context_tracking.c
>> +++ b/kernel/context_tracking.c
>> @@ -1,3 +1,19 @@
>> +/*
>> + * Context tracking: Probe on high level context boundaries such as kernel
>> + * and userspace. This includes syscalls and exceptions entry/exit.
>> + *
>> + * This is used by RCU to remove its dependency to the timer tick while a CPU
>> + * runs in userspace.
>
> "on the timer tick"

Oops, will fix, along with the other spelling issues you reported.

>
>> + *
>> + *  Started by Frederic Weisbecker:
>> + *
>> + * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@xxxxxxxxxx>
>> + *
>> + * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
>> + * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
>> + *
>> + */
>> +
>>  #include <linux/context_tracking.h>
>>  #include <linux/rcupdate.h>
>>  #include <linux/sched.h>
>> @@ -6,8 +22,8 @@
>>
>>  struct context_tracking {
>>       /*
>> -      * When active is false, hooks are not set to
>> -      * minimize overhead: TIF flags are cleared
>> +      * When active is false, hooks are unset in order
>> +      * to minimize overhead: TIF flags are cleared
>>        * and calls to user_enter/exit are ignored. This
>>        * may be further optimized using static keys.
>>        */
>> @@ -24,6 +40,16 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
>>  #endif
>>  };
>>
>> +/**
>> + * user_enter - Inform the context tracking that the CPU is going to
>> + *              enter in userspace mode.
>
> s/in //
>
>> + *
>> + * This function must be called right before we switch from the kernel
>> + * to the user space, when the last remaining kernel instructions to execute
>
> s/the user space/userspace/
>
>> + * are low level arch code that perform the resuming to userspace.
>
> This is a bit vague - what is "right before"?  What happens if this is
> done a few instructions early?  I mean, what exactly is the requirement
> here?  Might it be something like "after the last rcu_foo operation"?
>
> IOW, if the call to user_enter() were moved earlier and earlier, at
> what point would the kernel gain a bug?  What caused that bug?

That's indeed too vague. So as long as RCU is the only user of this,
the only rule is: call user_enter() when you're about to resume in
userspace and you're sure there will be no use of RCU until we return
to the kernel. Here the precision on when to call that wrt. kernel ->
user transition step doesn't matter much. This is only about RCU usage
correctness.

Now this context tracking will soon be used by the cputime subsystem
in order to implement a generic tickless cputime accounting. The
precision induced by the probe location in kernel/user transition will
have an effect on cputime accounting precision. But even there this
shouldn't matter much because this will have a per-jiffies
granularity. This may evolve in the future with a nanosec granularity
but then it will be up to archs to place the probes closer to the real
kernel/user boundaries.

Anyway, I'll comment on the RCU requirement for now and extend the
comments to explain the cputime precision issue when I'll add the
cputime bits.

>
>> + * This call supports re-entrancy.
>
> Presumably the explanation for user_exit() applies here.

Not sure what you mean here.

>
>> + */
>>  void user_enter(void)
>>  {
>>       unsigned long flags;
>> @@ -39,40 +65,68 @@ void user_enter(void)
>>       if (in_interrupt())
>>               return;
>>
>> +     /* Kernel thread aren't supposed to go to userspace */
>
> s/thread/threads/
>
>>       WARN_ON_ONCE(!current->mm);
>>
>>       local_irq_save(flags);
>>       if (__this_cpu_read(context_tracking.active) &&
>>           __this_cpu_read(context_tracking.state) != IN_USER) {
>>               __this_cpu_write(context_tracking.state, IN_USER);
>> +             /*
>> +              * At this stage, only low level arch entry code remains and
>> +              * then we'll run in userspace. We can assume there won't we
>
> s/we/be/
>
>> +              * any RCU read-side critical section until the next call to
>> +              * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
>> +              * on the tick.
>> +              */
>>               rcu_user_enter();
>>       }
>>       local_irq_restore(flags);
>>  }
>>
>> +
>> +/**
>> + * user_exit - Inform the context tracking that the CPU is
>> + *             exiting userspace mode and entering the kernel.
>> + *
>> + * This function must be called right before we run any high level kernel
>> + * code (ie: anything that is not low level arch entry code) after we entered
>> + * the kernel from userspace.
>
> Also a very vague spec.

You're right, as for user_enter(), I'll insist on the RCU and cputime
requirements.

[...]
>> +/**
>> + * context_tracking_task_switch - context switch the syscall hooks
>> + *
>> + * The context tracking uses the syscall slow path to implement its user-kernel
>> + * boundaries hooks on syscalls. This way it doesn't impact the syscall fast
>> + * path on CPUs that don't do context tracking.
>> + *
>> + * But we need to clear the flag on the previous task because it may later
>> + * migrate to some CPU that doesn't do the context tracking. As such the TIF
>> + * flag may not be desired there.
>> + */
>>  void context_tracking_task_switch(struct task_struct *prev,
>>                            struct task_struct *next)
>>  {
>
> It's mainly this bit which makes me wonder why the code is in lib/.  Is
> there any conceivable prospect that any other subsystem will use this
> code for anything?

So that's because of that cputime accounting on dynticks CPUs which
will need to know about user/kernel transitions. I'm preparing that
for the 3.9 merge window.

Thanks.
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