linux-kernel - Re: [PATCH v5 08/24] sched: Introduce per memory space current virtual cpu id

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Date:   Wed, 9 Nov 2022 10:04:33 -0500
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Christian Brauner <brauner@...nel.org>,
        Florian Weimer <fw@...eb.enyo.de>, David.Laight@...lab.com,
        carlos@...hat.com, Peter Oskolkov <posk@...k.io>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Chris Kennelly <ckennelly@...gle.com>
Subject: Re: [PATCH v5 08/24] sched: Introduce per memory space current
 virtual cpu id

On 2022-11-09 04:28, Peter Zijlstra wrote:
> On Thu, Nov 03, 2022 at 04:03:43PM -0400, Mathieu Desnoyers wrote:
> 
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index 08969f5aa38d..6a2323266942 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1047,6 +1047,10 @@ static struct task_struct *dup_task_struct(struct task_struct *orig, int node)
>>   	tsk->reported_split_lock = 0;
>>   #endif
>>   
>> +#ifdef CONFIG_SCHED_MM_VCPU
>> +	tsk->mm_vcpu = -1;
>> +	tsk->mm_vcpu_active = 0;
>> +#endif
>>   	return tsk;
>>   
>>   free_stack:
> 
> Note how the above hunk does exactly the same as the below thunk, and I
> think they're even on the same code-path.
> 
> How about moving all of this to __sched_fork() or something?
> 
>> @@ -1579,6 +1586,7 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
>>   
>>   	tsk->mm = mm;
>>   	tsk->active_mm = mm;
>> +	sched_vcpu_fork(tsk);
>>   	return 0;
>>   }
> 
>> +void sched_vcpu_fork(struct task_struct *t)
>> +{
>> +	WARN_ON_ONCE((t->flags & PF_KTHREAD) || !t->mm);
>> +	t->mm_vcpu = -1;
>> +	t->mm_vcpu_active = 1;
>> +}

Let's look at how things are brought up in copy_process():

         p = dup_task_struct(current, node);
           tsk->mm_vcpu = -1;
           tsk->mm_vcpu_active = 0;
[...]

         /* Perform scheduler related setup. Assign this task to a CPU. */
         retval = sched_fork(clone_flags, p);

-> I presume that from this point the task is observable by the scheduler. However, tsk->mm does not point to the new mm yet.
The "mm_vcpu_active" flag == 0 prevents the scheduler from trying to poke into the wrong mm vcpu_id bitmaps across early mm-struct
lifetime (clone/fork), late in the mm-struct lifetime (exit), and across reset of the mm-struct (exec).

[...]

         retval = copy_mm(clone_flags, p);
             new_mm = dup_mm(old_mm)
               mm_init(new_mm)
                  mm_init_vcpu(new_mm)
-> At this point it becomes OK for the scheduler to poke into the tsk->mm vcpu_id bitmaps. Therefore, sched_vcpu_fork() sets
mm_vcpu_active flag = 1.
              sched_vcpu_fork(tsk)
-> From this point the scheduler can poke into tsk->mm's vcpu_id bitmaps.

So what I think we should to do here is to remove the extra "t->mm_vcpu = -1;" assignment from sched_vcpu_fork(), because
it has already been set by dup_task_struct. We could actually turn that into a "WARN_ON_ONCE(t->mm_vcpu != -1)".

However, if my understanding is correct, keeping "tsk->mm_vcpu_active = 0;" early in dup_task_struct and "tsk->mm_vcpu_active = 1"
in sched_vcpu_fork() after the new mm is initialized is really important. Or am I missing something ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com