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Date:   Fri, 16 Mar 2018 10:51:13 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kathleen Chang <yt.chang@mediatek.com>
Cc:     mingo@redhat.com, matthias.bgg@gmail.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linux-mediatek@lists.infradead.org, Jonathan.JMChen@mediatek.com,
        wsd_upstream@mediatek.com, Matt Fleming <matt@codeblueprint.co.uk>
Subject: Re: update vruntime incorrectly When use rt_mutex
Message-ID: <20180316095113.GA4064@hirez.programming.kicks-ass.net>
References: <1521099370.7712.4.camel@mtkswgap22>
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On Thu, Mar 15, 2018 at 03:36:10PM +0800, Kathleen Chang wrote:
> hi,  
> 
> We found the vruntime might update incorrectly when use rt_mutex.

That's nice, on what kernel?

Also, your email is very hard to make sense of.

> <<abnormal case>>
> When the Task is waking, update vruntime incorrectly.
> 1.  When there is a CFS task (A) hold  rt_mutex_lock and the state is
> TASK_WAKING (on_rq=0),  a RT task (B) want to hold this rt_mutex_lock.
> Update vruntime incorrectly.
> 
>   RT task (B)
>      rt_mutex_setprio  (cfs->RT)  ->  Task is waking , and update
> vruntime
> 
>        queued = task_on_rq_queued(p); // task is waking, queued=0
>        running = task_current(rq, p);
>        if (queued)      /*   don't update vruntime here!   */
>                 dequeue_task(rq, p, queue_flag);                      
>        if (running)
>                 put_prev_task(rq, p);
>        
>        check_class_changed(rq, p, prev_class, oldprio);   -> 
>                 switched_from_fair -> 
>                       detach_task_cfs_rq  
>           ( due to task is waking, and bypass
> vruntime-=cfs_rq.min_vruntime) 
> 
> static void detach_task_cfs_rq(struct task_struct *p)
> {
>         struct sched_entity *se = &p->se;
>         struct cfs_rq *cfs_rq = cfs_rq_of(se);
> 
>         if (!vruntime_normalized(p)) {   // return 1, then p->state is
> TASK_WAKING
>                 /*
>                  * Fix up our vruntime so that the current sleep doesn't
>                  * cause 'unlimited' sleep bonus.
>                  */
>                 place_entity(cfs_rq, se, 0);
>                 check_vruntime(8, se, cfs_rq->min_vruntime);
>                 se->vruntime -= cfs_rq->min_vruntime;

So here we subtract min_vruntime,

>                 se->normalized = true;

this doesn't exist.. which makes me wonder what you're looking at,

>         }
> 
>         detach_entity_cfs_rq(se);
> }
> 
> // when p->state is TASK_WAKING, the task's vruntime is normalized 
> static inline bool vruntime_normalized(struct task_struct *p)
> {
> .....
>         if (!se->sum_exec_runtime || p->state == TASK_WAKING)
>                 return true;
> 
> }
> 
> 2.  When the task (A) which holds the rt_muex_lock unlock the
> rt_mutex_lock.
>      Task (A) must be on_rq=1 
> 
>     rt_mutex_setprio (RT->CFS)     
>        if (queued)
>                 enqueue_task(rq, p, queue_flag);    );     
>                     /* vruntime += cfs_rq.min_vruntime */

And here we're adding min_vruntime.

>        if (running)
>                 set_curr_task(rq, p);
> 
>     that result in vruntime accumulates

So what exactly is the problem?