linux-kernel - Re: [RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid starvation

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Message-ID: <20160825081141.GE10153@twins.programming.kicks-ass.net>
Date:   Thu, 25 Aug 2016 10:11:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <waiman.long@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>
Subject: Re: [RFC][PATCH 3/3] locking/mutex: Add lock handoff to avoid
 starvation

On Wed, Aug 24, 2016 at 03:50:10PM -0400, Waiman Long wrote:

> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -97,6 +97,8 @@ static void __mutex_handoff(struct mutex *lock, struct
> task_st
>         for (;;) {
>                 unsigned long old, new;
> 
> +               if ((owner & ~MUTEX_FLAG_ALL) != current)
> +                       break;
>                 new = (owner & MUTEX_FLAG_WAITERS);
>                 new |= (unsigned long)task;
> 
> I also think that the MUTEX_FLAG_HANDOFF bit needs to be cleared if the list
> is empty.
> 
> @@ -614,7 +633,7 @@ __mutex_lock_common(struct mutex *lock, long state,
> unsigned
>         mutex_remove_waiter(lock, &waiter, task);
>         /* set it to 0 if there are no waiters left: */
>         if (likely(list_empty(&lock->wait_list)))
> -               __mutex_clear_flag(lock, MUTEX_FLAG_WAITERS);
> +               __mutex_clear_flag(lock,
> MUTEX_FLAG_WAITERS|MUTEX_FLAG_HANDOFF);
> 
> Or we should try to reset the handoff bit after the while loop exit if the
> bit is still set.

Yes, I think you're right. I've also found another issue wrt WAITERS in
patch 1.

I'm not trying to get aim7 running to see if I can reproduce Jason's
results and verify things.