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Message-ID: <56A23195.4000802@hpe.com>
Date: Fri, 22 Jan 2016 08:41:41 -0500
From: Waiman Long <waiman.long@....com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Ding Tianhong <dingtianhong@...wei.com>,
Ingo Molnar <mingo@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Davidlohr Bueso <dave@...olabs.net>,
Linus Torvalds <torvalds@...ux-foundation.org>,
"Paul E. McKenney" <paulmck@...ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Will Deacon <Will.Deacon@....com>,
Jason Low <jason.low2@...com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
is not NULL.
On 01/22/2016 03:54 AM, Peter Zijlstra wrote:
> On Thu, Jan 21, 2016 at 06:02:34PM -0500, Waiman Long wrote:
>> This patch attempts to fix this live-lock condition by enabling the
>> a woken task in the wait list to enter optimistic spinning loop itself
>> with precedence over the ones in the OSQ. This should prevent the
>> live-lock
>> condition from happening.
>
> So I think having the top waiter going back in to contend on the OSQ is
> an excellent idea, but I'm not sure the wlh_spinning thing is important.
Yes, that is optional. I put it there just to make it is more likely for
the waiter spinner to get the lock. Without that, the chance will be
50/50 on average. I can certainly take that out.
> The OSQ itself is FIFO fair, and the waiters retain the wait_list
> position. So having the top wait_list entry contending on the OSQ
> ensures we cannot starve (I think).
>
> Also, as Davidlohr said, we cannot copy/paste this much code.
As I said in the previous mail, I do intend to refactor it before
sending out the official patch.
Cheers,
Longman
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