[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <4BBA6F0B.2060008@us.ibm.com>
Date: Mon, 05 Apr 2010 16:15:23 -0700
From: Darren Hart <dvhltc@...ibm.com>
To: Avi Kivity <avi@...hat.com>
CC: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...e.hu>,
Eric Dumazet <eric.dumazet@...il.com>,
"Peter W. Morreale" <pmorreale@...ell.com>,
Rik van Riel <riel@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Gregory Haskins <ghaskins@...ell.com>,
Sven-Thorsten Dietrich <sdietrich@...ell.com>,
Chris Mason <chris.mason@...cle.com>,
John Cooper <john.cooper@...rd-harmonic.com>,
Chris Wright <chrisw@...s-sol.org>
Subject: Re: [PATCH V2 0/6][RFC] futex: FUTEX_LOCK with optional adaptive
spinning
Avi Kivity wrote:
>>> An interesting (but perhaps difficult to achieve) optimization would
>>> be to spin in userspace.
>>
>> I couldn't think of a lightweight way to determine when the owner has
>> been scheduled out in userspace. Kernel assistance is required. You
>> could do this on the schedule() side of things, but I figured I'd get
>> some strong pushback if I tried to add a hook into descheduling that
>> flipped a bit in the futex value stating the owner was about to
>> deschedule(). Still, that might be something to explore.
>
> In the futex value it's hopeless (since a thread can hold many locks),
It can, but there is a futex value per lock. If the task_struct had a
list of held futex locks (as it does for pi futex locks) the
deschedule() path could walk that and mark the FUTEX_OWNER_SLEEPING bit.
> but I don't think it's unreasonable to set a bit in the thread local
> storage area. The futex format would then need to be extended to
> contain a pointer to this bit.
This appears to be 1 bit per task instead of 1 bit per lock. Also, the
value is thread-specific... so how would a potential waiter be able to
determine if the owner of a particular lock was running or not with this
method? ... maybe I'm missing some core bit about TLS... are you
talking about pthread_key_create() and pthread_getspecific() ?
Thanks,
--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists