linux-kernel - Re: [RFC,PATCH] mutex: mutex_is

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Message-ID: <1257846081.4648.11.camel@twins>
Date:	Tue, 10 Nov 2009 10:41:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC,PATCH] mutex: mutex_is_owner() helper

On Tue, 2009-11-10 at 00:21 +0100, Eric Dumazet wrote:
> Peter Zijlstra a écrit :
> > On Wed, 2009-11-04 at 18:19 +0100, Eric Dumazet wrote:
> >> BTW, I was thinking of a mutex_yield() implementation, but could not
> >> cook it without hard thinking, maybe you already have some nice
> >> implementation ?
> > 
> > Why? Yield sets off alarm bells, since 99.9%, and possibly more, of its
> > uses are wrong.
> 
> If I remember well, I had problems doing "modprobe dummy numdummies=30000",
> because it creates 30000 netdevices, and thanks to hotplug starts 30000 udev
> that all wait that my modprobe is finished... Nice to see load average going
> so big by the way :)

lol :-) With a bit of luck udev will spawn a python interpreter for each
of those things too..

> I tried following patch without success, because rtnl_unlock()/rtnl_lock()
> is too fast (awaken process(es) ha(s/ve) no chance to get the lock, as we
> take it immediately after releasing it)

Right, due to lock-stealing.

> diff --git a/drivers/net/dummy.c b/drivers/net/dummy.c
> index 37dcfdc..108c4fa 100644
> --- a/drivers/net/dummy.c
> +++ b/drivers/net/dummy.c
> @@ -138,8 +138,12 @@ static int __init dummy_init_module(void)
>  	rtnl_lock();
>  	err = __rtnl_link_register(&dummy_link_ops);
>  
> -	for (i = 0; i < numdummies && !err; i++)
> +	for (i = 0; i < numdummies && !err; i++) {
>  		err = dummy_init_one();
> +		rtnl_unlock();
> +		msleep(1);
> +		rtnl_lock();
> +	}
>  	if (err < 0)
>  		__rtnl_link_unregister(&dummy_link_ops);
>  	rtnl_unlock();
> 
> But if hotplug is disabled, this force a useless msleep(1) * 30000 -> this is bit slow
> 
> Yes, this code is stupid, but I use it to stress network stack
> with insane number of devices, to spot scalability problems.

Right...

> mutex_yield() could help in this situation.

Agreed, except I don't like the name, but I could be tained from
sched_yield().

> mutex is said to be FIFO, but its not exactly true : A new comer can take the mutex
> even if 10000 threads are waiting on mutex...

Yep, lock-stealing, you don't want to see the regression reports if you
'fix' that :-)

> I wont mention other problems, because mutex_{try}lock() has no timedwait variant

Nobody needed it I guess.. also I never quite understood the need for
timedwait, either you need to get the work done or you don't, not maybe.

Use mutex_lock_interruptible() and set a timer or something.

> , and funny code doing :
> 
> if (!rtnl_trylock())
> 	return restart_syscall();
> 
> Making 30000 processes running/fighting to get the mutex :(

Funny definition of funny ;-) That's some seriously fugly code there.


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