netdev - Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1) instead of yield() to wait for outstanding qdisc

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Message-ID: <20140307154739.GA18441@linutronix.de>
Date:	Fri, 7 Mar 2014 16:47:39 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Marc Kleine-Budde <mkl@...gutronix.de>
Cc:	David Miller <davem@...emloft.net>, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1) instead
 of yield() to wait for outstanding qdisc_run calls

* Marc Kleine-Budde | 2014-03-06 22:39:58 [+0100]:

>> Therefore it should allow lower priority threads to run, not just
>> equal or higher priority ones.
>
>Yes, we need a call that does what you described, however I'm not sure
>if yield() really does that. According to:
>
>http://lxr.free-electrons.com/source/kernel/sched/core.c#L3599
>
>> * Typical broken usage is:
>> *
>> * while (!event)
>> *      yield();
>> *
>> * where one assumes that yield() will let 'the other' process run that will
>> * make event true. If the current task is a SCHED_FIFO task that will never
>> * happen. Never use yield() as a progress guarantee!!
>
>My Process runs with SCHED_FIFO and prio > 50, with IRQ at default prio,
>which is 50.
>
>Maybe the RT guys can comment on this. I found another interesting
>function in the RT patch set: cpu_chill().

If you boot mainline without -RT, use threadirqs, start your application
do the same prio thing then you should end up with exactly the same
outcome. Please say so :)

msleep() is safe as long as it is used outside of the softirq. Nice that
you found cpu_chill() but on non-RT it turns to cpu_relax() and you do
not want this here.

wait_event() would be nice in the end to have. For now I take that patch
for -RT.

>Marc

Sebastian
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