linux-kernel - Re: [PATCH -rt] ipc/sem: Rework semaphore wakeups

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Date:	Thu, 15 Sep 2011 19:04:35 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH -rt] ipc/sem: Rework semaphore wakeups

On 09/14/2011 09:23 PM, Peter Zijlstra wrote:
> On Wed, 2011-09-14 at 20:48 +0200, Manfred Spraul wrote:
>> The code does:
>>
>>       spin_lock()
>>       preempt_disable();
>>       usually_very_simple_but_worstcase_O_2
>>       spin_unlock()
>>       usually_very_simple_but_worstcase_O_1
>>       preempt_enable();
>>
>> with your change, it becomes:
>>
>>       spin_lock()
>>       usually_very_simple_but_worstcase_O_2
>>       usually_very_simple_but_worstcase_O_1
>>       spin_unlock()
>>
>> The complex ops remain unchanged, they are still under a lock.
> preemptible lock (aka pi-mutex) on -rt, so no weird latencies.
But the change means that more operations are under spin_lock().
Acutally for a large SMP system with a simple semaphore operation, the 
wake_up_process() takes longer than the semaphore operation.
And for some databases, contention on the spin_lock() is an issue.


>> What about removing the preempt_disable?
>> It's only there to cover a rare race on uniprocessor preempt systems.
>> (a task is woken up simultaneously due to timeout of semtimedop() and a
>> true wakeup)
>>
>> Then fix the that race - something like the attached patch [obviously
>> buggy - see the fixme]
> sched_yield() is always a bug, as is it here. Its an life-lock if the
> woken task is of higher priority than the waking task. A higher prio
> FIFO task calling sched_yield() in a loop is just that, a loop, starving
> the lower prio waker.
>
> If you've got enough medium prio tasks around to occupy all other cpus,
> you're got indefinite priority inversion, so even on smp its a problem.
>
> But yeah its not the prettiest of solutions but it works.. see that
> other patch with the wake-list stuff for something that ought to work
> for both rt and mainline (except of course it doesn't actually work).
Wake lists are definitively the better approach.
[let's continue in that thread]

--
     Manfred
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