linux-kernel - Re: Subject: [PATCH 2/2] priority System V Semaphores

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Date:	Wed, 21 Dec 2011 22:48:35 +0200
From:	raz ben yehuda <raziebe@...il.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Lior Brafman <LBrafman@...z.com>,
	Torsten Scherer <TScherer@...z.com>,
	Rasty Slutsker <RSlutsker@...z.com>
Subject: Re: Subject: [PATCH 2/2] priority System V Semaphores

On Wed, 2011-12-21 at 19:31 +0100, Manfred Spraul wrote:
> Hi raz,
> 
> 
> On 12/20/2011 11:23 PM, raz ben yehuda wrote:
> > > From 25aa166505aff2561dd715c927c654d0bbb432ba Mon Sep 17 00:00:00 2001
> > From: raz<raziebe@...il.com>
> > Date: Tue, 20 Dec 2011 22:54:56 +0200
> >
> >
> > Add positioning routine find_pos. find_pos returns
> > the place where to put the sleeper before.
> > I sort only rt tasks, OTHER policy is pushed back in
> > queue. I do not distinct between SCHED_RR and SCHED_FIFO
> > policies and they are treated as a single policy
> > for the sorting algorithm.
> >
> > SETPRIO operates only when user issues a single semop
> > operation and not an array of opretions.
> As far as I can see, the advantages of sysvsem are backward 
> compatibility and the ability to use complex ops.
> You propose to add a new feature that doesn't work on complex ops.
> 
> Are there any apps that use SETPRIO?
> What is the use case?
Vxworks is the use case. And there are plenty of companies with
vxWorks software and in i believe they will migrate sooner or later to
PreemptRT.  My current company uses old wrapper software that implements
vxWorks semaphores as system V semaphores. vxWorks semaphores have a priority
feature which is widely used.
I will probably change it some time in the future to posix semaphores , but posix 
semaphores are implemented in glibc with futexes and atomic ops and i rather 
mess with kernel and not glibc. funny , but true. glibc is harder. 

> > SETFIFO is the default for backward compatbility.
> >
> > Signed-off-by: raz<raziebe@...il.com>
> > ---
> >   ipc/sem.c |   51 +++++++++++++++++++++++++++++++++++++++++++++++++--
> >   1 files changed, 49 insertions(+), 2 deletions(-)
> >e
> > diff --git a/ipc/sem.c b/ipc/sem.c
> > index 90dc5a1..921056d 100644
> > --- a/ipc/sem.c
> > +++ b/ipc/sem.c
> > @@ -1343,6 +1343,51 @@ static int get_queue_result(struct sem_queue *q)
> >   	return error;
> >   }
> >
> > +/*
> > + * find the best place to put the task sorted by rt prio
> > +*/
> > +static struct list_head *find_pos(struct sem *curr, int alter,
> > +		struct task_struct *p)
> > +{
> > +	struct sem_queue *q;
> > +	struct sem_queue *ret_pos = NULL;
> > +	struct list_head *tasks_queue =&curr->sem_pending;
> > +
> > +	if (!alter)
> > +		return tasks_queue;
> > +
> Tasks that do not alter anything end up first - IMHO correct.
> 
> > +	if (!(curr->flags&  PRIO_SEM))
> > +		return tasks_queue;
> > +	/*
> > +	 *  make no effort to sort SCHED_OTHER,
> > +	 *  just push task to the back of the queue.
> > +	*/
> > +	if (!(p->policy == SCHED_FIFO || p->policy == SCHED_RR))
> > +		return tasks_queue;
> > +	/*
> > +	 *  make no distinction between SCHED_FIFO
> > +	 *  and SCHED_RR policies.
> > +	 */
> > +	list_for_each_entry(q, tasks_queue, simple_list) {
> > +		struct task_struct *t;
> > +
> > +		t  = q->sleeper;
> > +		if (current->rt_priority == t->rt_priority) {
> > +			/*
> > +			* push in a FIFO manner
> > +			* tasks in same priority
> > +			*/
> > +			ret_pos = q;
> > +			continue;
> > +		}
> > +		if (current->rt_priority<  t->rt_priority)
> > +			continue;
> > +		return&q->simple_list;
> > +	}
> Here in the loop, non-alter tasks are evaluated as well.
> I think that's wrong, it could starve non-alter tasks.
> e.g. queue:
> - high prio non-alter
> - low prio non-alter.
> 
> Now a medium prio alter task is added.
> I think it will end up in the wrong position (before the low-prio 
> non-alter task), correct?
hammm.. correct. thanks. I did not check non-alter tasks at all. The entire queue might be a total mess. 
I will sort non alter tasks as well.
Also, this patch is missing scenario where a task priority is changed whilst it is waiting. 
I fixed that already. 

Who maintains this code ? 

> --
>      Manfred



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