linux-kernel - Re: [PATCH] RT: Add priority-queuing and priority-inheritance to workqueue infrastructure

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <1186407173.7182.16.camel@twins>
Date:	Mon, 06 Aug 2007 15:32:53 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Ingo Molnar <mingo@...e.hu>, Gregory Haskins <ghaskins@...ell.com>,
	Daniel Walker <dwalker@...sta.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RT: Add priority-queuing and priority-inheritance to
	workqueue infrastructure

On Mon, 2007-08-06 at 15:29 +0200, Peter Zijlstra wrote:
> On Mon, 2007-08-06 at 17:18 +0400, Oleg Nesterov wrote:
> 
> > Yes, I still disagree with the whole idea because I hope we can make
> > something more simpler to solve the problem, but I must admit I don't
> > quite understand what the problem is.
> > 
> > So, please consider a noise from my side as my attempt to help. And
> > in fact, I am very curious about -rt tree, just I never had a time
> > to study it :)
> 
> 
> Well, the thing is, suppose we have 2 drivers both using keventd say a
> NIC and some USB thingy.
> 
> Now the NIC is deemed important hand gets irq thread prio 90, and the
> USB could not be cared less about and gets 10 (note that on -rt irq
> handlers are threaded and run SCHED_FIFO).
> 
> So now you can get priority inversion in keventd. Say the USB thingy
> schedules a work item which will be executed. Then during the execution
> of this work the NIC will also schedule a work item. Now the NIC (fifo
> 90) will have to wait for the USB work (fifo 10) to complete.

/me hits himself.

of course today everything will run on whatever prio keventd ends up,
regardless of the prio of the submitter.

still this does not change the fundamental issue of a high prio piece of
work waiting on a lower prio task.

> The typical solution is priority inheritance, where the highest prio of
> any waiter is propagated to the currently running work, so that it can
> finish and get on with the more important work.
> 
> 
> So these patches aimed to provide proper PI in the workqueue structure
> to avoid this problem.
> 
> However as you rightly noted, this horribly breaks the barrier/flush
> semantics.
> 
> I suspect most of the barrier/flush semantics could be replaced with
> completions from specific work items. Doing this will be a lot of work
> though.
> 
> I hope this rambling is not confusing you any further :-)

-
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/