linux-kernel - Re: On migrate_disable() and latencies

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: <1311582110.2617.40.camel@laptop>
Date:	Mon, 25 Jul 2011 10:21:50 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Nicholas Mc Guire <der.herr@...r.at>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Carsten Emde <ce@...g.ch>,
	Clark Williams <williams@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Kumar Gala <galak@...e.crashing.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	rostedt <rostedt@...dmis.org>
Subject: Re: On migrate_disable() and latencies

On Fri, 2011-07-22 at 19:45 +0200, Nicholas Mc Guire wrote:

> > Therefore the worst case latency is in the order of
> > max-migrate-disable-period * nr-cpus.
> 
> + something like sum of (interrupt rate [n] /
> max-migrate-disable-period * nr-cpus) * top-half handler [n]. if you
> go on with theoretical WCET analysis on multicore systems you will
> always end up at the conclusion that only UP is suitable for RT....

+ preemption cost + migration costs etc.. :-)

> > Currently we have no means of measuring these latencies, this is
> > something we need to grow, I think Steven can fairly easy craft a
> > migrate_disable runtime tracer -- it needs to use t->se.sum_exec_runtime
> > for measure so as to only count the actual time spend on the task and
> > ignore any time it was blocked.
>  
> well this is a similar problem as with the WCET "calculations" - you can
> calculate theoretical worst cases - but the question is what the actual
> distribution of "stacking" is and thus what the probability is that you
> manage to stack tasks in this way.

Sure, and yes those are thing to look at.

> One could I guess put some relatively simple instrumentation in to monitor
> this stacking problem - quit independant of actually measuring the times 

Yes, that would be very easy to do.

> > Once we have this, its back to the old game of 'lock'-breaking.
> >
> if the stacking problem does not practically exist then it might not be worth
> the effort to resolve it with elaborate lock breaking.

Fully agreed, its just that I wanted to share my understanding of the
problem space (both Thomas and I thought on-list email was a better
location that private IRC logs ;-), and we need to get the know the
problem before we can decide to ignore it ;-)

Ignoring unknowns always leads to surprises.

--
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/