linux-kernel - Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for equal prio cases

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] [day] [month] [year] [list]

Message-ID: <1430225304.3378.44.camel@gmail.com>
Date:	Tue, 28 Apr 2015 14:48:24 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>, pang.xunlei@....com.cn,
	Juri Lelli <juri.lelli@...il.com>,
	linux-kernel@...r.kernel.org, linux-kernel-owner@...r.kernel.org,
	Xunlei Pang <pang.xunlei@...aro.org>
Subject: Re: [PATCH v6 2/3] sched/rt: Fix wrong SMP scheduler behavior for
 equal prio cases

On Tue, 2015-04-28 at 12:19 +0200, Peter Zijlstra wrote:
> On Sun, Apr 26, 2015 at 11:58:51AM -0400, Steven Rostedt wrote:
> > I think what Xunlei is trying to say, is that we don't currently keep
> > FIFO when preemption or migration is involved. If a task is currently
> > running, strict FIFO denotes that it should run ahead of all other
> > tasks queued at its priority or less until it decides to schedule out.
> > But the issue is, if it gets preempted or migrates, it gets placed
> > behind other tasks of the same priority as itself, but it never
> > voluntarily relinquished the CPU.
> 
> So 1) FIFO is only defined for UP, anything SMP is well outside of the
> FIFO spec and therefore we cannot break it.
> 
> 2) The 'head' of the queue only has meaning on UP, with SMP there's 'n'
> heads, which of those heads is is the foremost head? That is, we're
> already lost order, you cannot reconstruct. This cannot be done without
> first defining order and then implementing that.

Good luck with that :)  Trying to preserve run order across the box led
me to a seemingly _endless_ supply of deadlocks while piddling with
preemptible spinning locks in rt.  Maybe you can pull that off, me it
gave serious headaches.

	-Mike

--
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/