linux-kernel - Re: [PATCH v3 17/17] (Avoid overload)

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: <473EE1AE.BA47.005A.0@novell.com>
Date:	Sat, 17 Nov 2007 12:42:22 -0500
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>,
	"LKML" <linux-kernel@...r.kernel.org>
Cc:	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Christoph Lameter" <clameter@....com>
Subject: Re: [PATCH v3 17/17] (Avoid overload)

>>> On Sat, Nov 17, 2007 at  1:33 AM, in message
<20071117063318.GA31442@...dmis.org>, Steven Rostedt <rostedt@...dmis.org>
wrote: 

> Sorry!  I forgot to put in a prologue for this patch.
> 
> Here it is.
> 
> ====
> 
> This patch changes the searching for a run queue by a waking RT task
> to try to pick another runqueue if the currently running task
> is an RT task.
> 
> The reason is that RT tasks behave different than normal
> tasks. Preempting a normal task to run a RT task to keep
> its cache hot is fine, because the preempted non-RT task
> may wait on that same runqueue to run again unless the
> migration thread comes along and pulls it off.
> 
> RT tasks behave differently. If one is preempted, it makes
> an active effort to continue to run. So by having a high
> priority task preempt a lower priority RT task, that lower
> RT task will then quickly try to run on another runqueue.
> This will cause that lower RT task to replace its nice
> hot cache (and TLB) with a completely cold one. This is
> for the hope that the new high priority RT task will keep
>  its cache hot.
> 
> Remeber that this high priority RT task was just woken up.
> So it may likely have been sleeping for several milliseconds,
> and will end up with a cold cache anyway. RT tasks run till
> they voluntarily stop, or are preempted by a higher priority
> task. This means that it is unlikely that the woken RT task
> will have a hot cache to wake up to. So pushing off a lower
> RT task is just killing its cache for no good reason.

You make some excellent points here.  Out of curiosity, have you tried a comparison to see if it helps?

-Greg


-
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/