linux-kernel - Re: [PATCH 1/3] sched: enable interrupts and drop rq-lockduringnewidle balancing

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: <4860B2D5.BA47.005A.0@novell.com>
Date:	Tue, 24 Jun 2008 06:39:49 -0600
From:	"Gregory Haskins" <ghaskins@...ell.com>
To:	"Peter Zijlstra" <peterz@...radead.org>
Cc:	<mingo@...e.hu>, <rostedt@...dmis.org>, <tglx@...utronix.de>,
	"David Bahi" <DBahi@...ell.com>, <linux-kernel@...r.kernel.org>,
	<linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH 1/3] sched: enable interrupts and drop
	rq-lockduringnewidle balancing

>>> On Tue, Jun 24, 2008 at  8:24 AM, in message <1214310299.4351.27.camel@...ns>,
Peter Zijlstra <peterz@...radead.org> wrote: 
> On Tue, 2008-06-24 at 07:15 -0600, Gregory Haskins wrote:
>> >>> On Tue, Jun 24, 2008 at  6:13 AM, in message 
> <1214302405.4351.21.camel@...ns>,
>> Peter Zijlstra <peterz@...radead.org> wrote: 
>> > On Mon, 2008-06-23 at 17:04 -0600, Gregory Haskins wrote:
>> >> We do find_busiest_groups() et. al. without locks held for normal 
> balancing,
>> >> so lets do it for newidle as well.  It will allow other cpus to make
>> >> forward progress (against our RQ) while we try to balance and allow 
>> >> some interrupts to occur.
>> > 
>> > Is running f_b_g really that expensive? 
>> 
>> According to our oprofile data, yes.  I speculate that it works out that way 
> because most newidle
>> attempts result in "no imbalance".  But we were spending ~60%+ time in 
> find_busiest_groups()
>> because of all the heavy-context switching that goes on in PREEMPT_RT.  So 
> while f_b_g() is
>> probably cheaper than double-lock/move_tasks(), the ratio of occurrence is 
> off the charts in
>> comparison. Prior to this patch, those occurrences were 
> preempt-disabled/irq-disabled/rq->lock critical
>> sections.
>> 
>> So while it is not clear if f_b_g() is the actual cost, it is a convenient 
> (and legal, afaict) place to
>> deterministically reduce the rq->lock scope.  Additionally, doing so 
> measurably helps
>> performance, so I think its a win.  Without this patch you have to hope the 
> double_lock releases
>> this_rq, and even so were not checking for the NEEDS_RESCHED. 
> 
> See, having had this information in the changelog to begin with would
> have helped ;-)

What?  You can't read my mind?  :)

Good point, Peter.  Will fix on next drop.

-Greg

> 
> 
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html


--
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/