linux-kernel - Re: [PATCH 4/5] sched/fair: Remove double_lock_balance() from active_load_balance_cpu

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: <1406271955.3526.91.camel@tkhai>
Date:	Fri, 25 Jul 2014 11:05:55 +0400
From:	Kirill Tkhai <ktkhai@...allels.com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
CC:	<linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"Ingo Molnar" <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	<tkhai@...dex.ru>
Subject: Re: [PATCH 4/5] sched/fair: Remove double_lock_balance() from
 active_load_balance_cpu_stop()

В Чт, 24/07/2014 в 17:04 -0700, Tim Chen пишет:
> On Tue, 2014-07-22 at 15:30 +0400, Kirill Tkhai wrote:
> > Bad situation:
> > 
> > double_lock_balance() drops busiest_rq lock. The busiest_rq is *busiest*,
> > and a lot of tasks and context switches there. We are dropping the lock
> > and waiting for it again.
> > 
> > Let's just detach the task and once finally unlock it!
> > 
> > Warning: this admits unlocked using of can_migrate_task(), throttled_lb_pair(),
> > and task_hot(). I added comments about that.
> > 
> 
> Wonder if we should also consider removing double_lock_balance usage
> from rt.c and deadline.c? Then those two schedulers will also not
> lock both the source and destination queues at the same time
> for load balancing.

rt.c and deadline.c are similar, so we are able to discuss about one of them.

There are two places with double_lock_balance() in rt.c:

1)push_rt_task()->find_lock_lowest_rq()

We can't detach a task before we are locked lowest_rq. It's unknown whether
it will still be suitable to be attached to lowest_rq after we are locked it,
because the highest prioriry of lowest_rq may change. We have the race there.

2)pull_rt_task()

The same with here. The situation may change. We must keep both locks locked
to be sure the priorities won't change. For example, somebody may wake a high
priority task on src_rq, or somebody can pull a task there.

RT balancing is stricter than fair's..

Regards,
	Kirill

--
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/