linux-kernel - Re: [PATCH] sched: Check nr_running before calling pick_next

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]

Date:	Sat, 02 Jul 2011 17:35:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched: Check nr_running before calling pick_next_task
 in schedule().

On Sat, 2011-07-02 at 20:26 +0600, Rakib Mullick wrote:
> Well, yes - branching seems definitely have some side effects.

It adds the cost of the test as well as a possible branch mis-predict.

> Thinking from UP's perspective, it will only hit slow path -- going
> into idle.

Uhm, no, every time the machine is busy and does a schedule between
tasks you still get to do that extra nr_running test and branch.

>  In that case, that likely branch will just fail. And on an
> UP system that slow path -- going into idle -- is the only way, taking
> the fast path (trying picking a task) isn't the right thing, isn't
> it? 

I'm not at all sure I even understand what you're trying to say. I
really don't understand what's the problem with going the long way with
picking the idle task, the machine is idle, it doesn't have anything
useful to do, who cares.
--
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/