linux-kernel - Re: [RFC] per-cpu preempt

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: <20130812175830.GB18691@gmail.com>
Date:	Mon, 12 Aug 2013 19:58:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <bitbucket@...ine.de>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] per-cpu preempt_count


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Mon, Aug 12, 2013 at 4:51 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > The below boots to wanting to mount a root filesystem with
> > CONFIG_PREEMPT=y using kvm -smp 4.
> 
> But doesn't work in general? Or you just never tested?

(I think Peter never tested it on real hw - this is an RFC patch to show 
the concept .)

> > Adding TIF_NEED_RESCHED into the preempt count would allow a single 
> > test in preempt_check_resched() instead of still needing the TI. 
> > Removing PREEMPT_ACTIVE from preempt count should allow us to get rid 
> > of ti::preempt_count altogether.
> >
> > The only problem with TIF_NEED_RESCHED is that its cross-cpu which 
> > would make the entire thing atomic which would suck donkey balls so 
> > maybe we need two separate per-cpu variables?
> 
> Agreed. Making it atomic would suck, and cancel all advantages of the 
> better code generation to access it. Good point.

We could still have the advantages of NEED_RESCHED in preempt_count() by 
realizing that we only rarely actually set/clear need_resched and mostly 
read it from the highest freq user, the preempt_enable() check.

So we could have it atomic, but do atomic_read() in the preempt_enable() 
hotpath which wouldn't suck donkey balls, right?

That would allow a really sweet preempt_enable() fastpath, on x86 at 
least.

Thanks,

	Ingo
--
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/