linux-kernel - Re: [PATCH 2/7] nohz: New tick dependency mask

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: <20151202150908.GR3816@twins.programming.kicks-ass.net>
Date:	Wed, 2 Dec 2015 16:09:08 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/7] nohz: New tick dependency mask

On Wed, Dec 02, 2015 at 03:08:16PM +0100, Frederic Weisbecker wrote:
> On Wed, Dec 02, 2015 at 11:56:33AM +0100, Peter Zijlstra wrote:
> > On Tue, Dec 01, 2015 at 11:20:28PM +0100, Frederic Weisbecker wrote:
> > > > > +	prev = fetch_or(dep, BIT_MASK(bit));
> > > > > +	if (!prev) {
> > > > > +		/*
> > > > > +		* We need the IPIs to be sent from sane process context.
> > > > 
> > > > Why ?
> > > 
> > > Because posix timers code is all called with interrupts disabled and we can't
> > > send IPIs then.
> > > 
> > > > 
> > > > > +		* The posix cpu timers are always set with irqs disabled.
> > > > > +		*/
> > > > > +		schedule_work(&kick_all_work);
> > > > > +	}
> > > > > +}
> > > > > +
> > > > > +/*
> > > > > + * Set a global tick dependency. Lets do the wide IPI kick asynchronously
> > > > > + * for callers with irqs disabled.
> > > > 
> > > > This seems to suggest you can call this with IRQs disabled
> > > 
> > > Ah right, that's a misleading comment. We need to use the _delayed() version
> > > when interrupts are disabled.
> > 
> > Why can't you use tick_nohz_full_kick_cpu() for all that, which is
> > usable from IRQ context and avoid all that delayed muck?
> 
> Because I need to kick all the CPUs where the task/signal is running on. That's
> a bit difficult to do though. I think we had something to try to send an IPI to a task,
> but I can't retrieve it. Looks easy to do anyway. But in the signal case I'd need to do
> that for all tasks in the group. That sounds like a costly loop.
> 
> So I simplify that with a global IPI.

Sure, but what I meant was that:

	for_each_cpu(cpu, mask)
		tick_nohz_full_kick_cpu(cpu);

is IRQ safe, whereas the current thingy is not. Sure, its a wee bit less
efficient, but do we really care?
--
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/