linux-kernel - Re: [PATCH] hotplug: Optimize {get,put}_online

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 for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20131002163152.GA16233@redhat.com>
Date:	Wed, 2 Oct 2013 18:31:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

On 10/02, Peter Zijlstra wrote:
>
> On Wed, Oct 02, 2013 at 04:00:20PM +0200, Oleg Nesterov wrote:
> > And again, even
> >
> > 	for (;;) {
> > 		percpu_down_write();
> > 		percpu_up_write();
> > 	}
> >
> > should not completely block the readers.
>
> Sure there's a tiny window, but don't forget that a reader will have to
> wait for the gp_state cacheline to transfer to shared state and the
> per-cpu refcount cachelines to be brought back into exclusive mode and
> the above can be aggressive enough that by that time we'll observe
> state == blocked again.

Sure, but don't forget that other callers of cpu_down() do a lot more
work before/after they actually call cpu_hotplug_begin/end().

> So I'll stick to waitcount -- as you can see in the patches I've just
> posted.

I still do not believe we need this waitcount "in practice" ;)

But even if I am right this is minor and we can reconsider this later,
so please forget.

Oleg.

--
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/