From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932961AbdCWQ0p (ORCPT ); Thu, 23 Mar 2017 12:26:45 -0400 Received: from mx2.suse.de ([195.135.220.15]:58092 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751367AbdCWQ0o (ORCPT ); Thu, 23 Mar 2017 12:26:44 -0400 Date: Thu, 23 Mar 2017 17:26:40 +0100 From: Petr Mladek To: Sergey Senozhatsky Cc: Steven Rostedt , Andrew Morton , linux-kernel@vger.kernel.org, Sergey Senozhatsky , Thomas Gleixner , Sebastian Andrzej Siewior , Ingo Molnar Subject: Re: [PATCH] printk: use console_trylock() in console_cpu_notify() Message-ID: <20170323162640.GA10306@pathway.suse.cz> References: <20170121104729.8585-1-sergey.senozhatsky@gmail.com> <20170125150236.GF6620@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170125150236.GF6620@pathway.suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 2017-01-25 16:02:36, Petr Mladek wrote: > On Sat 2017-01-21 19:47:29, Sergey Senozhatsky wrote: > > There is no need to always call blocking console_lock() in > > console_cpu_notify(), it's quite possible that console_sem can > > be locked by other CPU on the system, either already printing > > or soon to begin printing the messages. console_lock() in this > > case can simply block CPU hotplug for unknown period of time > > (console_unlock() is time unbound). Not that hotplug is very > > fast, but still, with other CPUs being online and doing > > printk() console_cpu_notify() can stuck. > > > > Use console_trylock() instead and opt-out if console_sem is > > already acquired from another CPU, since that CPU will do > > the printing for us. > > > > Signed-off-by: Sergey Senozhatsky > > --- > > kernel/printk/printk.c | 11 ++++++----- > > 1 file changed, 6 insertions(+), 5 deletions(-) > > > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > > index 7180088cbb23..772eb16436ce 100644 > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -2028,15 +2028,16 @@ void resume_console(void) > > * @cpu: unused > > * > > * If printk() is called from a CPU that is not online yet, the messages > > - * will be spooled but will not show up on the console. This function is > > - * called when a new CPU comes online (or fails to come up), and ensures > > - * that any such output gets printed. > > + * will be printed on the console only if there are CON_ANYTIME consoles. > > + * This function is called when a new CPU comes online (or fails to come > > + * up) or goes offline. > > */ > > static int console_cpu_notify(unsigned int cpu) > > { > > if (!cpuhp_tasks_frozen) { > > - console_lock(); > > - console_unlock(); > > + /* If trylock fails, someone else is doing the printing */ > > + if (console_trylock()) > > + console_unlock(); > > } > > return 0; > > } > > All in all. The change looks fine to me because it most likely > improves the performance[1] and does not affect the reliability > that much. > > [1] https://lkml.kernel.org/r/20170119120744.GB435@tigerII.localdomain Acked-by: Petr Mladek I am going to add this patch into printk.git for 4.12. Best Regards, Petr