From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f70.google.com (mail-pg0-f70.google.com [74.125.83.70]) by kanga.kvack.org (Postfix) with ESMTP id E4B026B0033 for ; Mon, 27 Nov 2017 03:53:57 -0500 (EST) Received: by mail-pg0-f70.google.com with SMTP id i123so28501035pgd.2 for ; Mon, 27 Nov 2017 00:53:57 -0800 (PST) Received: from lgeamrelo11.lge.com (LGEAMRELO11.lge.com. [156.147.23.51]) by mx.google.com with ESMTP id d81si9264233pfj.183.2017.11.27.00.53.56 for ; Mon, 27 Nov 2017 00:53:56 -0800 (PST) Date: Mon, 27 Nov 2017 17:53:43 +0900 From: Byungchul Park Subject: Re: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes Message-ID: <20171127085343.GB15859@X58A-UD3R> References: <20171108102723.602216b1@gandalf.local.home> <20171124152857.ahnapnwmmsricunz@pathway.suse.cz> <20171124155816.pxp345ch4gevjqjm@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171124155816.pxp345ch4gevjqjm@pathway.suse.cz> Sender: owner-linux-mm@kvack.org List-ID: To: Petr Mladek Cc: Steven Rostedt , LKML , akpm@linux-foundation.org, linux-mm@kvack.org, Cong Wang , Dave Hansen , Johannes Weiner , Mel Gorman , Michal Hocko , Sergey Senozhatsky , Vlastimil Babka , Peter Zijlstra , Linus Torvalds , Jan Kara , Mathieu Desnoyers , Tetsuo Handa , rostedt@home.goodmis.org, kernel-team@lge.com On Fri, Nov 24, 2017 at 04:58:16PM +0100, Petr Mladek wrote: > On Fri 2017-11-24 16:54:16, Petr Mladek wrote: > > On Wed 2017-11-08 10:27:23, Steven Rostedt wrote: > > > If there is a waiter, then it breaks out of the loop, clears the waiter > > > flag (because that will release the waiter from its spin), and exits. > > > Note, it does *not* release the console semaphore. Because it is a > > > semaphore, there is no owner. Hello Petr, Thank you for adding me into this thread. You seem to change console_lock_dep_map to cross-release stuff. I will add my opinion after reviewing it :) Thanks, Byungchul > > > Index: linux-trace.git/kernel/printk/printk.c > > > =================================================================== > > > --- linux-trace.git.orig/kernel/printk/printk.c > > > +++ linux-trace.git/kernel/printk/printk.c > > > @@ -86,8 +86,15 @@ EXPORT_SYMBOL_GPL(console_drivers); > > > static struct lockdep_map console_lock_dep_map = { > > > .name = "console_lock" > > > }; > > > +static struct lockdep_map console_owner_dep_map = { > > > + .name = "console_owner" > > > +}; > > > #endif > > > > > > +static DEFINE_RAW_SPINLOCK(console_owner_lock); > > > +static struct task_struct *console_owner; > > > +static bool console_waiter; > > > + > > > enum devkmsg_log_bits { > > > __DEVKMSG_LOG_BIT_ON = 0, > > > __DEVKMSG_LOG_BIT_OFF, > > > @@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility > > > * semaphore. The release will print out buffers and wake up > > > * /dev/kmsg and syslog() users. > > > */ > > > - if (console_trylock()) > > > + if (console_trylock()) { > > > console_unlock(); > > > + } else { > > > + struct task_struct *owner = NULL; > > > + bool waiter; > > > + bool spin = false; > > > + > > > + printk_safe_enter_irqsave(flags); > > > + > > > + raw_spin_lock(&console_owner_lock); > > > + owner = READ_ONCE(console_owner); > > > + waiter = READ_ONCE(console_waiter); > > > + if (!waiter && owner && owner != current) { > > > + WRITE_ONCE(console_waiter, true); > > > + spin = true; > > > + } > > > + raw_spin_unlock(&console_owner_lock); > > > + > > > + /* > > > + * If there is an active printk() writing to the > > > + * consoles, instead of having it write our data too, > > > + * see if we can offload that load from the active > > > + * printer, and do some printing ourselves. > > > + * Go into a spin only if there isn't already a waiter > > > + * spinning, and there is an active printer, and > > > + * that active printer isn't us (recursive printk?). > > > + */ > > > + if (spin) { > > > + /* We spin waiting for the owner to release us */ > > > + spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_); > > > + /* Owner will clear console_waiter on hand off */ > > > + while (READ_ONCE(console_waiter)) > > > + cpu_relax(); > > > + > > > + spin_release(&console_owner_dep_map, 1, _THIS_IP_); > > > + printk_safe_exit_irqrestore(flags); > > > + > > > + /* > > > + * The owner passed the console lock to us. > > > + * Since we did not spin on console lock, annotate > > > + * this as a trylock. Otherwise lockdep will > > > + * complain. > > > + */ > > > + mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); > > > > I am not sure that this correctly imitates the real lock > > dependency. The trylock flag means that we are able to skip > > this section when the lock is taken elsewhere. But it is not > > the whole truth. In fact, we are blocked in this code path > > when console_sem is taken by another process. > > > > The use of console_owner_lock is not enough. The other > > console_sem calls a lot of code outside the section > > guarded by console_owner_lock. > > > > I think that we have actually entered the cross-release bunch > > of problems, see https://lwn.net/Articles/709849/ > > > > IMHO, we need to use struct lockdep_map_cross for > > console_lock_dep_map. Also we need to put somewhere > > lock_commit_crosslock(). > > > > I am going to play with it. Also I add Byungchul Park into CC. > > This is why I keep most of the context in this reply (I am sorry > > for it). > > See my first attempt below. I do not get any lockdep > warning but it is possible that I just did it wrong. > > > >From 0345785d4767f853ff2d733b565084c3339f9fe0 Mon Sep 17 00:00:00 2001 > From: Petr Mladek > Date: Fri, 24 Nov 2017 16:50:25 +0100 > Subject: [PATCH] printk: Try to describe real console_sem dependecies using > the crosslock feature > > console_sem might be newly taken and released by different > processes. This is an attempt to check the crossrelease > dependencies. > --- > kernel/printk/printk.c | 25 +++++++++---------------- > 1 file changed, 9 insertions(+), 16 deletions(-) > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 040fb948924e..bda25feae0d5 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -83,9 +83,9 @@ struct console *console_drivers; > EXPORT_SYMBOL_GPL(console_drivers); > > #ifdef CONFIG_LOCKDEP > -static struct lockdep_map console_lock_dep_map = { > - .name = "console_lock" > -}; > +static struct lockdep_map_cross console_lock_dep_map = > + STATIC_CROSS_LOCKDEP_MAP_INIT("console_lock", &console_sem); > + > static struct lockdep_map console_owner_dep_map = { > .name = "console_owner" > }; > @@ -218,7 +218,7 @@ static int nr_ext_console_drivers; > */ > #define down_console_sem() do { \ > down(&console_sem);\ > - mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\ > + mutex_acquire((struct lockdep_map *)&console_lock_dep_map, 0, 0, _RET_IP_);\ > } while (0) > > static int __down_trylock_console_sem(unsigned long ip) > @@ -237,7 +237,7 @@ static int __down_trylock_console_sem(unsigned long ip) > > if (lock_failed) > return 1; > - mutex_acquire(&console_lock_dep_map, 0, 1, ip); > + mutex_acquire((struct lockdep_map *)&console_lock_dep_map, 0, 1, ip); > return 0; > } > #define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_) > @@ -246,7 +246,7 @@ static void __up_console_sem(unsigned long ip) > { > unsigned long flags; > > - mutex_release(&console_lock_dep_map, 1, ip); > + mutex_release((struct lockdep_map *)&console_lock_dep_map, 1, ip); > > printk_safe_enter_irqsave(flags); > up(&console_sem); > @@ -1797,13 +1797,6 @@ asmlinkage int vprintk_emit(int facility, int level, > spin_release(&console_owner_dep_map, 1, _THIS_IP_); > printk_safe_exit_irqrestore(flags); > > - /* > - * The owner passed the console lock to us. > - * Since we did not spin on console lock, annotate > - * this as a trylock. Otherwise lockdep will > - * complain. > - */ > - mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_); > console_unlock(); > printk_safe_enter_irqsave(flags); > } > @@ -2334,10 +2327,10 @@ void console_unlock(void) > /* The waiter is now free to continue */ > spin_release(&console_owner_dep_map, 1, _THIS_IP_); > /* > - * Hand off console_lock to waiter. The waiter will perform > - * the up(). After this, the waiter is the console_lock owner. > + * Hand off console_lock to waiter. After this, the waiter > + * is the console_lock owner. > */ > - mutex_release(&console_lock_dep_map, 1, _THIS_IP_); > + lock_commit_crosslock((struct lockdep_map *)&console_lock_dep_map); > printk_safe_exit_irqrestore(flags); > /* Note, if waiter is set, logbuf_lock is not held */ > return; > -- > 2.13.6 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org