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Message-ID: <20160105144818.GD14464@quack.suse.cz>
Date: Tue, 5 Jan 2016 15:48:18 +0100
From: Jan Kara <jack@...e.cz>
To: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
Petr Mladek <pmladek@...e.cz>,
KY Sri nivasan <kys@...rosoft.com>,
Steven Rostedt <rostedt@...dmis.org>,
linux-kernel@...r.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH 1/7] printk: Hand over printing to console if printing
too long
On Thu 31-12-15 13:58:59, Sergey Senozhatsky wrote:
> On (12/31/15 12:13), Sergey Senozhatsky wrote:
> [..]
> > cond_resched() does its job there, of course. well, a user process still can
> > do a lot of call_console_drivers() calls. may be we can check who is calling
> > console_unlock() and if we have "!printk_sync && !oops_in_progress" (or just printk_sync
> > test) AND a user process then return from console_unlock() doing irq_work_queue()
> > and set PRINTK_PENDING_OUTPUT pending bit, the way vprintk_emit() does it.
>
> attached two patches, I ended up having on top of yours. just in case.
>
> printk: factor out can_printk_async()
>
> console_unlock() can be called directly or indirectly by a user
> space process, so it can end up doing call_console_drivers() loop,
> which will hold it from returning back to user-space from a syscall
> for unpredictable amount of time.
>
> Factor out can_printk_async() function, which queues an irq work and
> sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
> vprintk_emit() already does it, add can_printk_async() call to
> console_unlock() for !PF_KTHREAD processes.
I'd be cautious about changing this userspace visible behavior. Someone may
be relying on it... I agree that sometimes we can block userspace process
in kernel for a long time (e.g. in my testing I often see syslog process
doing the printing) but so far I didn't see / was notified about some real
problem with this. So unless I see some real user issues with user
processes doing printing for too long I would not touch this.
> and
>
> printk: introduce console_sync_mode
>
> console_sync_mode() should be called early in panic() to switch
> printk from async mode to sync. Otherwise, STOP IPIs can arrive
> to other CPUs too late and those CPUs will see oops_in_progress
> being 0 again.
So as I wrote, I like this in principle but there are much more places
calling console_verbose() and all of them want console_sync_mode() as well.
So I prefer hiding the sync printing in console_verbose() and possibly
renaming it to something better but I'm not sure renaming is worth it.
Honza
>
> -ss
> From c3fc955809adab8f497cdc7581e67e1fa29d6517 Mon Sep 17 00:00:00 2001
> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Date: Wed, 30 Dec 2015 20:39:12 +0900
> Subject: [PATCH 1/2] printk: introduce console_sync_mode
>
> console_sync_mode() should be called early in panic() to switch
> printk from async mode to sync. Otherwise, STOP IPIs can arrive
> to other CPUs too late and those CPUs will see oops_in_progress
> being 0 again.
> ---
> include/linux/console.h | 1 +
> kernel/panic.c | 1 +
> kernel/printk/printk.c | 5 +++++
> 3 files changed, 7 insertions(+)
>
> diff --git a/include/linux/console.h b/include/linux/console.h
> index bd19434..f068985 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -150,6 +150,7 @@ extern int console_trylock(void);
> extern void console_unlock(void);
> extern void console_conditional_schedule(void);
> extern void console_unblank(void);
> +extern void console_sync_mode(void);
> extern struct tty_driver *console_device(int *);
> extern void console_stop(struct console *);
> extern void console_start(struct console *);
> diff --git a/kernel/panic.c b/kernel/panic.c
> index b333380..04c8ff4 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -117,6 +117,7 @@ void panic(const char *fmt, ...)
> if (old_cpu != PANIC_CPU_INVALID && old_cpu != this_cpu)
> panic_smp_self_stop();
>
> + console_sync_mode();
> console_verbose();
> bust_spinlocks(1);
> va_start(args, fmt);
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index de9d31b..47a70a2 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2501,6 +2501,11 @@ void console_unblank(void)
> console_unlock();
> }
>
> +void console_sync_mode(void)
> +{
> + printk_sync = true;
> +}
> +
> /*
> * Return the console tty driver structure and its associated index
> */
> --
> 2.6.4
>
> From 92f2c0f2a5ed015caa2757dcfec4407d708f8628 Mon Sep 17 00:00:00 2001
> From: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Date: Thu, 31 Dec 2015 13:39:58 +0900
> Subject: [PATCH 2/2] printk: factor out can_printk_async()
>
> console_unlock() can be called directly or indirectly by a user
> space process, so it can end up doing call_console_drivers() loop,
> which will hold it from returning back to user-space from a syscall
> for unpredictable amount of time.
>
> Factor out can_printk_async() function, which queues an irq work and
> sets a PRINTK_PENDING_OUTPUT pending bit (if we can do async printk).
> vprintk_emit() already does it, add can_printk_async() call to
> console_unlock() for !PF_KTHREAD processes.
> ---
> kernel/printk/printk.c | 42 ++++++++++++++++++++++++++++--------------
> 1 file changed, 28 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 47a70a2..7d3a8e1 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -355,6 +355,26 @@ int printk_deferred(const char *fmt, ...)
> return r;
> }
>
> +static bool can_printk_async(bool sync)
> +{
> + /*
> + * By default we print message to console asynchronously so that kernel
> + * doesn't get stalled due to slow serial console. That can lead to
> + * softlockups, lost interrupts, or userspace timing out under heavy
> + * printing load.
> + *
> + * However we resort to synchronous printing of messages during early
> + * boot, when oops is in progress, or when synchronous printing was
> + * explicitely requested by kernel parameter.
> + */
> + if (keventd_up() && !oops_in_progress && !sync) {
> + __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> + irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> + return true;
> + }
> + return false;
> +}
> +
> /* Return log buffer address */
> char *log_buf_addr_get(void)
> {
> @@ -1889,20 +1909,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> logbuf_cpu = UINT_MAX;
> raw_spin_unlock(&logbuf_lock);
> lockdep_on();
> - /*
> - * By default we print message to console asynchronously so that kernel
> - * doesn't get stalled due to slow serial console. That can lead to
> - * softlockups, lost interrupts, or userspace timing out under heavy
> - * printing load.
> - *
> - * However we resort to synchronous printing of messages during early
> - * boot, when oops is in progress, or when synchronous printing was
> - * explicitely requested by kernel parameter.
> - */
> - if (keventd_up() && !oops_in_progress && !sync_print) {
> - __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
> - irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> - } else
> + if (!can_printk_async(sync_print))
> sync_print = true;
> local_irq_restore(flags);
>
> @@ -2328,6 +2335,13 @@ void console_unlock(void)
> return;
> }
>
> + if (!(current->flags & PF_KTHREAD) &&
> + can_printk_async(printk_sync)) {
> + console_locked = 0;
> + up_console_sem();
> + return;
> + }
> +
> /*
> * Console drivers are called under logbuf_lock, so
> * @console_may_schedule should be cleared before; however, we may
> --
> 2.6.4
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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