From: Sergey Senozhatsky <senozhatsky@chromium.org>
To: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
John Ogness <john.ogness@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH printk v1 03/13] printk: use percpu flag instead of cpu_online()
Date: Wed, 16 Feb 2022 12:29:29 +0900 [thread overview]
Message-ID: <YgxvmepaTqRxwn/o@google.com> (raw)
In-Reply-To: <YguCuFYeZ52mkr4r@alley>
On (22/02/15 11:38), Petr Mladek wrote:
> On Mon 2022-02-14 16:35:18, Sergey Senozhatsky wrote:
> > On (22/02/11 17:05), Petr Mladek wrote:
> > > On Mon 2022-02-07 20:49:13, John Ogness wrote:
> > [..]
> > > The problem is the commit ac25575203c11145066ea ("[PATCH] CPU hotplug
> > > printk fix"). It suggests that per-CPU data of slab are freed during
> > > hotplug.
> > >
> > > There are many other things that are manipulated during cpu hotplug.
> > > And there are the two notifiers "printk:dead" and "printk:online",
> > > see printk_late_init(). Maybe, we should use them to decide whether
> > > the non-trivial consoles are callable during CPU hotplug.
> >
> > Great findings. Looks like we only set __printk_percpu_data_ready to
> > true and never set it back to false, relying on cpu_online() in such
> > cases. But here's the thing: we have printk_percpu_data_ready() in
> > __printk_recursion_counter() and in wake_up_klogd() and in
> > defer_console_output(), but why we never check __printk_percpu_data_ready
> > in __down_trylock_console_sem()/__up_console_sem() and more importantly
> > in console_trylock_spinning() and those do access this_cpu() in printk safe
> > enter/exit. Am I missing something?
>
> Great point!
>
> I am not 100% sure. But it seems that static per-CPU variables might
> actually be used since the boot.
Wow, this is great to learn. Thanks!
> This is from mm/percpu.c
>
> * There is special consideration for the first chunk which must handle
> * the static percpu variables in the kernel image as allocation services
> * are not online yet. In short, the first chunk is structured like so:
> *
> * <Static | [Reserved] | Dynamic>
> *
> * The static data is copied from the original section managed by the
> * linker. The reserved section, if non-zero, primarily manages static
> * percpu variables from kernel modules. Finally, the dynamic section
> * takes care of normal allocations.
>
>
> I thought that it might work only for CPU0. But it seems that it
> probably works for each possible cpu, see:
>
> bool __is_kernel_percpu_address(unsigned long addr, unsigned long *can_addr)
> {
> #ifdef CONFIG_SMP
> const size_t static_size = __per_cpu_end - __per_cpu_start;
> void __percpu *base = __addr_to_pcpu_ptr(pcpu_base_addr);
> unsigned int cpu;
>
> for_each_possible_cpu(cpu) {
> void *start = per_cpu_ptr(base, cpu);
> void *va = (void *)addr;
>
> if (va >= start && va < start + static_size) {
> [...]
> }
>
> and
>
> /**
> * is_kernel_percpu_address - test whether address is from static percpu area
> * @addr: address to test
> *
> * Test whether @addr belongs to in-kernel static percpu area. Module
> * static percpu areas are not considered. For those, use
> * is_module_percpu_address().
> *
> * RETURNS:
> * %true if @addr is from in-kernel static percpu area, %false otherwise.
> */
> bool is_kernel_percpu_address(unsigned long addr)
> {
> return __is_kernel_percpu_address(addr, NULL);
> }
>
>
> Most likely, only dynamically allocated per-cpu variables have to wait
> until the per-cpu areas are initialized.
>
> This might explain why there is no generic
> are_per_cpu_variables_ready() callback.
>
> We should probably revisit the code and remove the fallback to
> normal static variables.
Agreed.
next prev parent reply other threads:[~2022-02-16 3:29 UTC|newest]
Thread overview: 87+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-07 19:43 [PATCH printk v1 00/13] implement threaded console printing John Ogness
2022-02-07 19:43 ` [PATCH printk v1 01/13] printk: rename cpulock functions John Ogness
2022-02-11 12:44 ` Petr Mladek
2022-02-11 14:42 ` John Ogness
2022-02-11 20:57 ` Steven Rostedt
2022-02-11 21:04 ` Peter Zijlstra
2022-02-15 9:32 ` Petr Mladek
2022-02-15 9:13 ` Petr Mladek
2022-02-14 6:49 ` Sergey Senozhatsky
2022-02-14 9:45 ` John Ogness
2022-02-15 9:29 ` Petr Mladek
2022-02-16 3:27 ` Sergey Senozhatsky
2022-02-17 14:34 ` John Ogness
2022-02-07 19:43 ` [PATCH printk v1 02/13] printk: cpu sync always disable interrupts John Ogness
2022-02-11 12:58 ` Petr Mladek
2022-02-14 6:36 ` Sergey Senozhatsky
2022-02-07 19:43 ` [PATCH printk v1 03/13] printk: use percpu flag instead of cpu_online() John Ogness
2022-02-11 16:05 ` Petr Mladek
2022-02-14 7:08 ` Sergey Senozhatsky
2022-02-14 7:35 ` Sergey Senozhatsky
2022-02-15 10:38 ` Petr Mladek
2022-02-16 3:29 ` Sergey Senozhatsky [this message]
2022-03-02 14:21 ` John Ogness
2022-03-04 15:56 ` Petr Mladek
2022-03-05 17:05 ` Jason A. Donenfeld
2022-03-07 16:14 ` Petr Mladek
2022-02-16 13:58 ` two locations: was: " Petr Mladek
2022-03-02 14:49 ` John Ogness
2022-03-04 16:14 ` Petr Mladek
2022-03-07 10:06 ` John Ogness
2022-03-08 16:08 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 04/13] printk: get caller_id/timestamp after migration disable John Ogness
2022-02-15 5:53 ` Sergey Senozhatsky
2022-02-15 11:56 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 05/13] printk: call boot_delay_msec() in printk_delay() John Ogness
2022-02-15 5:58 ` Sergey Senozhatsky
2022-02-15 14:59 ` Petr Mladek
2022-02-16 3:21 ` Sergey Senozhatsky
2022-02-15 15:03 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 06/13] printk: refactor and rework printing logic John Ogness
2022-02-16 15:43 ` Petr Mladek
2022-03-02 16:10 ` John Ogness
2022-02-07 19:43 ` [PATCH printk v1 07/13] printk: move buffer definitions into console_emit_next_record() caller John Ogness
2022-02-16 16:10 ` Petr Mladek
2022-03-02 16:25 ` John Ogness
2022-02-07 19:43 ` [PATCH printk v1 08/13] printk: add pr_flush() John Ogness
2022-02-17 10:11 ` Petr Mladek
2022-03-02 17:23 ` John Ogness
2022-03-04 13:24 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 09/13] printk: add functions to allow direct printing John Ogness
2022-02-17 12:52 ` Petr Mladek
2022-02-18 9:00 ` David Laight
2022-02-18 12:52 ` Petr Mladek
2022-03-03 14:37 ` John Ogness
2022-02-07 19:43 ` [PATCH printk v1 10/13] printk: add kthread console printers John Ogness
2022-02-18 9:00 ` early start: was: " Petr Mladek
2022-02-18 9:04 ` start&stop: " Petr Mladek
2022-02-18 9:08 ` main loop: " Petr Mladek
2022-02-18 9:12 ` wake_up_all: " Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 11/13] printk: reimplement console_lock for proper kthread support John Ogness
2022-02-18 16:20 ` Petr Mladek
2022-02-18 21:41 ` John Ogness
2022-02-18 22:03 ` John Ogness
2022-02-22 11:42 ` Petr Mladek
2022-02-23 17:20 ` John Ogness
2022-02-24 8:27 ` Petr Mladek
2022-02-23 10:19 ` Petr Mladek
2022-03-09 13:56 ` John Ogness
2022-03-10 14:34 ` Petr Mladek
2022-03-10 16:08 ` John Ogness
2022-03-11 10:26 ` Petr Mladek
2022-03-11 13:28 ` John Ogness
2022-03-11 16:17 ` Petr Mladek
2022-03-11 22:21 ` John Ogness
2022-03-14 14:08 ` Petr Mladek
2022-03-14 14:43 ` John Ogness
2022-03-14 15:53 ` Petr Mladek
2022-03-11 18:41 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 12/13] printk: remove @console_locked John Ogness
2022-02-23 12:17 ` Petr Mladek
2022-02-07 19:43 ` [PATCH printk v1 13/13] console: introduce CON_MIGHT_SLEEP for vt John Ogness
2022-02-23 13:37 ` Petr Mladek
2022-02-23 18:31 ` Greg Kroah-Hartman
[not found] ` <20220208083620.2736-1-hdanton@sina.com>
2022-02-08 11:08 ` [PATCH printk v1 10/13] printk: add kthread console printers John Ogness
2022-02-08 14:53 ` Petr Mladek
2022-02-14 6:12 ` Sergey Senozhatsky
2022-02-14 10:02 ` Petr Mladek
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