linux-kernel - Re: [PATCH printk v1 03/13] printk: use percpu flag instead of cpu

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Date:   Wed, 16 Feb 2022 12:29:29 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        John Ogness <john.ogness@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH printk v1 03/13] printk: use percpu flag instead of
 cpu_online()

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.