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

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Message-ID: <YguCuFYeZ52mkr4r@alley>
Date:   Tue, 15 Feb 2022 11:38:48 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     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 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.

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.

Best Regards,
Petr