linux-kernel - Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd

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Date:	Sun, 14 Jun 2015 09:36:52 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Brian Gerst <brgerst@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <Waiman.Long@...com>,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the
 memory hotplug code


* Oleg Nesterov <oleg@...hat.com> wrote:

> On 06/13, Ingo Molnar wrote:
> >
> > @@ -169,29 +169,40 @@ void sync_global_pgds(unsigned long start, unsigned long end, int removed)
> >
> >  	for (address = start; address <= end; address += PGDIR_SIZE) {
> >  		const pgd_t *pgd_ref = pgd_offset_k(address);
> > -		struct page *page;
> > +		struct task_struct *g, *p;
> >
> >  		/*
> > -		 * When it is called after memory hot remove, pgd_none()
> > -		 * returns true. In this case (removed == 1), we must clear
> > -		 * the PGD entries in the local PGD level page.
> > +		 * When this function is called after memory hot remove,
> > +		 * pgd_none() already returns true, but only the reference
> > +		 * kernel PGD has been cleared, not the process PGDs.
> > +		 *
> > +		 * So clear the affected entries in every process PGD as well:
> >  		 */
> >  		if (pgd_none(*pgd_ref) && !removed)
> >  			continue;
> >
> > -		spin_lock(&pgd_lock);
> > -		list_for_each_entry(page, &pgd_list, lru) {
> > +		spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */
>                                          ^^^^^^^^^^^^^^^^^^^^^^^
> 
> Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand how it 
> actually works ;) But, say, rcu_check_callbacks() can be called from irq and 
> since spin_lock() doesn't increment current->rcu_read_lock_nesting this can lead 
> to rcu_preempt_qs()?

No, RCU grace periods are still defined by 'heavy' context boundaries such as 
context switches, entering idle or user-space mode.

PREEMPT_RCU is like traditional RCU, except that blocking is allowed within the 
RCU read critical section - that is why it uses a separate nesting counter 
(current->rcu_read_lock_nesting), not the preempt count.

But if a piece of kernel code is non-preemptible, such as a spinlocked region or 
an irqs-off region, then those are still natural RCU read lock regions, regardless 
of the RCU model, and need no additional RCU locking.

rcu_check_callbacks() can be called from irq context, but only to observe whether 
the current CPU is in quiescent state. If it interrupts a spinlocked region it 
won't register a quiesent state.

> > +		for_each_process_thread(g, p) {
> > +			struct mm_struct *mm;
> >  			pgd_t *pgd;
> >  			spinlock_t *pgt_lock;
> >
> > -			pgd = (pgd_t *)page_address(page) + pgd_index(address);
> > -			/* the pgt_lock only for Xen */
> > -			pgt_lock = &pgd_page_get_mm(page)->page_table_lock;
> > +			task_lock(p);
> > +			mm = p->mm;
> > +			if (!mm) {
> > +				task_unlock(p);
> > +				continue;
> > +			}
> 
> Again, you can simplify this code and avoid for_each_process_thread() if you use 
> for_each_process() + find_lock_task_mm().

True!

So I looked at this when you first mentioned it but mis-read find_lock_task_mm(), 
which as you insist is exactly what this iteration needs to become faster and 
simpler. Thanks for the reminder - I have fixed it, will be part of -v3.

Thanks,

	Ingo
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