* Oleg Nesterov <oleg@xxxxxxxxxx> 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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