Re: [PATCH v18 06/32] mm/thp: narrow lru locking

[Matthew Wilcox]

On Fri, Sep 11, 2020 at 11:37:50AM +0800, Alex Shi wrote:
> 
> 
> 在 2020/9/10 下午9:49, Matthew Wilcox 写道:
> > On Mon, Aug 24, 2020 at 08:54:39PM +0800, Alex Shi wrote:
> >> lru_lock and page cache xa_lock have no reason with current sequence,
> >> put them together isn't necessary. let's narrow the lru locking, but
> >> left the local_irq_disable to block interrupt re-entry and statistic update.
> > 
> > What stats are you talking about here?
> 
> Hi Matthew,
> 
> Thanks for comments!
> 
> like __dec_node_page_state(head, NR_SHMEM_THPS); will have preemptive warning...

OK, but those stats are guarded by 'if (mapping)', so this patch doesn't
produce that warning because we'll have taken the xarray lock and disabled
interrupts.

> > How about this patch instead?  It occurred to me we already have
> > perfectly good infrastructure to track whether or not interrupts are
> > already disabled, and so we should use that instead of ensuring that
> > interrupts are disabled, or tracking that ourselves.
> 
> So your proposal looks like;
> 1, xa_lock_irq(&mapping->i_pages); (optional)
> 2, spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
> 3, spin_lock_irqsave(&pgdat->lru_lock, flags);
> 
> Is there meaningful for the 2nd and 3rd flags?

Yes.  We want to avoid doing:

	if (mapping)
		spin_lock(&ds_queue->split_queue_lock);
	else
		spin_lock_irq(&ds_queue->split_queue_lock);
...
	if (mapping)
		spin_unlock(&ds_queue->split_queue_lock);
	else
		spin_unlock_irq(&ds_queue->split_queue_lock);

Just using _irqsave has the same effect and is easier to reason about.

> IIRC, I had a similar proposal as your, the flags used in xa_lock_irqsave(),
> but objected by Hugh.

I imagine Hugh's objection was that we know it's safe to disable/enable
interrupts here because we're in a sleepable context.  But for the
other two locks, we'd rather not track whether we've already disabled
interrupts or not.

Maybe you could dig up the email from Hugh?  I can't find it.