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Date:   Sun, 13 Sep 2020 16:27:03 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     akpm@linux-foundation.org, mgorman@techsingularity.net,
        tj@kernel.org, hughd@google.com, khlebnikov@yandex-team.ru,
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        linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, shakeelb@google.com,
        iamjoonsoo.kim@lge.com, richard.weiyang@gmail.com,
        kirill@shutemov.name, alexander.duyck@gmail.com,
        rong.a.chen@intel.com, mhocko@suse.com, vdavydov.dev@gmail.com,
        shy828301@gmail.com, Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v18 06/32] mm/thp: narrow lru locking
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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.