linux-kernel - Re: [PATCH v1 10/14] mm: multigenerational lru: core

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Date:   Sun, 14 Mar 2021 21:37:27 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     linux-mm@...ck.org, Alex Shi <alex.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...e.com>,
        Roman Gushchin <guro@...com>, Vlastimil Babka <vbabka@...e.cz>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Yang Shi <shy828301@...il.com>,
        Ying Huang <ying.huang@...el.com>,
        linux-kernel@...r.kernel.org, page-reclaim@...gle.com
Subject: Re: [PATCH v1 10/14] mm: multigenerational lru: core

On Sun, Mar 14, 2021 at 07:02:01PM -0700, Andi Kleen wrote:
> Yu Zhao <yuzhao@...gle.com> writes:

Hi Andi!

Recovering the context a bit:

		err = -EBUSY;

> > +
> > +#ifdef CONFIG_MEMCG
> > +		if (memcg && atomic_read(&memcg->moving_account))
> > +			goto contended;
> > +#endif
> > +		if (!mmap_read_trylock(mm))
> > +			goto contended;
> 
> These are essentially spinloops. Surely you need a cpu_relax() somewhere?

contended:
		...
		cond_resched();
	} while (err == -EAGAIN && !mm_is_oom_victim(mm) && !mm_has_migrated(mm, memcg));

So if it's contended, we break the loop.

> In general for all of spinloop like constructs it would be useful to
> consider how to teach lockdep about them.
> 
> > +	do {
> > +		old_flags = READ_ONCE(page->flags);
> > +		new_gen = ((old_flags & LRU_GEN_MASK) >> LRU_GEN_PGOFF) - 1;
> > +		VM_BUG_ON_PAGE(new_gen < 0, page);
> > +		if (new_gen >= 0 && new_gen != old_gen)
> > +			goto sort;
> > +
> > +		new_gen = (old_gen + 1) % MAX_NR_GENS;
> > +		new_flags = (old_flags & ~LRU_GEN_MASK) | ((new_gen + 1UL) << LRU_GEN_PGOFF);
> > +		/* mark page for reclaim if pending writeback */
> > +		if (front)
> > +			new_flags |= BIT(PG_reclaim);
> > +	} while (cmpxchg(&page->flags, old_flags, new_flags) !=
> > old_flags);
> 
> I see this cmpxchg flags pattern a lot. Could there be some common code
> factoring?

Thanks for noticing this. A shorthand macro would be nice. Hmm... let
me investigate. I don't know how to do this off the top of my head:

A macro can be used like

  cmpxchg_macro() {
     func();
  }

without backslashes and it'll be expanded to

  do {
     func();
   } while ();