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

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Message-ID: <871rchw4x2.fsf@linux.intel.com>
Date:   Sun, 14 Mar 2021 19:02:01 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Yu Zhao <yuzhao@...gle.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

Yu Zhao <yuzhao@...gle.com> writes:
> +
> +#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?

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?

-Andi