linux-kernel - Re: [PATCH v6 1/3] workingset: refactor LRU refault to expose refault recency check

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Message-ID: <Y8rjMHXNH8KVKapZ@bfoster>
Date:   Fri, 20 Jan 2023 13:53:36 -0500
From:   Brian Foster <bfoster@...hat.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Nhat Pham <nphamcs@...il.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        willy@...radead.org, linux-api@...r.kernel.org,
        kernel-team@...a.com, Yu Zhao <yuzhao@...gle.com>
Subject: Re: [PATCH v6 1/3] workingset: refactor LRU refault to expose
 refault recency check

On Fri, Jan 20, 2023 at 11:27:12AM -0500, Johannes Weiner wrote:
> On Fri, Jan 20, 2023 at 09:34:18AM -0500, Brian Foster wrote:
> > On Tue, Jan 17, 2023 at 11:59:57AM -0800, Nhat Pham wrote:
> > > +	int memcgid;
> > > +	struct pglist_data *pgdat;
> > > +	unsigned long token;
> > > +
> > > +	unpack_shadow(shadow, &memcgid, &pgdat, &token, workingset);
> > > +	eviction_memcg = mem_cgroup_from_id(memcgid);
> > > +
> > > +	lruvec = mem_cgroup_lruvec(eviction_memcg, pgdat);
> > > +	lrugen = &lruvec->lrugen;
> > > +
> > > +	min_seq = READ_ONCE(lrugen->min_seq[file]);
> > > +	return !((token >> LRU_REFS_WIDTH) != (min_seq & (EVICTION_MASK >> LRU_REFS_WIDTH)));
> > 
> > I think this might be more readable without the double negative.
> > 
> > Also it looks like this logic is pulled from lru_gen_refault(). Any
> > reason the caller isn't refactored to use this helper, similar to how
> > workingset_refault() is modified? It seems like a potential landmine to
> > duplicate the logic here for cachestat purposes and somewhere else for
> > actual workingset management.
> 
> The initial version was refactored. Yu explicitly requested it be
> duplicated [1] to cut down on some boiler plate.
> 

Ah, sorry for missing the previous discussion. TBH I wasn't terribly
comfortable reviewing this one until I had made enough passes at the
second patch..

> I have to agree with Brian on this one, though. The factored version
> is better for maintenance than duplicating the core logic here. Even
> if it ends up a bit more boiler plate - it's harder to screw that up,
> and easier to catch at compile time, than the duplicates diverging.
> 

It seems more elegant to me, FWIW. Glad I'm not totally off the rails at
least. ;) But I'll defer to those who know the code better and the
author, so that's just my .02. I don't want to cause this to go around
in circles..

Brian

> [1] https://lore.kernel.org/lkml/CAOUHufZKTqoD2rFwrX9-eCknBmeWqP88rZ7X7A_5KHHbGBUP=A@mail.gmail.com/
>