linux-kernel - Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork

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Message-Id: <20220426183448.3538d53eb9aafb48d1bfdb2b@linux-foundation.org>
Date:   Tue, 26 Apr 2022 18:34:48 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yu Zhao <yuzhao@...gle.com>
Cc:     Stephen Rothwell <sfr@...hwell.id.au>,
        Linux-MM <linux-mm@...ck.org>, Andi Kleen <ak@...ux.intel.com>,
        Aneesh Kumar <aneesh.kumar@...ux.ibm.com>,
        Barry Song <21cnbao@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hillf Danton <hdanton@...a.com>, Jens Axboe <axboe@...nel.dk>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Michael Larabel <Michael@...haellarabel.com>,
        Michal Hocko <mhocko@...nel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
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        Ying Huang <ying.huang@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
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        Holger Hoffstätte 
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        Shuang Zhai <szhai2@...rochester.edu>,
        Sofia Trinh <sofia.trinh@....works>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH v10 05/14] mm: multi-gen LRU: groundwork

On Tue, 26 Apr 2022 19:18:21 -0600 Yu Zhao <yuzhao@...gle.com> wrote:

> > For example, lru_gen_add_folio() is huge and has 4(?) call sites.  This
> > may well produce slower code due to the icache footprint.
> >
> > Experiment: moving lru_gen_del_folio() into mm/vmscan.c shrinks that
> > file's .text from 80612 bytes to 78956.
> >
> > I tend to think that out-of-line regular old C functions should be the
> > default and that the code should be inlined only when a clear benefit
> > is demonstrable, or has at least been seriously thought about.
> 
> I can move those functions to vmscan.c if you think it would improve
> performance. I don't have a strong opinion here -- I was able to
> measure the bloat but not the performance impact.

This seems to be more an act of faith than anything else.  Unlikely
that any difference will be measurable.

If there is a difference, the inlined version should win on
microbenchmarks because all four copies of the function will be in
cache.  But a more realistic, broader test might suffer a slowdown due
to having to move the larger text in more frequently.  And inter-build
alignment changes seem to make a larger difference than anything else,
thus confounding measurement attempts.