linux-kernel - Re: [PATCH v2] mm: fs: invalidate bh

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Message-Id: <20210601161540.9f449314965bd94c84725481@linux-foundation.org>
Date:   Tue, 1 Jun 2021 16:15:40 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Chris Goldsworthy <cgoldswo@...eaurora.org>,
        Laura Abbott <labbott@...nel.org>,
        Oliver Sang <oliver.sang@...el.com>,
        David Hildenbrand <david@...hat.com>,
        John Dias <joaodias@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...el.com, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v2] mm: fs: invalidate bh_lrus for only cold path

On Tue,  1 Jun 2021 07:54:25 -0700 Minchan Kim <minchan@...nel.org> wrote:

> kernel test robot reported the regression of fio.write_iops[1]
> with [2].
> 
> Since lru_add_drain is called frequently, invalidate bh_lrus
> there could increase bh_lrus cache miss ratio, which needs
> more IO in the end.
> 
> This patch moves the bh_lrus invalidation from the hot path(
> e.g., zap_page_range, pagevec_release) to cold path(i.e.,
> lru_add_drain_all, lru_cache_disable).

This code is starting to hurt my brain.

What are the locking/context rules for invalidate_bh_lrus_cpu()? 
AFAICT it offers no protection against two CPUs concurrently running
__invalidate_bh_lrus() against the same bh_lru.

So when CONFIG_SMP=y, invalidate_bh_lrus_cpu() must always and only be
run on the cpu which owns the bh_lru.  In which case why does it have
the `cpu' arg?

Your new lru_add_and_bh_lrus_drain() follows these rules by calling
invalidate_bh_lrus_cpu() from a per-cpu worker or when CONFIG_SMP=n.

I think.  It's all as clear as mud and undocumented.  Could you please
take a look at this?  Comment the locking/context rules thoroughly and
check that they are being followed?  Not forgetting cpu hotplug...  See if
there's a way of simplifying/clarifying the code?

The fact that swap.c has those #ifdef CONFIG_SMPs in there is a hint
that we're doing something wrong (or poorly) in there.  Perhaps that's
unavoidable because of all the fancy footwork in __lru_add_drain_all().