Date:   Wed, 09 Jun 2021 13:52:46 -0700
From:   Chris Goldsworthy <cgoldswo@...eaurora.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.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>,
        Minchan Kim <minchan.kim@...il.com>
Subject: Re: [PATCH v2] mm: fs: invalidate bh_lrus for only cold path

On 2021-06-02 15:45, Minchan Kim wrote:
> On Tue, Jun 01, 2021 at 04:15:40PM -0700, Andrew Morton wrote:
>> 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.
> 
> The lru_add_drain_per_cpu will run on per-cpu since it's per-cpu work
> and invalidate_bh_lrus_cpu will run under bh_lru_lock so I couldn't
> imagine that race can happen.
> 
>> 
>> 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?
> 
> I just wanted to express both lru_add_drain_cpu and 
> invalidate_bh_lrus_cpu
> in lru_add_and_bh_lrus_drain run in the same cpu but look like a bad 
> idea
> since it makes people confused. Let me remove the cpu argument from
> invalidate_bh_lrus_cpu.
> 
>> 
>> 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().
>> 
> 
> Hopefully, this is better.
> 
> From 8d58e7ade3ed6c080995dec1395b1e130b3d16b3 Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@...nel.org>
> Date: Tue, 25 May 2021 08:19:17 -0700
> Subject: [PATCH] mm: fs: invalidate bh_lrus for only cold path
> 
> 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).
> 
> [1] 
> https://lore.kernel.org/lkml/20210520083144.GD14190@xsang-OptiPlex-9020/
> [2] 8cc621d2f45d, mm: fs: invalidate BH LRU during page migration
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Signed-off-by: Minchan Kim <minchan@...nel.org>
> ---
>  fs/buffer.c                 |  8 ++++++--
>  include/linux/buffer_head.h |  4 ++--
>  mm/swap.c                   | 19 ++++++++++++++++---
>  3 files changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 673cfbef9eec..bdaffed39030 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1487,12 +1487,16 @@ void invalidate_bh_lrus(void)
>  }
>  EXPORT_SYMBOL_GPL(invalidate_bh_lrus);
> 
> -void invalidate_bh_lrus_cpu(int cpu)
> +/*
> + * It's called from workqueue context so we need a bh_lru_lock to 
> close
> + * the race with preemption/irq.
> + */
> +void invalidate_bh_lrus_cpu(void)
>  {
>  	struct bh_lru *b;
> 
>  	bh_lru_lock();
> -	b = per_cpu_ptr(&bh_lrus, cpu);
> +	b = this_cpu_ptr(&bh_lrus);
>  	__invalidate_bh_lrus(b);
>  	bh_lru_unlock();
>  }
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index e7e99da31349..b04d34bab124 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -194,7 +194,7 @@ void __breadahead_gfp(struct block_device *,
> sector_t block, unsigned int size,
>  struct buffer_head *__bread_gfp(struct block_device *,
>  				sector_t block, unsigned size, gfp_t gfp);
>  void invalidate_bh_lrus(void);
> -void invalidate_bh_lrus_cpu(int cpu);
> +void invalidate_bh_lrus_cpu(void);
>  bool has_bh_in_lru(int cpu, void *dummy);
>  struct buffer_head *alloc_buffer_head(gfp_t gfp_flags);
>  void free_buffer_head(struct buffer_head * bh);
> @@ -408,7 +408,7 @@ static inline int inode_has_buffers(struct inode
> *inode) { return 0; }
>  static inline void invalidate_inode_buffers(struct inode *inode) {}
>  static inline int remove_inode_buffers(struct inode *inode) { return 
> 1; }
>  static inline int sync_mapping_buffers(struct address_space *mapping)
> { return 0; }
> -static inline void invalidate_bh_lrus_cpu(int cpu) {}
> +static inline void invalidate_bh_lrus_cpu(void) {}
>  static inline bool has_bh_in_lru(int cpu, void *dummy) { return 0; }
>  #define buffer_heads_over_limit 0
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 1958d5feb148..4d9ec3c3c5a9 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -642,7 +642,6 @@ void lru_add_drain_cpu(int cpu)
>  		pagevec_lru_move_fn(pvec, lru_lazyfree_fn);
> 
>  	activate_page_drain(cpu);
> -	invalidate_bh_lrus_cpu(cpu);
>  }
> 
>  /**
> @@ -725,6 +724,20 @@ void lru_add_drain(void)
>  	local_unlock(&lru_pvecs.lock);
>  }
> 
> +/*
> + * It's called from per-cpu workqueue context in SMP case so
> + * lru_add_drain_cpu and invalidate_bh_lrus_cpu should run on
> + * the same cpu. It shouldn't be a problem in !SMP case since
> + * the core is only one and the locks will disable preemption.
> + */
> +static void lru_add_and_bh_lrus_drain(void)
> +{
> +	local_lock(&lru_pvecs.lock);
> +	lru_add_drain_cpu(smp_processor_id());
> +	local_unlock(&lru_pvecs.lock);
> +	invalidate_bh_lrus_cpu();
> +}
> +
>  void lru_add_drain_cpu_zone(struct zone *zone)
>  {
>  	local_lock(&lru_pvecs.lock);
> @@ -739,7 +752,7 @@ static DEFINE_PER_CPU(struct work_struct,
> lru_add_drain_work);
> 
>  static void lru_add_drain_per_cpu(struct work_struct *dummy)
>  {
> -	lru_add_drain();
> +	lru_add_and_bh_lrus_drain();
>  }
> 
>  /*
> @@ -880,7 +893,7 @@ void lru_cache_disable(void)
>  	 */
>  	__lru_add_drain_all(true);
>  #else
> -	lru_add_drain();
> +	lru_add_and_bh_lrus_drain();
>  #endif
>  }

Hi Minchan,

This looks good to me.  Feel free to add:

Reviewed-by: Chris Goldsworthy <cgoldswo@...eaurora.org>

Thanks,

Chris.

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