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@xxxxxxxxxx>
> > 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@xxxxxxxxxx>
> 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@xxxxxxxxx>
> Signed-off-by: Minchan Kim <minchan@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxx>
