Re: [PATCH v2] mm: fs: invalidate bh_lrus for only cold path

[Minchan Kim <minchan@kernel.org>]

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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@kernel.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@kernel.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(a)xsang-OptiPlex-9020/
[2] 8cc621d2f45d, mm: fs: invalidate BH LRU during page migration
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Minchan Kim <minchan@kernel.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
 }
 
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2.32.0.rc0.204.g9fa02ecfa5-goog