diff mbox series

[4/8] mm/swap: Use local_lock for protection

Message ID 20200519201912.1564477-5-bigeasy@linutronix.de (mailing list archive)
State New, archived
Headers show
Series None | expand

Commit Message

Sebastian Andrzej Siewior May 19, 2020, 8:19 p.m. UTC 
From: Ingo Molnar <mingo@kernel.org>

The various struct pagevec per CPU variables are protected by disabling
either preemption or interrupts across the critical sections. Inside
these sections spinlocks have to be acquired.

These spinlocks are regular spinlock_t types which are converted to
"sleeping" spinlocks on PREEMPT_RT enabled kernels. Obviously sleeping
locks cannot be acquired in preemption or interrupt disabled sections.

local locks provide a trivial way to substitute preempt and interrupt
disable instances. On a non PREEMPT_RT enabled kernel local_lock() maps
to preempt_disable() and local_lock_irq() to local_irq_disable().

Add swapvec_lock to protect the per-CPU lru_add_pvec and
lru_lazyfree_pvecs variables and rotate_lock to protect the per-CPU
lru_rotate_pvecs variable

Change the relevant call sites to acquire these locks instead of using
preempt_disable() / get_cpu() / get_cpu_var() and local_irq_disable() /
local_irq_save().

There is neither a functional change nor a change in the generated
binary code for non PREEMPT_RT enabled non-debug kernels.

When lockdep is enabled local locks have lockdep maps embedded. These
allow lockdep to validate the protections, i.e. inappropriate usage of a
preemption only protected sections would result in a lockdep warning
while the same problem would not be noticed with a plain
preempt_disable() based protection.

local locks also improve readability as they provide a named scope for
the protections while preempt/interrupt disable are opaque scopeless.

Finally local locks allow PREEMPT_RT to substitute them with real
locking primitives to ensure the correctness of operation in a fully
preemptible kernel.
No functional change.

[ bigeasy: Adopted to use local_lock ]

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-mm@kvack.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 include/linux/swap.h |  2 ++
 mm/compaction.c      |  7 +++---
 mm/swap.c            | 59 ++++++++++++++++++++++++++++++--------------
 3 files changed, 45 insertions(+), 23 deletions(-)

Comments

Andrew Morton May 19, 2020, 11:58 p.m. UTC | #1 
On Tue, 19 May 2020 22:19:08 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:

> From: Ingo Molnar <mingo@kernel.org>
> 
> The various struct pagevec per CPU variables are protected by disabling
> either preemption or interrupts across the critical sections. Inside
> these sections spinlocks have to be acquired.
> 
> These spinlocks are regular spinlock_t types which are converted to
> "sleeping" spinlocks on PREEMPT_RT enabled kernels. Obviously sleeping
> locks cannot be acquired in preemption or interrupt disabled sections.
> 
> local locks provide a trivial way to substitute preempt and interrupt
> disable instances. On a non PREEMPT_RT enabled kernel local_lock() maps
> to preempt_disable() and local_lock_irq() to local_irq_disable().
> 
> Add swapvec_lock to protect the per-CPU lru_add_pvec and
> lru_lazyfree_pvecs variables and rotate_lock to protect the per-CPU
> lru_rotate_pvecs variable
> 
> Change the relevant call sites to acquire these locks instead of using
> preempt_disable() / get_cpu() / get_cpu_var() and local_irq_disable() /
> local_irq_save().
> 
> There is neither a functional change nor a change in the generated
> binary code for non PREEMPT_RT enabled non-debug kernels.
> 
> When lockdep is enabled local locks have lockdep maps embedded. These
> allow lockdep to validate the protections, i.e. inappropriate usage of a
> preemption only protected sections would result in a lockdep warning
> while the same problem would not be noticed with a plain
> preempt_disable() based protection.
> 
> local locks also improve readability as they provide a named scope for
> the protections while preempt/interrupt disable are opaque scopeless.
> 
> Finally local locks allow PREEMPT_RT to substitute them with real
> locking primitives to ensure the correctness of operation in a fully
> preemptible kernel.
> No functional change.
>
> ...
>
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -12,6 +12,7 @@
>  #include <linux/fs.h>
>  #include <linux/atomic.h>
>  #include <linux/page-flags.h>
> +#include <linux/locallock.h>

Could we please make these local_lock.h and local_lock_internal.h?  Making
the filenames different from everything else is just irritating!

> +				local_lock(swapvec_lock);

It's quite peculiar that these operations appear to be pass-by-value. 
All other locking operations are pass-by-reference - spin_lock(&lock),
not spin_lock(lock).  This is what the eye expects to see and it's
simply more logical - calling code shouldn't have to "know" that the
locking operations are implemented as cpp macros.  And we'd be in a
mess if someone tried to convert these to real C functions.

Which prompts the question: why were all these operations implemented
in the processor anyway?  afaict they could have been written in C.
Matthew Wilcox May 20, 2020, 2:17 a.m. UTC | #2 
On Tue, May 19, 2020 at 04:58:37PM -0700, Andrew Morton wrote:
> On Tue, 19 May 2020 22:19:08 +0200 Sebastian Andrzej Siewior <bigeasy@linutronix.de> wrote:
> > +				local_lock(swapvec_lock);
> 
> It's quite peculiar that these operations appear to be pass-by-value. 
> All other locking operations are pass-by-reference - spin_lock(&lock),
> not spin_lock(lock).  This is what the eye expects to see and it's
> simply more logical - calling code shouldn't have to "know" that the
> locking operations are implemented as cpp macros.  And we'd be in a
> mess if someone tried to convert these to real C functions.

The funny thing is that the documentation gets this right:

+The mapping of local_lock to spinlock_t on PREEMPT_RT kernels has a few
+implications. For example, on a non-PREEMPT_RT kernel the following code
+sequence works as expected::
+
+  local_lock_irq(&local_lock);
+  raw_spin_lock(&lock);

but apparently the implementation changed without the documentation matching.
Peter Zijlstra May 20, 2020, 10:53 a.m. UTC | #3 
On Tue, May 19, 2020 at 10:19:08PM +0200, Sebastian Andrzej Siewior wrote:
> diff --git a/mm/swap.c b/mm/swap.c
> index bf9a79fed62d7..03c97d15fcd69 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -44,8 +44,14 @@
>  /* How many pages do we try to swap or page in/out together? */
>  int page_cluster;
>  
> -static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
> +
> +/* Protecting lru_rotate_pvecs */
> +static DEFINE_LOCAL_LOCK(rotate_lock);
>  static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
> +
> +/* Protecting the following struct pagevec */
> +DEFINE_LOCAL_LOCK(swapvec_lock);
> +static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
>  static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);
>  static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
>  static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);

So personally I'd prefer to have this look like:

struct lru_vecs {
	struct local_lock lock;
	struct pagevec add;
	struct pagevec rotate;
	struct pagevec deact_file;
	struct pagevec deact;
	struct pagevec lazyfree;
#ifdef CONFIG_SMP
	struct pagevec active;
#endif
};

DEFINE_PER_CPU(struct lru_pvec, lru_pvec);

or something, but I realize that is a lot of churn (although highly
automated), so I'll leave that to the mm folks.
diff mbox series

Patch

diff --git a/include/linux/swap.h b/include/linux/swap.h
index e1bbf7a16b276..540b52c71bc95 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -12,6 +12,7 @@ 
 #include <linux/fs.h>
 #include <linux/atomic.h>
 #include <linux/page-flags.h>
+#include <linux/locallock.h>
 #include <asm/page.h>
 
 struct notifier_block;
@@ -328,6 +329,7 @@  extern unsigned long nr_free_pagecache_pages(void);
 
 
 /* linux/mm/swap.c */
+DECLARE_LOCAL_LOCK(swapvec_lock);
 extern void lru_cache_add(struct page *);
 extern void lru_cache_add_anon(struct page *page);
 extern void lru_cache_add_file(struct page *page);
diff --git a/mm/compaction.c b/mm/compaction.c
index 46f0fcc93081e..77972c8d4dead 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -2243,15 +2243,14 @@  compact_zone(struct compact_control *cc, struct capture_control *capc)
 		 * would succeed.
 		 */
 		if (cc->order > 0 && last_migrated_pfn) {
-			int cpu;
 			unsigned long current_block_start =
 				block_start_pfn(cc->migrate_pfn, cc->order);
 
 			if (last_migrated_pfn < current_block_start) {
-				cpu = get_cpu();
-				lru_add_drain_cpu(cpu);
+				local_lock(swapvec_lock);
+				lru_add_drain_cpu(smp_processor_id());
 				drain_local_pages(cc->zone);
-				put_cpu();
+				local_unlock(swapvec_lock);
 				/* No more flushing until we migrate again */
 				last_migrated_pfn = 0;
 			}
diff --git a/mm/swap.c b/mm/swap.c
index bf9a79fed62d7..03c97d15fcd69 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -44,8 +44,14 @@ 
 /* How many pages do we try to swap or page in/out together? */
 int page_cluster;
 
-static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
+
+/* Protecting lru_rotate_pvecs */
+static DEFINE_LOCAL_LOCK(rotate_lock);
 static DEFINE_PER_CPU(struct pagevec, lru_rotate_pvecs);
+
+/* Protecting the following struct pagevec */
+DEFINE_LOCAL_LOCK(swapvec_lock);
+static DEFINE_PER_CPU(struct pagevec, lru_add_pvec);
 static DEFINE_PER_CPU(struct pagevec, lru_deactivate_file_pvecs);
 static DEFINE_PER_CPU(struct pagevec, lru_deactivate_pvecs);
 static DEFINE_PER_CPU(struct pagevec, lru_lazyfree_pvecs);
@@ -254,11 +260,11 @@  void rotate_reclaimable_page(struct page *page)
 		unsigned long flags;
 
 		get_page(page);
-		local_irq_save(flags);
+		local_lock_irqsave(rotate_lock, flags);
 		pvec = this_cpu_ptr(&lru_rotate_pvecs);
 		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_move_tail(pvec);
-		local_irq_restore(flags);
+		local_unlock_irqrestore(rotate_lock, flags);
 	}
 }
 
@@ -308,12 +314,14 @@  void activate_page(struct page *page)
 {
 	page = compound_head(page);
 	if (PageLRU(page) && !PageActive(page) && !PageUnevictable(page)) {
-		struct pagevec *pvec = &get_cpu_var(activate_page_pvecs);
+		struct pagevec *pvec;
 
+		local_lock(swapvec_lock);
+		pvec = this_cpu_ptr(&activate_page_pvecs);
 		get_page(page);
 		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, __activate_page, NULL);
-		put_cpu_var(activate_page_pvecs);
+		local_unlock(swapvec_lock);
 	}
 }
 
@@ -335,9 +343,12 @@  void activate_page(struct page *page)
 
 static void __lru_cache_activate_page(struct page *page)
 {
-	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
+	struct pagevec *pvec;
 	int i;
 
+	local_lock(swapvec_lock);
+	pvec = this_cpu_ptr(&lru_add_pvec);
+
 	/*
 	 * Search backwards on the optimistic assumption that the page being
 	 * activated has just been added to this pagevec. Note that only
@@ -357,7 +368,7 @@  static void __lru_cache_activate_page(struct page *page)
 		}
 	}
 
-	put_cpu_var(lru_add_pvec);
+	local_unlock(swapvec_lock);
 }
 
 /*
@@ -404,12 +415,14 @@  EXPORT_SYMBOL(mark_page_accessed);
 
 static void __lru_cache_add(struct page *page)
 {
-	struct pagevec *pvec = &get_cpu_var(lru_add_pvec);
+	struct pagevec *pvec;
 
+	local_lock(swapvec_lock);
+	pvec = this_cpu_ptr(&lru_add_pvec);
 	get_page(page);
 	if (!pagevec_add(pvec, page) || PageCompound(page))
 		__pagevec_lru_add(pvec);
-	put_cpu_var(lru_add_pvec);
+	local_unlock(swapvec_lock);
 }
 
 /**
@@ -603,9 +616,9 @@  void lru_add_drain_cpu(int cpu)
 		unsigned long flags;
 
 		/* No harm done if a racing interrupt already did this */
-		local_irq_save(flags);
+		local_lock_irqsave(rotate_lock, flags);
 		pagevec_move_tail(pvec);
-		local_irq_restore(flags);
+		local_unlock_irqrestore(rotate_lock, flags);
 	}
 
 	pvec = &per_cpu(lru_deactivate_file_pvecs, cpu);
@@ -641,11 +654,14 @@  void deactivate_file_page(struct page *page)
 		return;
 
 	if (likely(get_page_unless_zero(page))) {
-		struct pagevec *pvec = &get_cpu_var(lru_deactivate_file_pvecs);
+		struct pagevec *pvec;
+
+		local_lock(swapvec_lock);
+		pvec = this_cpu_ptr(&lru_deactivate_file_pvecs);
 
 		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, lru_deactivate_file_fn, NULL);
-		put_cpu_var(lru_deactivate_file_pvecs);
+		local_unlock(swapvec_lock);
 	}
 }
 
@@ -660,12 +676,14 @@  void deactivate_file_page(struct page *page)
 void deactivate_page(struct page *page)
 {
 	if (PageLRU(page) && PageActive(page) && !PageUnevictable(page)) {
-		struct pagevec *pvec = &get_cpu_var(lru_deactivate_pvecs);
+		struct pagevec *pvec;
 
+		local_lock(swapvec_lock);
+		pvec = this_cpu_ptr(&lru_deactivate_pvecs);
 		get_page(page);
 		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, lru_deactivate_fn, NULL);
-		put_cpu_var(lru_deactivate_pvecs);
+		local_unlock(swapvec_lock);
 	}
 }
 
@@ -680,19 +698,22 @@  void mark_page_lazyfree(struct page *page)
 {
 	if (PageLRU(page) && PageAnon(page) && PageSwapBacked(page) &&
 	    !PageSwapCache(page) && !PageUnevictable(page)) {
-		struct pagevec *pvec = &get_cpu_var(lru_lazyfree_pvecs);
+		struct pagevec *pvec;
 
+		local_lock(swapvec_lock);
+		pvec = this_cpu_ptr(&lru_lazyfree_pvecs);
 		get_page(page);
 		if (!pagevec_add(pvec, page) || PageCompound(page))
 			pagevec_lru_move_fn(pvec, lru_lazyfree_fn, NULL);
-		put_cpu_var(lru_lazyfree_pvecs);
+		local_unlock(swapvec_lock);
 	}
 }
 
 void lru_add_drain(void)
 {
-	lru_add_drain_cpu(get_cpu());
-	put_cpu();
+	local_lock(swapvec_lock);
+	lru_add_drain_cpu(smp_processor_id());
+	local_unlock(swapvec_lock);
 }
 
 #ifdef CONFIG_SMP