From: Feng Tang <feng.tang@intel.com>
Subject: [PATCH v5 3/3] mm: adjust vm_committed_as_batch according to vm overcommit policy
Date: Sun, 21 Jun 2020 15:36:40 +0800
Patch-mainline: v5.9-rc1
Git-commit: 56f3547bfa4d361148aa748ccb86073bc57f5e6c
References: bnc#1173271

When checking a performance change for will-it-scale scalability mmap test
[1], we found very high lock contention for spinlock of percpu counter
'vm_committed_as': 

    94.14%     0.35%  [kernel.kallsyms]         [k] _raw_spin_lock_irqsave
    48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
    45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;

Actually this heavy lock contention is not always necessary.  The
'vm_committed_as' needs to be very precise when the strict
OVERCOMMIT_NEVER policy is set, which requires a rather small batch number
for the percpu counter.

So keep 'batch' number unchanged for strict OVERCOMMIT_NEVER policy, and
lift it to 64X for OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS policies.  Also
add a sysctl handler to adjust it when the policy is reconfigured.

Benchmark with the same testcase in [1] shows 53% improvement on a 8C/16T
desktop, and 2097%(20X) on a 4S/72C/144T server.  We tested with test
platforms in 0day (server, desktop and laptop), and 80%+ platforms shows
improvements with that test.  And whether it shows improvements depends on
if the test mmap size is bigger than the batch number computed.

And if the lift is 16X, 1/3 of the platforms will show improvements,
though it should help the mmap/unmap usage generally, as Michal Hocko
Mentioned: 

: I believe that there are non-synthetic worklaods which would benefit from
: a larger batch.  E.g.  large in memory databases which do large mmaps
: during startups from multiple threads.

[1] https://lore.kernel.org/lkml/20200305062138.GI5972@shao2-debian/

Link: http://lkml.kernel.org/r/1589611660-89854-4-git-send-email-feng.tang@intel.com
Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Tim Chen <tim.c.chen@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>

---
 include/linux/mm.h   |    3 +++
 include/linux/mman.h |    4 ++++
 kernel/sysctl.c      |    2 +-
 mm/mm_init.c         |   16 +++++++++++++---
 mm/util.c            |   12 ++++++++++++
 5 files changed, 33 insertions(+), 4 deletions(-)

--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -210,6 +210,9 @@ extern int overcommit_ratio_handler(stru
 extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
 				    size_t *, loff_t *);
 
+int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
+		loff_t *);
+
 #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
 
 /* to align the pointer to the (next) page boundary */
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -57,8 +57,12 @@ extern struct percpu_counter vm_committe
 
 #ifdef CONFIG_SMP
 extern s32 vm_committed_as_batch;
+extern void mm_compute_batch(void);
 #else
 #define vm_committed_as_batch 0
+static inline void mm_compute_batch(void)
+{
+}
 #endif
 
 unsigned long vm_memory_committed(void);
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1314,7 +1314,7 @@ static struct ctl_table vm_table[] = {
 		.data		= &sysctl_overcommit_memory,
 		.maxlen		= sizeof(sysctl_overcommit_memory),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_minmax,
+		.proc_handler	= overcommit_policy_handler,
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= &two,
 	},
--- a/mm/mm_init.c
+++ b/mm/mm_init.c
@@ -13,6 +13,7 @@
 #include <linux/memory.h>
 #include <linux/notifier.h>
 #include <linux/sched.h>
+#include <linux/mman.h>
 #include "internal.h"
 
 #ifdef CONFIG_DEBUG_MEMORY_INIT
@@ -140,14 +141,23 @@ EXPORT_SYMBOL_GPL(mm_kobj);
 #ifdef CONFIG_SMP
 s32 vm_committed_as_batch = 32;
 
-static void __meminit mm_compute_batch(void)
+void mm_compute_batch(void)
 {
 	u64 memsized_batch;
 	s32 nr = num_present_cpus();
 	s32 batch = max_t(s32, nr*2, 32);
+	unsigned long ram_pages = totalram_pages();
 
-	/* batch size set to 0.4% of (total memory/#cpus), or max int32 */
-	memsized_batch = min_t(u64, (totalram_pages()/nr)/256, 0x7fffffff);
+	/*
+	 * For policy of OVERCOMMIT_NEVER, set batch size to 0.4%
+	 * of (total memory/#cpus), and lift it to 25% for other
+	 * policies to easy the possible lock contention for percpu_counter
+	 * vm_committed_as, while the max limit is INT_MAX
+	 */
+	if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
+		memsized_batch = min_t(u64, ram_pages/nr/256, INT_MAX);
+	else
+		memsized_batch = min_t(u64, ram_pages/nr/4, INT_MAX);
 
 	vm_committed_as_batch = max_t(s32, memsized_batch, batch);
 }
--- a/mm/util.c
+++ b/mm/util.c
@@ -624,6 +624,18 @@ int overcommit_ratio_handler(struct ctl_
 	return ret;
 }
 
+int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer,
+		size_t *lenp, loff_t *ppos)
+{
+	int ret;
+
+	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+	if (ret == 0 && write)
+		mm_compute_batch();
+
+	return ret;
+}
+
 int overcommit_kbytes_handler(struct ctl_table *table, int write,
 			     void __user *buffer, size_t *lenp,
 			     loff_t *ppos)