Re: [PATCH 3/3] mm: adjust vm_committed_as_batch according to vm overcommit policy

[Michal Hocko]

On Fri 08-05-20 15:25:17, Feng Tang wrote:
> 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 lift the batch number to 16X for OVERCOMMIT_ALWAYS and
> OVERCOMMIT_GUESS policies, and add a sysctl handler to adjust it
> when the policy is reconfigured.

Increasing the batch size for weaker overcommit modes makes sense. But
your patch is also tuning OVERCOMMIT_NEVER without any explanation why
that is still "small enough to be precise".

> Benchmark with the same testcase in [1] shows 53% improvement on a
> 8C/16T desktop, and 2097%(20X) on a 4S/72C/144T server. And no change
> for some platforms, due to the test mmap size of the case is bigger
> than the batch number computed, though the patch will help mmap/munmap
> generally.
> 
> [1] https://lkml.org/lkml/2020/3/5/57

Please do not use lkml.org links in the changelog. Use
http://lkml.kernel.org/r/$msg instead.

> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> ---
>  include/linux/mm.h   |  2 ++
>  include/linux/mman.h |  4 ++++
>  kernel/sysctl.c      |  2 +-
>  mm/mm_init.c         | 19 +++++++++++++++----
>  mm/util.c            | 13 +++++++++++++
>  5 files changed, 35 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5a32342..bc3722f 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -205,6 +205,8 @@ extern int overcommit_ratio_handler(struct ctl_table *, int, void __user *,
>  				    size_t *, loff_t *);
>  extern int overcommit_kbytes_handler(struct ctl_table *, int, void __user *,
>  				    size_t *, loff_t *);
> +extern int overcommit_policy_handler(struct ctl_table *, int, void __user *,
> +				    size_t *, loff_t *);
>  
>  #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
>  
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 4b08e9c..91c93c1 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -57,8 +57,12 @@ extern struct percpu_counter vm_committed_as;
>  
>  #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);
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index 8a176d8..6fa552d 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -1278,7 +1278,7 @@ 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,
>  	},
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 7da6991..1654358 100644
> --- 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
> @@ -140,16 +141,26 @@ 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);
> -
> -	/* batch size set to 0.4% of (total memory/#cpus), or max int32 */
> -	memsized_batch = min_t(u64, (totalram_pages()/nr)/256, 0x7fffffff);
> +	unsigned long ram_pages = totalram_pages();
> +
> +	/*
> +	 * For policy of OVERCOMMIT_NEVER, set batch size to 0.4%
> +	 * of (total memory/#cpus), and lift it to 6.25% 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/16, INT_MAX);
>  
>  	vm_committed_as_batch = max_t(s32, memsized_batch, batch);
> +	printk("vm_committed_as_batch = %d\n", vm_committed_as_batch);
>  }
>  
>  static int __meminit mm_compute_batch_notifier(struct notifier_block *self,
> diff --git a/mm/util.c b/mm/util.c
> index 3de78e9..99936d3 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -729,6 +729,19 @@ int overcommit_ratio_handler(struct ctl_table *table, int write,
>  	return ret;
>  }
>  
> +int overcommit_policy_handler(struct ctl_table *table, int write,
> +			     void __user *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)
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs