Re: [PATCH v4 4/4] mm: adjust vm_committed_as_batch according to vm overcommit policy

[Michal Hocko]

On Fri 29-05-20 09:06:10, 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 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@xxxxxxxxx
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Andi Kleen <andi.kleen@xxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  include/linux/mm.h   |  2 ++
>  include/linux/mman.h |  4 ++++
>  kernel/sysctl.c      |  2 +-
>  mm/mm_init.c         | 18 ++++++++++++++----
>  mm/util.c            | 12 ++++++++++++
>  5 files changed, 33 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 573947c..c2efea6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -206,6 +206,8 @@ int overcommit_ratio_handler(struct ctl_table *, int, void *, size_t *,
>  		loff_t *);
>  int overcommit_kbytes_handler(struct ctl_table *, int, void *, 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))
>  
> 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 db1ce7a..9456c86 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -2650,7 +2650,7 @@ 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 435e5f7..c5a6fb1 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
> @@ -144,14 +145,23 @@ 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);
> -
> -	/* 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 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);
>  }
> diff --git a/mm/util.c b/mm/util.c
> index fe63271..580d268 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -746,6 +746,18 @@ int overcommit_ratio_handler(struct ctl_table *table, int write, void *buffer,
>  	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 *buffer,
>  		size_t *lenp, loff_t *ppos)
>  {
> -- 
> 2.7.4
> 

-- 
Michal Hocko
SUSE Labs