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Date: Fri, 15 May 2020 09:41:25 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Feng Tang <feng.tang@...el.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Johannes Weiner <hannes@...xchg.org>,
Matthew Wilcox <willy@...radead.org>,
Mel Gorman <mgorman@...e.de>,
Kees Cook <keescook@...omium.org>,
Luis Chamberlain <mcgrof@...nel.org>,
Iurii Zaikin <yzaikin@...gle.com>, andi.kleen@...el.com,
tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: adjust vm_committed_as_batch according to vm
overcommit policy
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@...el.com>
> ---
> 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
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