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 <[email protected]> > --- > 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
