Performance analysis of software compilation by Gentoo portage on an Intel E5-2620 with 64GB of RAM revealed that a sizeable amount of time, anywhere from 5% to 15%, was spent in get_vmalloc_info(), with at least 40% of that time spent in the _raw_spin_lock() invoked by it. The spinlock call is done on vmap_area_lock to protect vmap_area_list, but changes to vmap_area_list are made under RCU. The only consumer that requires a spinlock on an RCU-ified list is /proc/vmallocinfo. That is only intended for use by kernel developers doing debugging, but even few kernel developers appear to use it. Introducing DEBUG_VMALLOCINFO allows us to fully RCU-ify the list, which eliminates this list as a source of contention. This patch brings a substantial reduction in time spent in spinlocks on my system. Flame graphs from my early analysis are available on my developer space. They were created by profiling the system under concurrent package builds done by emerge at a sample rate of 99Hz for 10 seconds and using Brendan Gregg's scripts to process the data: http://dev.gentoo.org/~ryao/emerge.svg http://dev.gentoo.org/~ryao/emerge-patched.svg In this example, 6.64% of system time is spent in get_vmalloc_info() with 2.59% spent in the spinlock. The patched version sees only 0.50% of time spent in get_vmalloc_info() with neligible time spent in spin locks. The low utilization of get_vmalloc_info() in this is partly attributable to measurement error, but the reduction in time spent spinning is clear. Signed-off-by: Richard Yao <ryao@xxxxxxxxxx> --- lib/Kconfig.debug | 11 +++++++++++ mm/vmalloc.c | 32 ++++++++++++++++++++++++++++++++ 2 files changed, 43 insertions(+) diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index dd7f885..a3e6967 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -492,6 +492,17 @@ config DEBUG_STACK_USAGE This option will slow down process creation somewhat. +config DEBUG_VMALLOCINFO + bool "Provide /proc/vmallocinfo" + depends on PROC_FS + help + Provides a userland interface to view kernel virtual memory mappings. + Enabling this places a RCU-ified list under spinlock protection. That + hurts performance in concurrent workloads. + + If unsure, say N. + + config DEBUG_VM bool "Debug VM" depends on DEBUG_KERNEL diff --git a/mm/vmalloc.c b/mm/vmalloc.c index bf233b2..12ab34b 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c @@ -1988,8 +1988,13 @@ long vread(char *buf, char *addr, unsigned long count) if ((unsigned long) addr + count < count) count = -(unsigned long) addr; +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_lock(&vmap_area_lock); list_for_each_entry(va, &vmap_area_list, list) { +#else + rcu_read_lock(); + list_for_each_entry_rcu(va, &vmap_area_list, list) { +#endif if (!count) break; @@ -2020,7 +2025,11 @@ long vread(char *buf, char *addr, unsigned long count) count -= n; } finished: +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_unlock(&vmap_area_lock); +#else + rcu_read_unlock(); +#endif if (buf == buf_start) return 0; @@ -2070,8 +2079,13 @@ long vwrite(char *buf, char *addr, unsigned long count) count = -(unsigned long) addr; buflen = count; +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_lock(&vmap_area_lock); list_for_each_entry(va, &vmap_area_list, list) { +#else + rcu_read_lock(); + list_for_each_entry_rcu(va, &vmap_area_list, list) { +#endif if (!count) break; @@ -2101,7 +2115,11 @@ long vwrite(char *buf, char *addr, unsigned long count) count -= n; } finished: +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_unlock(&vmap_area_lock); +#else + rcu_read_unlock(); +#endif if (!copied) return 0; return buflen; @@ -2531,6 +2549,7 @@ void pcpu_free_vm_areas(struct vm_struct **vms, int nr_vms) #endif /* CONFIG_SMP */ #ifdef CONFIG_PROC_FS +#ifdef CONFIG_DEBUG_VMALLOCINFO static void *s_start(struct seq_file *m, loff_t *pos) __acquires(&vmap_area_lock) { @@ -2677,6 +2696,7 @@ static int __init proc_vmalloc_init(void) return 0; } module_init(proc_vmalloc_init); +#endif void get_vmalloc_info(struct vmalloc_info *vmi) { @@ -2689,14 +2709,22 @@ void get_vmalloc_info(struct vmalloc_info *vmi) prev_end = VMALLOC_START; +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_lock(&vmap_area_lock); +#else + rcu_read_lock(); +#endif if (list_empty(&vmap_area_list)) { vmi->largest_chunk = VMALLOC_TOTAL; goto out; } +#ifdef CONFIG_DEBUG_VMALLOCINFO list_for_each_entry(va, &vmap_area_list, list) { +#else + list_for_each_entry_rcu(va, &vmap_area_list, list) { +#endif unsigned long addr = va->va_start; /* @@ -2723,7 +2751,11 @@ void get_vmalloc_info(struct vmalloc_info *vmi) vmi->largest_chunk = VMALLOC_END - prev_end; out: +#ifdef CONFIG_DEBUG_VMALLOCINFO spin_unlock(&vmap_area_lock); +#else + rcu_read_unlock(); +#endif } #endif -- 1.8.3.2 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
