* Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> One more detail: I just realized that with the read barriers, the READ_ONCE()
> accesses are not needed anymore - the barriers and the control dependencies are
> enough.
>
> This will further simplify the code.
I.e. something like the updated patch below. (We still need the WRITE_ONCE() for
vmap_info_gen update.)
Thanks,
Ingo
========================>
>From 46a0507e0a395a7bc2fe4b46a4766e7457ac0140 Mon Sep 17 00:00:00 2001
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Sat, 22 Aug 2015 12:28:01 +0200
Subject: [PATCH] mm/vmalloc: Cache the vmalloc memory info
Linus reported that for scripting-intense workloads such as the
Git build, glibc's qsort will read /proc/meminfo for every process
created (by way of get_phys_pages()), which causes the Git build
to generate a surprising amount of kernel overhead.
A fair chunk of the overhead is due to get_vmalloc_info() - which
walks a potentially long list to do its statistics.
Modify Linus's jiffies based patch to use generation counters
to cache the vmalloc info: vmap_unlock() increases the generation
counter, and the get_vmalloc_info() reads it and compares it
against a cached generation counter.
Also use a seqlock to make sure we always print a consistent
set of vmalloc statistics.
Reported-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
mm/vmalloc.c | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 79 insertions(+), 3 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 605138083880..2f8d9660e007 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -276,7 +276,21 @@ EXPORT_SYMBOL(vmalloc_to_pfn);
#define VM_LAZY_FREEING 0x02
#define VM_VM_AREA 0x04
-static DEFINE_SPINLOCK(vmap_area_lock);
+static __cacheline_aligned_in_smp DEFINE_SPINLOCK(vmap_area_lock);
+
+#ifdef CONFIG_PROC_FS
+/*
+ * A seqlock and two generation counters for a simple cache of the
+ * vmalloc allocation statistics info printed in /proc/meminfo.
+ *
+ * ( The assumption of the optimization is that it's read frequently, but
+ * modified infrequently. )
+ */
+static DEFINE_SPINLOCK(vmap_info_lock);
+static int vmap_info_gen = 1;
+static int vmap_info_cache_gen;
+static struct vmalloc_info vmap_info_cache;
+#endif
static inline void vmap_lock(void)
{
@@ -285,6 +299,9 @@ static inline void vmap_lock(void)
static inline void vmap_unlock(void)
{
+#ifdef CONFIG_PROC_FS
+ WRITE_ONCE(vmap_info_gen, vmap_info_gen+1);
+#endif
spin_unlock(&vmap_area_lock);
}
@@ -2699,7 +2716,7 @@ static int __init proc_vmalloc_init(void)
}
module_init(proc_vmalloc_init);
-void get_vmalloc_info(struct vmalloc_info *vmi)
+static void calc_vmalloc_info(struct vmalloc_info *vmi)
{
struct vmap_area *va;
unsigned long free_area_size;
@@ -2746,5 +2763,64 @@ void get_vmalloc_info(struct vmalloc_info *vmi)
out:
rcu_read_unlock();
}
-#endif
+/*
+ * Return a consistent snapshot of the current vmalloc allocation
+ * statistics, for /proc/meminfo:
+ */
+void get_vmalloc_info(struct vmalloc_info *vmi)
+{
+ int gen = vmap_info_gen;
+
+ /*
+ * If the generation counter of the cache matches that of
+ * the vmalloc generation counter then return the cache:
+ */
+ if (vmap_info_cache_gen == gen) {
+ int gen_after;
+
+ /*
+ * The two read barriers make sure that we read
+ * 'gen', 'vmap_info_cache' and 'gen_after' in
+ * precisely that order:
+ */
+ smp_rmb();
+ *vmi = vmap_info_cache;
+
+ smp_rmb();
+ gen_after = vmap_info_gen;
+
+ /* The cache is still valid: */
+ if (gen == gen_after)
+ return;
+
+ /* Ok, the cache got invalidated just now, regenerate it */
+ gen = gen_after;
+ }
+
+ /* Make sure 'gen' is read before the vmalloc info */
+ smp_rmb();
+
+ calc_vmalloc_info(vmi);
+
+ /*
+ * All updates to vmap_info_cache_gen go through this spinlock,
+ * so when the cache got invalidated, we'll only mark it valid
+ * again if we first fully write the new vmap_info_cache.
+ *
+ * This ensures that partial results won't be used.
+ */
+ spin_lock(&vmap_info_lock);
+ if (gen-vmap_info_cache_gen > 0) {
+ vmap_info_cache = *vmi;
+ /*
+ * Make sure the new cached data is visible before
+ * the generation counter update:
+ */
+ smp_wmb();
+ vmap_info_cache_gen = gen;
+ }
+ spin_unlock(&vmap_info_lock);
+}
+
+#endif /* CONFIG_PROC_FS */
--
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