* George Spelvin <linux@xxxxxxxxxxx> wrote:
> Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> > I think this is too complex.
> >
> > How about something simple like the patch below (on top of the third patch)?
>
> > It makes the vmalloc info transactional - /proc/meminfo will always print a
> > consistent set of numbers. (Not that we really care about races there, but it
> > looks really simple to solve so why not.)
>
> Looks like a huge simplification!
>
> It needs a comment about the approximate nature of the locking and
> the obvious race conditions:
> 1) The first caller to get_vmalloc_info() clears vmap_info_changed
> before updating vmap_info_cache, so a second caller is likely to
> get stale data for the duration of a calc_vmalloc_info call.
> 2) Although unlikely, it's possible for two threads to race calling
> calc_vmalloc_info, and the one that computes fresher data updates
> the cache first, so the later write leaves stale data.
>
> Other issues:
> 3) Me, I'd make vmap_info_changed a bool, for documentation more than
> any space saving.
> 4) I wish there were a trylock version of write_seqlock, so we could
> avoid blocking entirely. (You *could* hand-roll it, but that eats
> into the simplicity.)
Ok, fair enough - so how about the attached approach instead, which uses a 64-bit
generation counter to track changes to the vmalloc state.
This is still very simple, but should not suffer from stale data being returned
indefinitely in /proc/meminfo. We might race - but that was true before as well
due to the lock-less RCU list walk - but we'll always return a correct and
consistent version of the information.
Lightly tested. This is a replacement patch to make it easier to read via email.
I also made sure there's no extra overhead in the !CONFIG_PROC_FS case.
Note that there's an even simpler variant possible I think: we could use just the
two generation counters and barriers to remove the seqlock.
Thanks,
Ingo
==============================>
>From f9fd770e75e2edb4143f32ced0b53d7a77969c94 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 glibc (rather stupidly) reads /proc/meminfo
for every sysinfo() call, which causes the Git build to use
a surprising amount of CPU time, mostly due to the overhead
of get_vmalloc_info() - which walks a 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.
Cc: 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 | 59 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 56 insertions(+), 3 deletions(-)
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 605138083880..d72b23436906 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_SEQLOCK(vmap_info_lock);
+static u64 vmap_info_gen;
+static u64 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 +2763,41 @@ 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)
+{
+ u64 gen = READ_ONCE(vmap_info_gen);
+
+ /*
+ * If the generation counter of the cache matches that of
+ * the vmalloc generation counter then return the cache:
+ */
+ if (READ_ONCE(vmap_info_cache_gen) == gen) {
+ unsigned int seq;
+
+ do {
+ seq = read_seqbegin(&vmap_info_lock);
+ *vmi = vmap_info_cache;
+ } while (read_seqretry(&vmap_info_lock, seq));
+
+ return;
+ }
+
+ calc_vmalloc_info(vmi);
+
+ /*
+ * If are racing with a new vmalloc() then we might write
+ * the old generation counter here - and the next call to
+ * get_vmalloc_info() will fix things up:
+ */
+ write_seqlock(&vmap_info_lock);
+ vmap_info_cache = *vmi;
+ WRITE_ONCE(vmap_info_cache_gen, gen);
+ write_sequnlock(&vmap_info_lock);
+}
+
+#endif /* CONFIG_PROC_FS */
--
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