Le jeudi 30 septembre 2010 Ã 09:45 -0700, Andrew Morton a Ãcrit :
> Could eliminate `p' I guess, but that would involve using
> __get_cpu_var() as an lval, which looks vile and might generate worse
> code.
>
Hmm, I see, please check this new patch, using the most modern stuff ;)
> Readers of this code won't know why last_ino_get() was marked noinline.
> It looks wrong, really.
Oops sorry, this was a temporary hack of mine to ease disassembly
analysis. Good catch !
Here is the new generated code on i686 (with the noinline) :
pretty good ;)
c02e5930 <last_ino_get>:
c02e5930: 55 push %ebp
c02e5931: 89 e5 mov %esp,%ebp
c02e5933: 64 a1 44 29 7d c0 mov %fs:0xc07d2944,%eax
c02e5939: a9 ff 03 00 00 test $0x3ff,%eax
c02e593e: 74 09 je c02e5949 <last_ino_get+0x19>
c02e5940: 40 inc %eax
c02e5941: 64 a3 44 29 7d c0 mov %eax,%fs:0xc07d2944
c02e5947: c9 leave
c02e5948: c3 ret
c02e5949: b8 00 04 00 00 mov $0x400,%eax
c02e594e: f0 0f c1 05 80 c8 92 c0 lock xadd %eax,0xc092c880
c02e5956: eb e8 jmp c02e5940 <last_ino_get+0x10>
Thanks
[PATCH] fs: inode per-cpu last_ino allocator
new_inode() dirties a contended cache line to get increasing
inode numbers.
Solve this problem by providing to each cpu a per_cpu variable,
feeded by the shared last_ino, but once every 1024 allocations.
This reduces contention on the shared last_ino, and give same
spreading ino numbers than before (i.e. same wraparound after 2^32
allocations).
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
fs/inode.c | 47 ++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 40 insertions(+), 7 deletions(-)
diff --git a/fs/inode.c b/fs/inode.c
index 8646433..5c233f0 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -624,6 +624,45 @@ void inode_add_to_lists(struct super_block *sb, struct inode *inode)
}
EXPORT_SYMBOL_GPL(inode_add_to_lists);
+#define LAST_INO_BATCH 1024
+
+/*
+ * Each cpu owns a range of LAST_INO_BATCH numbers.
+ * 'shared_last_ino' is dirtied only once out of LAST_INO_BATCH allocations,
+ * to renew the exhausted range.
+ *
+ * This does not significantly increase overflow rate because every CPU can
+ * consume at most LAST_INO_BATCH-1 unused inode numbers. So there is
+ * NR_CPUS*(LAST_INO_BATCH-1) wastage. At 4096 and 1024, this is ~0.1% of the
+ * 2^32 range, and is a worst-case. Even a 50% wastage would only increase
+ * overflow rate by 2x, which does not seem too significant.
+ *
+ * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
+ * error if st_ino won't fit in target struct field. Use 32bit counter
+ * here to attempt to avoid that.
+ */
+static DEFINE_PER_CPU(unsigned int, last_ino);
+
+static unsigned int last_ino_get(void)
+{
+ unsigned int res;
+
+ get_cpu();
+ res = __this_cpu_read(last_ino);
+#ifdef CONFIG_SMP
+ if (unlikely((res & (LAST_INO_BATCH - 1)) == 0)) {
+ static atomic_t shared_last_ino;
+ int next = atomic_add_return(LAST_INO_BATCH, &shared_last_ino);
+
+ res = next - LAST_INO_BATCH;
+ }
+#endif
+ res++;
+ __this_cpu_write(last_ino, res);
+ put_cpu();
+ return res;
+}
+
/**
* new_inode - obtain an inode
* @sb: superblock
@@ -638,12 +677,6 @@ EXPORT_SYMBOL_GPL(inode_add_to_lists);
*/
struct inode *new_inode(struct super_block *sb)
{
- /*
- * On a 32bit, non LFS stat() call, glibc will generate an EOVERFLOW
- * error if st_ino won't fit in target struct field. Use 32bit counter
- * here to attempt to avoid that.
- */
- static unsigned int last_ino;
struct inode *inode;
spin_lock_prefetch(&inode_lock);
@@ -652,7 +685,7 @@ struct inode *new_inode(struct super_block *sb)
if (inode) {
spin_lock(&inode_lock);
__inode_add_to_lists(sb, NULL, inode);
- inode->i_ino = ++last_ino;
+ inode->i_ino = last_ino_get();
inode->i_state = 0;
spin_unlock(&inode_lock);
}
--
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