Hi Al, Benjamin, David,
struct kiocb is placed on stack by, for example, do_sync_write().
Eventually it contributes to xfs writeout path's stack usage, among others.
This is *the* path which causes 4k stack overflows on i386 with xfs.
This patch trivially reorders fields of this structure,
and makes some of them smaller.
Reordering helps 64-bit architectures:
int, void*, int, void* - bad,
int, int, void*, void* - better.
These fields are made smaller:
ki_flags: long -> short: possible values are 0,1,2, so short is enough.
ki_nr_segs: ulong -> uint: nobody uses 4 billion element writev's
(and it would not work anyway)
ki_cur_seg: same
For 32bit x86, it makes this struct only 4 bytes smaller.
This isn't much, but it helps not only xfs, but all filesystems.
For 64-bit case savings are a bit more significant,
as ulong -> uint actually makes a difference, and reordering
of 64-bit fields eliminates some padding.
Only compile tested. Observed stack reductions on 32 bits:
-sock_recvmsg [vmlinux]: 196
-sock_sendmsg [vmlinux]: 196
+sock_recvmsg [vmlinux]: 192
+sock_sendmsg [vmlinux]: 192
-do_sync_write [vmlinux]: 140
-do_sync_read [vmlinux]: 140
+do_sync_write [vmlinux]: 136
+do_sync_read [vmlinux]: 136
-do_sync_readv_writev [vmlinux]: 132
+do_sync_readv_writev [vmlinux]: 128
Signed-off-by: Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx>
--
vda
diff -urpN linux-2.6-xfs1/include/linux/aio.h linux-2.6-xfs1.stk4/include/linux/aio.h
--- linux-2.6-xfs1/include/linux/aio.h 2008-03-30 03:27:54.000000000 +0200
+++ linux-2.6-xfs1.stk4/include/linux/aio.h 2008-04-27 05:47:09.000000000 +0200
@@ -86,7 +86,8 @@ struct kioctx;
*/
struct kiocb {
struct list_head ki_run_list;
- unsigned long ki_flags;
+ unsigned short ki_flags; /* range: 0..2 */
+ unsigned short ki_opcode;
int ki_users;
unsigned ki_key; /* id of this request */
@@ -102,23 +103,22 @@ struct kiocb {
} ki_obj;
__u64 ki_user_data; /* user's data for completion */
- wait_queue_t ki_wait;
loff_t ki_pos;
+ wait_queue_t ki_wait;
void *private;
/* State that we remember to be able to restart/retry */
- unsigned short ki_opcode;
+ /*unsigned short ki_opcode; - moved up for denser packing */
size_t ki_nbytes; /* copy of iocb->aio_nbytes */
- char __user *ki_buf; /* remaining iocb->aio_buf */
size_t ki_left; /* remaining bytes */
+ unsigned ki_nr_segs;
+ unsigned ki_cur_seg;
struct iovec ki_inline_vec; /* inline vector */
+ char __user *ki_buf; /* remaining iocb->aio_buf */
struct iovec *ki_iovec;
- unsigned long ki_nr_segs;
- unsigned long ki_cur_seg;
struct list_head ki_list; /* the aio core uses this
* for cancellation */
-
/*
* If the aio_resfd field of the userspace iocb is not zero,
* this is the underlying file* to deliver event to.
