On Tue, 2015-10-27 at 23:17 +0000, Al Viro wrote:
> * [Linux-specific aside] our __alloc_fd() can degrade quite badly
> with some use patterns. The cacheline pingpong in the bitmap is probably
> inevitable, unless we accept considerably heavier memory footprint,
> but we also have a case when alloc_fd() takes O(n) and it's _not_ hard
> to trigger - close(3);open(...); will have the next open() after that
> scanning the entire in-use bitmap. I think I see a way to improve it
> without slowing the normal case down, but I'll need to experiment a
> bit before I post patches. Anybody with examples of real-world loads
> that make our descriptor allocator to degrade is very welcome to post
> the reproducers...
Well, I do have real-world loads, but quite hard to setup in a lab :(
Note that we also hit the 'struct cred'->usage refcount for every
open()/close()/sock_alloc(), and simply moving uid/gid out of the first
cache line really helps, as current_fsuid() and current_fsgid() no
longer forces a pingpong.
I moved seldom used fields on the first cache line, so that overall
memory usage did not change (192 bytes on 64 bit arches)
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 8d70e1361ecd..460efae83522 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -124,7 +124,17 @@ struct cred {
#define CRED_MAGIC 0x43736564
#define CRED_MAGIC_DEAD 0x44656144
#endif
- kuid_t uid; /* real UID of the task */
+ struct rcu_head rcu; /* RCU deletion hook */
+
+ kernel_cap_t cap_inheritable; /* caps our children can inherit */
+ kernel_cap_t cap_permitted; /* caps we're permitted */
+ kernel_cap_t cap_effective; /* caps we can actually use */
+ kernel_cap_t cap_bset; /* capability bounding set */
+ kernel_cap_t cap_ambient; /* Ambient capability set */
+
+ kuid_t uid ____cacheline_aligned_in_smp;
+ /* real UID of the task */
+
kgid_t gid; /* real GID of the task */
kuid_t suid; /* saved UID of the task */
kgid_t sgid; /* saved GID of the task */
@@ -133,11 +143,6 @@ struct cred {
kuid_t fsuid; /* UID for VFS ops */
kgid_t fsgid; /* GID for VFS ops */
unsigned securebits; /* SUID-less security management */
- kernel_cap_t cap_inheritable; /* caps our children can inherit */
- kernel_cap_t cap_permitted; /* caps we're permitted */
- kernel_cap_t cap_effective; /* caps we can actually use */
- kernel_cap_t cap_bset; /* capability bounding set */
- kernel_cap_t cap_ambient; /* Ambient capability set */
#ifdef CONFIG_KEYS
unsigned char jit_keyring; /* default keyring to attach requested
* keys to */
@@ -152,7 +157,6 @@ struct cred {
struct user_struct *user; /* real user ID subscription */
struct user_namespace *user_ns; /* user_ns the caps and keyrings are
relative to. */
struct group_info *group_info; /* supplementary groups for euid/fsgid
*/
- struct rcu_head rcu; /* RCU deletion hook */
};
extern void __put_cred(struct cred *);
--
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