fs/file.c: don't acquire files->file_lock in fd_install()
Mateusz Guzik reported :
Currently obtaining a new file descriptor results in locking fdtable
twice - once in order to reserve a slot and second time to fill it.
Holding the spinlock in __fd_install() is needed in case a resize is
done, or to prevent a resize.
Mateusz provided an RFC patch and a micro benchmark :
http://people.redhat.com/~mguzik/pipebench.c
A resize is an unlikely operation in a process lifetime,
as table size is at least doubled at every resize.
We can use RCU instead of the spinlock.
__fd_install() must wait if a resize is in progress.
The resize must block new __fd_install() callers from starting,
and wait that ongoing install are finished (synchronize_sched())
resize should be attempted by a single thread to not waste resources.
rcu_sched variant is used, as __fd_install() and expand_fdtable() run
from process context.
It gives us a ~30% speedup using pipebench on a dual Intel(R) Xeon(R)
CPU E5-2696 v2 @ 2.50GHz
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Mateusz Guzik <mguzik@redhat.com>
Acked-by: Mateusz Guzik <mguzik@redhat.com>
Tested-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Documentation/filesystems/porting b/Documentation/filesystems/porting
index 3eae250..ec54561 100644
--- a/Documentation/filesystems/porting
+++ b/Documentation/filesystems/porting
@@ -500,3 +500,7 @@
dentry, it does not get nameidata at all and it gets called only when cookie
is non-NULL. Note that link body isn't available anymore, so if you need it,
store it as cookie.
+--
+[mandatory]
+ __fd_install() & fd_install() can now sleep. Callers should not
+ hold a spinlock or other resources that do not allow a schedule.
