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authorNeilBrown <neilb@suse.com>2018-11-30 02:33:18 +0300
committerAl Viro <viro@zeniv.linux.org.uk>2018-11-30 02:55:10 +0300
commit22cb7405fada5305926d9bbcb476c1cd638d2dab (patch)
treec21cde60f75baa4f607d8d04ffd22ba7fb30b9ad /fs/isofs
parent651022382c7f8da46cb4872a545ee1da6d097d2a (diff)
downloadlinux-22cb7405fada5305926d9bbcb476c1cd638d2dab.tar.xz
VFS: use synchronize_rcu_expedited() in namespace_unlock()
The synchronize_rcu() in namespace_unlock() is called every time a filesystem is unmounted. If a great many filesystems are mounted, this can cause a noticable slow-down in, for example, system shutdown. The sequence: mkdir -p /tmp/Mtest/{0..5000} time for i in /tmp/Mtest/*; do mount -t tmpfs tmpfs $i ; done time umount /tmp/Mtest/* on a 4-cpu VM can report 8 seconds to mount the tmpfs filesystems, and 100 seconds to unmount them. Boot the same VM with 1 CPU and it takes 18 seconds to mount the tmpfs filesystems, but only 36 to unmount. If we change the synchronize_rcu() to synchronize_rcu_expedited() the umount time on a 4-cpu VM drop to 0.6 seconds I think this 200-fold speed up is worth the slightly high system impact of using synchronize_rcu_expedited(). Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> (from general rcu perspective) Signed-off-by: NeilBrown <neilb@suse.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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