From: Eric Dumazet Date: Tue, 30 Jun 2015 13:54:08 +0000 (+0200) Subject: fs/file.c: don't acquire files->file_lock in fd_install() X-Git-Tag: omap-for-v4.2/fixes-rc1^2~6^2~8 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=8a81252b774b53e628a8a0fe18e2b8fc236d92cc;p=pandora-kernel.git 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 Reported-by: Mateusz Guzik Acked-by: Mateusz Guzik Tested-by: Mateusz Guzik Signed-off-by: Al Viro --- Reading git-diff-tree failed