[Devel] [PATCH RHEL7 COMMIT] ms/fs/file.c: don't acquire files->file_lock in fd_install()
Konstantin Khorenko
khorenko at virtuozzo.com
Thu Jan 18 18:05:13 MSK 2018
The commit is pushed to "branch-rh7-3.10.0-693.11.6.vz7.42.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.11.6.vz7.42.1
------>
commit c6fb234a45adbc8c1dc68103bced123d9055282e
Author: Eric Dumazet <edumazet at google.com>
Date: Thu Jan 18 18:05:13 2018 +0300
ms/fs/file.c: don't acquire files->file_lock in fd_install()
Patchset description:
Shrink big fdtable on criu restore
This patchset allows to avoid memory overuse introduced by service fds on criu
restore.
The solution is simple: smartly check for closed fd number, and shrink fdtable
if this could be made. The checks are happen in is_pseudosuper mode, so we do
not affect performance on normal work mode.
The problem is we can't solve this for 100% case in userspace.
Kernel allows to fix that completely.
https://jira.sw.ru/browse/PSBM-78827
Eric Dumazet (1):
ms/fs/file.c: don't acquire files->file_lock in fd_install()
Kirill Tkhai (3):
files: Add new argument to expand_files()
files: Add fdtable_align() helper
files: Shrink big fdtable on close in is_pseudosuper mode
Mateusz Guzik (1):
ms/vfs: grab the lock instead of blocking in __fd_install during resizing
============================================================
This patch description:
ms commit 8a81252b774b5
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 at google.com>
Reported-by: Mateusz Guzik <mguzik at redhat.com>
Acked-by: Mateusz Guzik <mguzik at redhat.com>
Tested-by: Mateusz Guzik <mguzik at redhat.com>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov at openvz.org>
---
fs/file.c | 67 +++++++++++++++++++++++++++++++++++--------------
include/linux/fdtable.h | 3 +++
2 files changed, 51 insertions(+), 19 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index 1db59527294f..adc42eccfd67 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -152,6 +152,13 @@ static int expand_fdtable(struct files_struct *files, int nr)
spin_unlock(&files->file_lock);
new_fdt = alloc_fdtable(nr);
+
+ /* make sure all __fd_install() have seen resize_in_progress
+ * or have finished their rcu_read_lock_sched() section.
+ */
+ if (atomic_read(&files->count) > 1)
+ synchronize_sched();
+
spin_lock(&files->file_lock);
if (!new_fdt)
return -ENOMEM;
@@ -163,21 +170,14 @@ static int expand_fdtable(struct files_struct *files, int nr)
__free_fdtable(new_fdt);
return -EMFILE;
}
- /*
- * Check again since another task may have expanded the fd table while
- * we dropped the lock
- */
cur_fdt = files_fdtable(files);
- if (nr >= cur_fdt->max_fds) {
- /* Continue as planned */
- copy_fdtable(new_fdt, cur_fdt);
- rcu_assign_pointer(files->fdt, new_fdt);
- if (cur_fdt != &files->fdtab)
- call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
- } else {
- /* Somebody else expanded, so undo our attempt */
- __free_fdtable(new_fdt);
- }
+ BUG_ON(nr < cur_fdt->max_fds);
+ copy_fdtable(new_fdt, cur_fdt);
+ rcu_assign_pointer(files->fdt, new_fdt);
+ if (cur_fdt != &files->fdtab)
+ call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
+ /* coupled with smp_rmb() in __fd_install() */
+ smp_wmb();
return 1;
}
@@ -190,21 +190,38 @@ static int expand_fdtable(struct files_struct *files, int nr)
* The files->file_lock should be held on entry, and will be held on exit.
*/
static int expand_files(struct files_struct *files, int nr)
+ __releases(files->file_lock)
+ __acquires(files->file_lock)
{
struct fdtable *fdt;
+ int expanded = 0;
+repeat:
fdt = files_fdtable(files);
/* Do we need to expand? */
if (nr < fdt->max_fds)
- return 0;
+ return expanded;
/* Can we expand? */
if (nr >= sysctl_nr_open)
return -EMFILE;
+ if (unlikely(files->resize_in_progress)) {
+ spin_unlock(&files->file_lock);
+ expanded = 1;
+ wait_event(files->resize_wait, !files->resize_in_progress);
+ spin_lock(&files->file_lock);
+ goto repeat;
+ }
+
/* All good, so we try */
- return expand_fdtable(files, nr);
+ files->resize_in_progress = true;
+ expanded = expand_fdtable(files, nr);
+ files->resize_in_progress = false;
+
+ wake_up_all(&files->resize_wait);
+ return expanded;
}
static inline void __set_close_on_exec(int fd, struct fdtable *fdt)
@@ -261,6 +278,8 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
atomic_set(&newf->count, 1);
spin_lock_init(&newf->file_lock);
+ newf->resize_in_progress = false;
+ init_waitqueue_head(&newf->resize_wait);
newf->next_fd = 0;
new_fdt = &newf->fdtab;
new_fdt->max_fds = NR_OPEN_DEFAULT;
@@ -566,11 +585,21 @@ void __fd_install(struct files_struct *files, unsigned int fd,
struct file *file)
{
struct fdtable *fdt;
- spin_lock(&files->file_lock);
- fdt = files_fdtable(files);
+
+ might_sleep();
+ rcu_read_lock_sched();
+
+ while (unlikely(files->resize_in_progress)) {
+ rcu_read_unlock_sched();
+ wait_event(files->resize_wait, !files->resize_in_progress);
+ rcu_read_lock_sched();
+ }
+ /* coupled with smp_wmb() in expand_fdtable() */
+ smp_rmb();
+ fdt = rcu_dereference_sched(files->fdt);
BUG_ON(fdt->fd[fd] != NULL);
rcu_assign_pointer(fdt->fd[fd], file);
- spin_unlock(&files->file_lock);
+ rcu_read_unlock_sched();
}
void fd_install(unsigned int fd, struct file *file)
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index eaaa0e790351..5f7c6c376799 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -47,6 +47,9 @@ struct files_struct {
* read mostly part
*/
atomic_t count;
+ bool resize_in_progress;
+ wait_queue_head_t resize_wait;
+
struct fdtable __rcu *fdt;
struct fdtable fdtab;
/*
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