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fs/epoll: use a per-cpu counter for user's watches count 

This counter tracks the number of watches a user has, to compare against
the 'max_user_watches' limit. This causes a scalability bottleneck on
SPECjbb2015 on large systems as there is only one user. Changing to a
per-cpu counter increases throughput of the benchmark by about 30% on a
16-socket, > 1000 thread system.

[rdunlap@infradead.org: fix build errors in kernel/user.c when CONFIG_EPOLL=n]
[npiggin@gmail.com: move ifdefs into wrapper functions, slightly improve panic message]
  Link: https://lkml.kernel.org/r/1628051945.fens3r99ox.astroid@bobo.none
[akpm@linux-foundation.org: tweak user_epoll_alloc(), per Guenter]
  Link: https://lkml.kernel.org/r/20210804191421.GA1900577@roeck-us.net

Link: https://lkml.kernel.org/r/20210802032013.2751916-1-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reported-by: Anton Blanchard <anton@ozlabs.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nicholas Piggin 2021-09-07 20:00:00 -07:00 committed by Linus Torvalds
parent 4ce9f97045
commit 1e1c15839d
3 changed files with 37 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
fs/eventpoll.c
View File

@ -723,7 +723,7 @@ static int ep_remove(struct eventpoll *ep, struct epitem *epi)
*/
call_rcu(&epi->rcu, epi_rcu_free);
atomic_long_dec(&ep->user->epoll_watches);
percpu_counter_dec(&ep->user->epoll_watches);
return 0;
}
@ -1439,7 +1439,6 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
{
int error, pwake = 0;
__poll_t revents;
long user_watches;
struct epitem *epi;
struct ep_pqueue epq;
struct eventpoll *tep = NULL;
@ -1449,11 +1448,15 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
lockdep_assert_irqs_enabled();
user_watches = atomic_long_read(&ep->user->epoll_watches);
if (unlikely(user_watches >= max_user_watches))
if (unlikely(percpu_counter_compare(&ep->user->epoll_watches,
max_user_watches) >= 0))
return -ENOSPC;
if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL)))
percpu_counter_inc(&ep->user->epoll_watches);
if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL))) {
percpu_counter_dec(&ep->user->epoll_watches);
return -ENOMEM;
}
/* Item initialization follow here ... */
INIT_LIST_HEAD(&epi->rdllink);
@ -1466,17 +1469,16 @@ static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
mutex_lock_nested(&tep->mtx, 1);
/* Add the current item to the list of active epoll hook for this file */
if (unlikely(attach_epitem(tfile, epi) < 0)) {
kmem_cache_free(epi_cache, epi);
if (tep)
mutex_unlock(&tep->mtx);
kmem_cache_free(epi_cache, epi);
percpu_counter_dec(&ep->user->epoll_watches);
return -ENOMEM;
}
if (full_check && !tep)
list_file(tfile);
atomic_long_inc(&ep->user->epoll_watches);
/*
* Add the current item to the RB tree. All RB tree operations are
* protected by "mtx", and ep_insert() is called with "mtx" held.

3
include/linux/sched/user.h
View File

@ -4,6 +4,7 @@
#include <linux/uidgid.h>
#include <linux/atomic.h>
#include <linux/percpu_counter.h>
#include <linux/refcount.h>
#include <linux/ratelimit.h>
@ -13,7 +14,7 @@
struct user_struct {
refcount_t __count; /* reference count */
#ifdef CONFIG_EPOLL
atomic_long_t epoll_watches; /* The number of file descriptors currently watched */
struct percpu_counter epoll_watches; /* The number of file descriptors currently watched */
#endif
unsigned long unix_inflight; /* How many files in flight in unix sockets */
atomic_long_t pipe_bufs; /* how many pages are allocated in pipe buffers */

25
kernel/user.c
View File

@ -129,6 +129,22 @@ static struct user_struct *uid_hash_find(kuid_t uid, struct hlist_head *hashent)
return NULL;
}
static int user_epoll_alloc(struct user_struct *up)
{
#ifdef CONFIG_EPOLL
return percpu_counter_init(&up->epoll_watches, 0, GFP_KERNEL);
#else
return 0;
#endif
}
static void user_epoll_free(struct user_struct *up)
{
#ifdef CONFIG_EPOLL
percpu_counter_destroy(&up->epoll_watches);
#endif
}
/* IRQs are disabled and uidhash_lock is held upon function entry.
* IRQ state (as stored in flags) is restored and uidhash_lock released
* upon function exit.
@ -138,6 +154,7 @@ static void free_user(struct user_struct *up, unsigned long flags)
{
uid_hash_remove(up);
spin_unlock_irqrestore(&uidhash_lock, flags);
user_epoll_free(up);
kmem_cache_free(uid_cachep, up);
}
@ -185,6 +202,10 @@ struct user_struct *alloc_uid(kuid_t uid)
new->uid = uid;
refcount_set(&new->__count, 1);
if (user_epoll_alloc(new)) {
kmem_cache_free(uid_cachep, new);
return NULL;
}
ratelimit_state_init(&new->ratelimit, HZ, 100);
ratelimit_set_flags(&new->ratelimit, RATELIMIT_MSG_ON_RELEASE);
@ -195,6 +216,7 @@ struct user_struct *alloc_uid(kuid_t uid)
spin_lock_irq(&uidhash_lock);
up = uid_hash_find(uid, hashent);
if (up) {
user_epoll_free(new);
kmem_cache_free(uid_cachep, new);
} else {
uid_hash_insert(new, hashent);
@ -216,6 +238,9 @@ static int __init uid_cache_init(void)
for(n = 0; n < UIDHASH_SZ; ++n)
INIT_HLIST_HEAD(uidhash_table + n);
if (user_epoll_alloc(&root_user))
panic("root_user epoll percpu counter alloc failed");
/* Insert the root user immediately (init already runs as root) */
spin_lock_irq(&uidhash_lock);
uid_hash_insert(&root_user, uidhashentry(GLOBAL_ROOT_UID));