On 10/17/2017 11:53 AM, Davidlohr Bueso wrote: > On Mon, 16 Oct 2017, Jason Baron wrote: > >> Hi, >> >> I posted a patch to completely remove the lock here from the >> ep_poll_safewake() patch here: >> >> http://lkml.iu.edu/hypermail/linux/kernel/1710.1/05834.html > > Kernel development never ceases to amaze me. Two complementary > fixes to a 8+ y/o performance issue on the same day - not that > nested epolls are that common, but it also comes from two real > workloads... > > Getting rid of the lock altogether is always the best way. > >> >> So these are going to conflict. The reason its safe to remove the lock >> is that there are loop and depth checks now that are performed during >> EPOLL_CTL_ADD. Specifically, ep_loop_check(). I would prefer to these >> checks once add add time as opposed to at each wakeup (even if they can >> be scaled better). > > Wrt conflicts, no worries, I'll rebase -- and hopefully we can get > the dlock stuff in for v4.15 as well. > >> >> I also have a pending patch to do something similar for >> poll_readywalk_ncalls, but I think that poll_safewake_ncalls is the most >> egregious one here? > > The customer's workload issues are for the loop_ncalls and readywalk_ncalls > lists, so I'd be interested in seeing your patch for the later. The reason > your patch above is likely not to help my scenario is that most of the time > is spent at a dispatcher level doing epoll_wait, not too many > EPOLL_CTL_ADDs > going on afaict. If there are not many EPOLL_CTL_ADDs, then I wouldn't think loop_ncalls would be highly contented (since it should only be called from the add path)? Thanks, -Jason > > Thanks, > Davidlohr > >> >> Thanks, >> >> -Jason >> >> On 10/13/2017 11:45 AM, Davidlohr Bueso wrote: >>> A customer reported massive contention on the ncalls->lock in which >>> the workload is designed around nested epolls (where the fd is already >>> an epoll). >>> >>> 83.49% [kernel] [k] __pv_queued_spin_lock_slowpath >>> 2.70% [kernel] [k] ep_call_nested.constprop.13 >>> 1.94% [kernel] [k] _raw_spin_lock_irqsave >>> 1.83% [kernel] [k] >>> __raw_callee_save___pv_queued_spin_unlock >>> 1.45% [kernel] [k] _raw_spin_unlock_irqrestore >>> 0.41% [kernel] [k] ep_scan_ready_list.isra.8 >>> 0.36% [kernel] [k] pvclock_clocksource_read >>> >>> The application running on kvm, is using a shared memory IPC >>> communication >>> with a pipe wakeup mechanism, and a two level dispatcher both built >>> around >>> 'epoll_wait'. There is one process per physical core and a full mesh of >>> pipes >>> between them, so on a 18 core system (the actual case), there are 18*18 >>> pipes >>> for the IPCs alone. >>> >>> This patch proposes replacing the nested calls global linked list with a >>> dlock >>> interface, for which we can benefit from pcpu lists when doing >>> ep_poll_safewake(), >>> and hashing for the current task, which provides two benefits: >>> >>> 1. Speeds up the process of loop and max-depth checks from O(N) lookups >>> to O(1) >>> (albeit possible collisions, which we have to iterate); and, >>> >>> 2. Provides smaller lock granularity. >>> >>> cpus before after diff >>> 1 1409370 1344804 -4.58% >>> 2 1015690 1337053 31.63% >>> 3 721009 1273254 76.59% >>> 4 380959 1128931 196.33% >>> 5 287603 1028362 257.56% >>> 6 221104 894943 304.76% >>> 7 173592 976395 462.46% >>> 8 145860 922584 532.51% >>> 9 127877 925900 624.05% >>> 10 112603 791456 602.87% >>> 11 97926 724296 639.63% >>> 12 80732 730485 804.82% >>> >>> With the exception of a single cpu, where the overhead of jhashing >>> influences), we >>> get some pretty good raw throughput increase. Similarly, the amount of >>> time spent >>> decreases immensely as well. >>> >>> Passes ltp related testcases. >>> >>> Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> >>> --- >>> fs/eventpoll.c | 88 >>> +++++++++++++++++++++++++++++++++++----------------------- >>> 1 file changed, 53 insertions(+), 35 deletions(-) >>> >>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c >>> index 2fabd19cdeea..675c97fdc5da 100644 >>> --- a/fs/eventpoll.c >>> +++ b/fs/eventpoll.c >>> @@ -22,7 +22,7 @@ >>> #include <linux/slab.h> >>> #include <linux/poll.h> >>> #include <linux/string.h> >>> -#include <linux/list.h> >>> +#include <linux/dlock-list.h> >>> #include <linux/hash.h> >>> #include <linux/spinlock.h> >>> #include <linux/syscalls.h> >>> @@ -119,7 +119,7 @@ struct epoll_filefd { >>> * and loop cycles. >>> */ >>> struct nested_call_node { >>> - struct list_head llink; >>> + struct dlock_list_node llink; >>> void *cookie; >>> void *ctx; >>> }; >>> @@ -129,8 +129,7 @@ struct nested_call_node { >>> * maximum recursion dept and loop cycles. >>> */ >>> struct nested_calls { >>> - struct list_head tasks_call_list; >>> - spinlock_t lock; >>> + struct dlock_list_heads tasks_call_list; >>> }; >>> >>> /* >>> @@ -371,10 +370,14 @@ static inline int ep_op_has_event(int op) >>> } >>> >>> /* Initialize the poll safe wake up structure */ >>> -static void ep_nested_calls_init(struct nested_calls *ncalls) >>> +static inline int ep_nested_calls_init(struct nested_calls *ncalls) >>> +{ >>> + return alloc_dlock_list_heads(&ncalls->tasks_call_list, true); >>> +} >>> + >>> +static inline void ep_nested_calls_free(struct nested_calls *ncalls) >>> { >>> - INIT_LIST_HEAD(&ncalls->tasks_call_list); >>> - spin_lock_init(&ncalls->lock); >>> + free_dlock_list_heads(&ncalls->tasks_call_list); >>> } >>> >>> /** >>> @@ -483,45 +486,50 @@ static int ep_call_nested(struct nested_calls >>> *ncalls, int max_nests, >>> { >>> int error, call_nests = 0; >>> unsigned long flags; >>> - struct list_head *lsthead = &ncalls->tasks_call_list; >>> - struct nested_call_node *tncur; >>> - struct nested_call_node tnode; >>> + struct dlock_list_head *head; >>> + struct nested_call_node *tncur, tnode; >>> >>> - spin_lock_irqsave(&ncalls->lock, flags); >>> + head = dlock_list_hash(&ncalls->tasks_call_list, ctx); >>> >>> + spin_lock_irqsave(&head->lock, flags); >>> /* >>> * Try to see if the current task is already inside this wakeup >>> call. >>> - * We use a list here, since the population inside this set is >>> always >>> - * very much limited. >>> + * >>> + * We use a chained hash table with linked lists here, and take the >>> + * lock to avoid racing when collisions (where ctx pointer is the >>> key). >>> + * Calls for which context is the cpu number, avoid hashing and >>> act as >>> + * pcpu add/removal. >>> + * >>> + * Since the population inside this set is always very much >>> limited, >>> + * list scanning should be short. >>> */ >>> - list_for_each_entry(tncur, lsthead, llink) { >>> - if (tncur->ctx == ctx && >>> - (tncur->cookie == cookie || ++call_nests > max_nests)) { >>> - /* >>> - * Ops ... loop detected or maximum nest level reached. >>> - * We abort this wake by breaking the cycle itself. >>> - */ >>> - error = -1; >>> - goto out_unlock; >>> - } >>> - } >>> + list_for_each_entry(tncur, &head->list, llink.list) { >>> + if (tncur->ctx == ctx && >>> + (tncur->cookie == cookie || ++call_nests > EP_MAX_NESTS)) { >>> + /* >>> + * Ops ... loop detected or maximum nest level reached. >>> + * We abort this wake by breaking the cycle itself. >>> + */ >>> + error = -1; >>> + spin_unlock_irqrestore(&head->lock, flags); >>> + goto done; >>> + } >>> + } >>> + >>> >>> /* Add the current task and cookie to the list */ >>> tnode.ctx = ctx; >>> tnode.cookie = cookie; >>> - list_add(&tnode.llink, lsthead); >>> - >>> - spin_unlock_irqrestore(&ncalls->lock, flags); >>> + tnode.llink.head = head; >>> + list_add(&tnode.llink.list, &head->list); >>> + spin_unlock_irqrestore(&head->lock, flags); >>> >>> /* Call the nested function */ >>> error = (*nproc)(priv, cookie, call_nests); >>> >>> /* Remove the current task from the list */ >>> - spin_lock_irqsave(&ncalls->lock, flags); >>> - list_del(&tnode.llink); >>> -out_unlock: >>> - spin_unlock_irqrestore(&ncalls->lock, flags); >>> - >>> + dlock_lists_del(&tnode.llink); >>> +done: >>> return error; >>> } >>> >>> @@ -2313,13 +2321,16 @@ static int __init eventpoll_init(void) >>> * Initialize the structure used to perform epoll file descriptor >>> * inclusion loops checks. >>> */ >>> - ep_nested_calls_init(&poll_loop_ncalls); >>> + if (ep_nested_calls_init(&poll_loop_ncalls)) >>> + goto err; >>> >>> /* Initialize the structure used to perform safe poll wait head wake >>> ups */ >>> - ep_nested_calls_init(&poll_safewake_ncalls); >>> + if (ep_nested_calls_init(&poll_safewake_ncalls)) >>> + goto err_free1; >>> >>> /* Initialize the structure used to perform file's f_op->poll() >>> calls */ >>> - ep_nested_calls_init(&poll_readywalk_ncalls); >>> + if (ep_nested_calls_init(&poll_readywalk_ncalls)) >>> + goto err_free0; >>> >>> /* >>> * We can have many thousands of epitems, so prevent this from >>> @@ -2336,5 +2347,12 @@ static int __init eventpoll_init(void) >>> sizeof(struct eppoll_entry), 0, SLAB_PANIC, NULL); >>> >>> return 0; >>> + >>> +err_free0: >>> + ep_nested_calls_free(&poll_safewake_ncalls); >>> +err_free1: >>> + ep_nested_calls_free(&poll_loop_ncalls); >>> +err: >>> + return -ENOMEM; >>> } >>> fs_initcall(eventpoll_init);
