By moving the events field epitem, we can avoid dirtying (or even loading) an extra cache line on 64-bit machines with 64-byte cache lines. Since EPOLLWAKEUP is uncommonly used, we add an additional check for the EPOLLWAKEUP flag to avoid reading a second cache line for the wakeup_source.
This allows ep_send_events to only read/write the top 64-bytes of an epitem in common cases. This patch was only made possible by the smaller footprint required by wfcqueue. epwbench test timings: Before (without wfcq at all): AVG: 5.448400 SIG: 0.003056 Before (with wfcq local): AVG: 5.532024 SIG: 0.000244 After (this commit): AVG: 5.331539 SIG: 0.000234 Even with the variability between runs on my KVM, I'm confident this wfcqueue epoll series introduces no performance regressions in the common single-threaded use cases of epoll. ref: http://www.xmailserver.org/epwbench.c Somewhat-tested-by: Eric Wong <normalper...@yhbt.net> Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com> Cc: Davide Libenzi <davi...@xmailserver.org> Cc: Al Viro <v...@zeniv.linux.org.uk> Cc: Andrew Morton <a...@linux-foundation.org> --- fs/eventpoll.c | 27 +++++++++++++++++++++++---- 1 file changed, 23 insertions(+), 4 deletions(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 1e04175..82bf483 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -155,12 +155,27 @@ struct epitem { /* The file descriptor information this item refers to */ struct epoll_filefd ffd; - /* Number of active wait queue attached to poll operations */ + /* + * Number of active wait queue attached to poll operations + * This is infrequently used, it pads well here but may be + * removed in the future + */ int nwait; /* state of this item */ enum epoll_item_state state; + /* The structure that describe the interested events and the source fd */ + struct epoll_event event; + + /* + * --> 64-byte boundary for 64-bit systems <-- + * frequently accessed (read/written) items ar above this comment + * infrequently accessed items are below this comment + * Keeping frequently accessed items within the 64-byte boundary + * prevents extra cache line usage on common x86-64 machines + */ + /* List containing poll wait queues */ struct list_head pwqlist; @@ -172,9 +187,6 @@ struct epitem { /* wakeup_source used when EPOLLWAKEUP is set */ struct wakeup_source __rcu *ws; - - /* The structure that describe the interested events and the source fd */ - struct epoll_event event; }; /* @@ -596,6 +608,13 @@ static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi) /* call only when ep->mtx is held */ static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi) { + /* + * avoid loading the extra cache line on machines with + * <= 64-byte cache lines + */ + if (!(epi->event.events & EPOLLWAKEUP)) + return NULL; + return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx)); } -- Eric Wong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
