Messages in this thread | | | Subject | Re: [PATCH 1/2] io_uring: avoid ring quiesce while registering/unregistering eventfd | From | Usama Arif <> | Date | Thu, 3 Feb 2022 17:47:27 +0000 |
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On 03/02/2022 15:48, Pavel Begunkov wrote: > On 2/3/22 15:11, Usama Arif wrote: >> This is done by creating a new RCU data structure (io_ev_fd) as part of >> io_ring_ctx that holds the eventfd_ctx. >> >> The function io_eventfd_signal is executed under rcu_read_lock with a >> single rcu_dereference to io_ev_fd so that if another thread unregisters >> the eventfd while io_eventfd_signal is still being executed, the >> eventfd_signal for which io_eventfd_signal was called completes >> successfully. >> >> The process of registering/unregistering eventfd is done under a lock >> so multiple threads don't enter a race condition while >> registering/unregistering eventfd. >> >> With the above approach ring quiesce can be avoided which is much more >> expensive then using RCU lock. On the system tested, io_uring_reigster >> with >> IORING_REGISTER_EVENTFD takes less than 1ms with RCU lock, compared to >> 15ms >> before with ring quiesce. >> >> Signed-off-by: Usama Arif <usama.arif@bytedance.com> >> --- >> fs/io_uring.c | 103 +++++++++++++++++++++++++++++++++++++++----------- >> 1 file changed, 80 insertions(+), 23 deletions(-) >> >> diff --git a/fs/io_uring.c b/fs/io_uring.c >> index 2e04f718319d..f07cfbb387a6 100644 >> --- a/fs/io_uring.c >> +++ b/fs/io_uring.c >> @@ -326,6 +326,12 @@ struct io_submit_state { >> struct blk_plug plug; >> }; > >> -static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx) >> +static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx, >> struct io_ev_fd *ev_fd) >> { >> - if (likely(!ctx->cq_ev_fd)) >> + if (likely(!ev_fd)) >> return false; >> if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED) >> return false; >> return !ctx->eventfd_async || io_wq_current_is_worker(); >> } >> +static void io_eventfd_signal(struct io_ring_ctx *ctx) >> +{ >> + struct io_ev_fd *ev_fd; >> + >> + rcu_read_lock(); > > Please always think about the fast path, which is not set eventfd. > We don't want extra overhead here. >
I guess this should be ok now from v3?
Thanks, Usama > if (ctx->ev_fd) { > rcu_read_lock(); > ev_fd = rcu_deref(...); > ... > rcu_read_unlock(); > } >
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