From: Marcelo Diop-Gonzalez <[email protected]>
To: Pavel Begunkov <[email protected]>
Cc: [email protected], [email protected]
Subject: Re: [PATCH v2 2/2] io_uring: flush timeouts that should already have expired
Date: Fri, 8 Jan 2021 10:57:26 -0500 [thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
On Sat, Jan 02, 2021 at 08:26:26PM +0000, Pavel Begunkov wrote:
> On 02/01/2021 19:54, Pavel Begunkov wrote:
> > On 19/12/2020 19:15, Marcelo Diop-Gonzalez wrote:
> >> Right now io_flush_timeouts() checks if the current number of events
> >> is equal to ->timeout.target_seq, but this will miss some timeouts if
> >> there have been more than 1 event added since the last time they were
> >> flushed (possible in io_submit_flush_completions(), for example). Fix
> >> it by recording the starting value of ->cached_cq_overflow -
> >> ->cq_timeouts instead of the target value, so that we can safely
> >> (without overflow problems) compare the number of events that have
> >> happened with the number of events needed to trigger the timeout.
>
> https://www.spinics.net/lists/kernel/msg3475160.html
>
> The idea was to replace u32 cached_cq_tail with u64 while keeping
> timeout offsets u32. Assuming that we won't ever hit ~2^62 inflight
> requests, complete all requests falling into some large enough window
> behind that u64 cached_cq_tail.
>
> simplifying:
>
> i64 d = target_off - ctx->u64_cq_tail
> if (d <= 0 && d > -2^32)
> complete_it()
>
> Not fond of it, but at least worked at that time. You can try out
> this approach if you want, but would be perfect if you would find
> something more elegant :)
>
What do you think about something like this? I think it's not totally
correct because it relies on having ->completion_lock in io_timeout() so
that ->cq_last_tm_flushed is updated, but in case of IORING_SETUP_IOPOLL,
io_iopoll_complete() doesn't take that lock, and ->uring_lock will not
be held if io_timeout() is called from io_wq_submit_work(), but maybe
could still be worth it since that was already possibly a problem?
diff --git a/fs/io_uring.c b/fs/io_uring.c
index cb57e0360fcb..50984709879c 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -353,6 +353,7 @@ struct io_ring_ctx {
unsigned cq_entries;
unsigned cq_mask;
atomic_t cq_timeouts;
+ unsigned cq_last_tm_flush;
unsigned long cq_check_overflow;
struct wait_queue_head cq_wait;
struct fasync_struct *cq_fasync;
@@ -1633,19 +1634,26 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
static void io_flush_timeouts(struct io_ring_ctx *ctx)
{
+ u32 seq = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
+
while (!list_empty(&ctx->timeout_list)) {
+ u32 events_needed, events_got;
struct io_kiocb *req = list_first_entry(&ctx->timeout_list,
struct io_kiocb, timeout.list);
if (io_is_timeout_noseq(req))
break;
- if (req->timeout.target_seq != ctx->cached_cq_tail
- - atomic_read(&ctx->cq_timeouts))
+
+ events_needed = req->timeout.target_seq - ctx->cq_last_tm_flush;
+ events_got = seq - ctx->cq_last_tm_flush;
+ if (events_got < events_needed)
break;
list_del_init(&req->timeout.list);
io_kill_timeout(req);
}
+
+ ctx->cq_last_tm_flush = seq;
}
static void io_commit_cqring(struct io_ring_ctx *ctx)
--
2.20.1
> >>
> >> Signed-off-by: Marcelo Diop-Gonzalez <[email protected]>
> >> ---
> >> fs/io_uring.c | 30 +++++++++++++++++++++++-------
> >> 1 file changed, 23 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/fs/io_uring.c b/fs/io_uring.c
> >> index f394bf358022..f62de0cb5fc4 100644
> >> --- a/fs/io_uring.c
> >> +++ b/fs/io_uring.c
> >> @@ -444,7 +444,7 @@ struct io_cancel {
> >> struct io_timeout {
> >> struct file *file;
> >> u32 off;
> >> - u32 target_seq;
> >> + u32 start_seq;
> >> struct list_head list;
> >> /* head of the link, used by linked timeouts only */
> >> struct io_kiocb *head;
> >> @@ -1629,6 +1629,24 @@ static void __io_queue_deferred(struct io_ring_ctx *ctx)
> >> } while (!list_empty(&ctx->defer_list));
> >> }
> >>
> >> +static inline u32 io_timeout_events_left(struct io_kiocb *req)
> >> +{
> >> + struct io_ring_ctx *ctx = req->ctx;
> >> + u32 events;
> >> +
> >> + /*
> >> + * events -= req->timeout.start_seq and the comparison between
> >> + * ->timeout.off and events will not overflow because each time
> >> + * ->cq_timeouts is incremented, ->cached_cq_tail is incremented too.
> >> + */
> >> +
> >> + events = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
> >> + events -= req->timeout.start_seq;
> >
> > It looks to me that events before the start_seq subtraction can have got wrapped
> > around start_seq.
> >
> > e.g.
> > 1) you submit a timeout with off=0xff...ff (start_seq=0 for convenience)
> >
> > 2) some time has passed, let @events = 0xff..ff - 1
> > so the timeout still waits
> >
> > 3) we commit 5 requests at once and call io_commit_cqring() only once for
> > them, so we get @events == 0xff..ff - 1 + 5, i.e. 4
> >
> > @events == 4 < off == 0xff...ff,
> > so we didn't trigger out timeout even though should have
> >
> >> + if (req->timeout.off > events)
> >> + return req->timeout.off - events;
> >> + return 0;
> >> +}
> >> +
> >> static void io_flush_timeouts(struct io_ring_ctx *ctx)
> >> {
> >> while (!list_empty(&ctx->timeout_list)) {
> >> @@ -1637,8 +1655,7 @@ static void io_flush_timeouts(struct io_ring_ctx *ctx)
> >>
> >> if (io_is_timeout_noseq(req))
> >> break;
> >> - if (req->timeout.target_seq != ctx->cached_cq_tail
> >> - - atomic_read(&ctx->cq_timeouts))
> >> + if (io_timeout_events_left(req) > 0)
> >> break;
> >>
> >> list_del_init(&req->timeout.list);
> >> @@ -5785,7 +5802,6 @@ static int io_timeout(struct io_kiocb *req)
> >> struct io_ring_ctx *ctx = req->ctx;
> >> struct io_timeout_data *data = req->async_data;
> >> struct list_head *entry;
> >> - u32 tail, off = req->timeout.off;
> >>
> >> spin_lock_irq(&ctx->completion_lock);
> >>
> >> @@ -5799,8 +5815,8 @@ static int io_timeout(struct io_kiocb *req)
> >> goto add;
> >> }
> >>
> >> - tail = ctx->cached_cq_tail - atomic_read(&ctx->cq_timeouts);
> >> - req->timeout.target_seq = tail + off;
> >> + req->timeout.start_seq = ctx->cached_cq_tail -
> >> + atomic_read(&ctx->cq_timeouts);
> >>
> >> /*
> >> * Insertion sort, ensuring the first entry in the list is always
> >> @@ -5813,7 +5829,7 @@ static int io_timeout(struct io_kiocb *req)
> >> if (io_is_timeout_noseq(nxt))
> >> continue;
> >> /* nxt.seq is behind @tail, otherwise would've been completed */
> >> - if (off >= nxt->timeout.target_seq - tail)
> >> + if (req->timeout.off >= io_timeout_events_left(nxt))
> >> break;
> >> }
> >> add:
> >>
> >
>
> --
> Pavel Begunkov
next prev parent reply other threads:[~2021-01-08 15:58 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-19 19:15 [PATCH v2 0/2] io_uring: fix skipping of old timeout events Marcelo Diop-Gonzalez
2020-12-19 19:15 ` [PATCH v2 1/2] io_uring: only increment ->cq_timeouts along with ->cached_cq_tail Marcelo Diop-Gonzalez
2021-01-02 20:03 ` Pavel Begunkov
2021-01-04 16:49 ` Marcelo Diop-Gonzalez
2020-12-19 19:15 ` [PATCH v2 2/2] io_uring: flush timeouts that should already have expired Marcelo Diop-Gonzalez
2021-01-02 19:54 ` Pavel Begunkov
2021-01-02 20:26 ` Pavel Begunkov
2021-01-08 15:57 ` Marcelo Diop-Gonzalez [this message]
2021-01-11 4:57 ` Pavel Begunkov
2021-01-11 15:28 ` Marcelo Diop-Gonzalez
2021-01-12 20:47 ` Pavel Begunkov
2021-01-13 14:41 ` Marcelo Diop-Gonzalez
2021-01-13 15:20 ` Pavel Begunkov
2021-01-14 0:46 ` Marcelo Diop-Gonzalez
2021-01-14 21:04 ` Pavel Begunkov
2021-01-04 17:56 ` Marcelo Diop-Gonzalez
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