[PATCH] HTTP: trigger lingering close when keepalive connection will be closed
Maxim Dounin
mdounin at mdounin.ru
Fri Jan 27 21:57:11 UTC 2023
Hello!
On Fri, Jan 27, 2023 at 08:11:34PM +0800, Miao Wang wrote:
> > 2023年1月27日 12:01，Maxim Dounin <mdounin at mdounin.ru> 写道：
> >
> > On Wed, Jan 25, 2023 at 12:49:10PM +0800, Miao Wang wrote:
> >
> >>> 2023年1月25日 10:17，Maxim Dounin <mdounin at mdounin.ru> 写道：
> >>>
> >>> On Mon, Jan 23, 2023 at 07:01:16PM +0800, Miao Wang wrote:
> >>>
> >>>>> 2023年1月23日 12:05，Maxim Dounin <mdounin at mdounin.ru> 写道：
> >>>>>
> >>>>> On Wed, Jan 18, 2023 at 11:28:52PM +0800, Miao Wang wrote:
> >>>>>
> >>>>>> # HG changeset patch
> >>>>>> # User Miao Wang <shankerwangmiao at gmail.com>
> >>>>>> # Date 1674055068 -28800
> >>>>>> # Wed Jan 18 23:17:48 2023 +0800
> >>>>>> # Node ID 73aa64bd29f3dec9e43e97560d6b5a07cdf40063
> >>>>>> # Parent 07b0bee87f32be91a33210bc06973e07c4c1dac9
> >>>>>> HTTP: trigger lingering close when keepalive connection will be closed
> >>>>>>
> >>>>>> When finalizing a request, if the request is not keepalive but
> >>>>>> its connection has served more than one request, then the connection
> >>>>>> has been a keepalive connection previously and this connection will
> >>>>>> be closed after this response. In this condition, it is likely that
> >>>>>> there are pipelined requests following this request, which we should
> >>>>>> ignore. As a result, lingering close is necessary in this case.
> >>>>>>
> >>>>>> Without this patch, nginx (with its default configuration) will send
> >>>>>> out TCP RST when there are more pipelined requests. The symptom is
> >>>>>> obvious when nginx is serving a debian repository and apt is
> >>>>>> downloading massive of packages. See [1]. It becomes more obvious
> >>>>>> when `keepalive_requests` is lower or nginx is under a relative
> >>>>>> higher load, and it disappears when specifying
> >>>>>> `lingering_close always`.
> >>>>>>
> >>>>>> [1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10
> >>>>>>
> >>>>>> diff -r 07b0bee87f32 -r 73aa64bd29f3 src/http/ngx_http_request.c
> >>>>>> --- a/src/http/ngx_http_request.c Wed Dec 21 14:53:27 2022 +0300
> >>>>>> +++ b/src/http/ngx_http_request.c Wed Jan 18 23:17:48 2023 +0800
> >>>>>> @@ -2749,6 +2749,10 @@
> >>>>>> return;
> >>>>>> }
> >>>>>>
> >>>>>> + if (!r->keepalive && r->connection->requests > 1) {
> >>>>>> + r->lingering_close = 1;
> >>>>>> + }
> >>>>>> +
> >>>>>> if (clcf->lingering_close == NGX_HTTP_LINGERING_ALWAYS
> >>>>>> || (clcf->lingering_close == NGX_HTTP_LINGERING_ON
> >>>>>> && (r->lingering_close
> >>>>>
> >>>>> Thanks for the patch and the link to the Debian bug report.
> >>>>>
> >>>>> Lingering close implies noticeable additional resource usage: even
> >>>>> if nothing happens on the connection, it will be kept open for
> >>>>> lingering_timeout, which is 5 seconds by default. Given that
> >>>>> pipelining is not used by most of the clients, forcing lingering
> >>>>> close for all clients which are using keepalive does not look like
> >>>>> a good solution to me.
> >>>>>
> >>>>> In general, nginx tries hard to determine if any additional data
> >>>>> are expected on the connection, and uses lingering close if there
> >>>>> is a good chance there will be some, but avoids lingering close by
> >>>>> default if additional data are unlikely. If this logic does not
> >>>>> work for some reason, lingering close can be explicitly requested
> >>>>> with "lingering_close always;".
> >>>>
> >>>> That's true since the symptom I described can be worked around with
> >>>> that option.
> >>>>
> >>>>>
> >>>>> In particular, note the "r->header_in->pos < r->header_in->last"
> >>>>> and "r->connection->read->ready" checks - these are expected to
> >>>>> catch connections with additional pipelined requests (see revision
> >>>>> 3981:77604e9a1ed8). And from the log provided in the report it
> >>>>> looks like it works most of the time - there are more than 6k HTTP
> >>>>> requests, and 60+ connections. But sometimes it fails - there are
> >>>>> two RST errors logged (and one "Undetermined Error", which looks
> >>>>> like a bug in apt, but might be related).
> >>>>>
> >>>>> It looks like when apt is downloading many resources, it does not
> >>>>> send all the requests at once (or in batches), but instead tries
> >>>>> to maintain a constant "depth", a number of pipelined requests in
> >>>>> flight. This essentially means that after reading of a response
> >>>>> it sends an additional request.
> >>>>
> >>>> That's right. From a traffic dump, I can see apt first sends one
> >>>> request, and after receiving the response, it will send out 10
> >>>> more requests, and maintain a depth of 10, since by default
> >>>> Acquire::http::Pipeline-Depth is 10.
> >>>>
> >>>>>
> >>>>> I see at least two possible cases which can result in nginx not
> >>>>> using lingering close with such a load:
> >>>>>
> >>>>> 1. If a response where keepalive_requests is reached happens to
> >>>>> be the last request in the r->header_in buffer (so the
> >>>>> "r->header_in->pos < r->header_in->last" won't be effective), and
> >>>>> there is a chance that nginx wasn't yet got an event from kernel
> >>>>> about additional data (and therefore "r->connection->read->ready"
> >>>>> will not be set). As such, nginx won't use lingering close, and
> >>>>> might close connection with unread data in the socket buffer,
> >>>>> resulting in RST.
> >>>>>
> >>>>> 2. Similarly, if nginx happens to be faster than apt, and socket
> >>>>> buffers are large enough, it might sent all the responses,
> >>>>> including the last one with "Connection: close", and close the
> >>>>> connection (since there are no pending pipelined requests at the
> >>>>> moment) even before an additional request is sent by apt. When
> >>>>> later apt will send an additional request after reading some of
> >>>>> the responses, it will send the request to already closed
> >>>>> connection, again resulting in RST.
> >>>>
> >>>> Actually, comparing the debug log and the pcap, nginx calls
> >>>> close() after writing the last response. However, at that time,
> >>>> that response is not fully transmitted to the client and there
> >>>> seems to be more requests not processed in the kernel buffer.
> >>>> Thus close() triggers an immediate RST.
> >>>
> >>> Thanks for the details. This looks more like the first case, and
> >>> probably can be addressed by improving likelihood of detecting the
> >>> read event.
> >>>
> >>> Could you please test if the patch below fixes the particular
> >>> issue you are seeing? It is somewhat unrelated, but it might be
> >>> a good enough solution (and is more or less equivalent to
> >>> checking r->pipeline).
> >>>
> >>>>> It would be interesting to see more details, such as tcpdump
> >>>>> and/or nginx debug logs, to find out what actually goes on here.
> >>>>
> >>>> The tcpdump and debug logs are too large to send in this mail list.
> >>>> I wonder if I can directly email it to you.
> >>>
> >>> Feel free to, my email should accept up to 100M messages.
> >>> Alternatively, a good solution might be to make the files
> >>> available for download and post a link here.
> >>>
> >>>>> Overall, given how apt uses pipelining, I tend to think that at
> >>>>> least (2) is unavoidable and can happen with certain sizes of the
> >>>>> responses.
> >>>>>
> >>>>> A good enough solution might be check for r->pipeline, which is
> >>>>> set by nginx as long as it reads a pipelined request. It might
> >>>>> not be enough though, since r->pipeline is only set for requests
> >>>>> seen by nginx as pipelined, which might not be true for the last
> >>>>> request.
> >>>>>
> >>>>> A more complete solution might be to introduce something like
> >>>>> c->pipeline flag and use lingering close if any pipelined requests
> >>>>> were seen on the connection.
> >>>
> >>> The following patch reworks handling of pipelined requests by
> >>> postponing them to the next event loop iteration. It is expected
> >>> make it more likely for nginx to know there are any additional
> >>> unread data in the socket buffer (and right now is mostly
> >>> equivalent to checking r->pipeline, since c->read->ready is always
> >>> set for pipelined requests):
> >>>
> >>> # HG changeset patch
> >>> # User Maxim Dounin <mdounin at mdounin.ru>
> >>> # Date 1674610218 -10800
> >>> # Wed Jan 25 04:30:18 2023 +0300
> >>> # Node ID 8cfd22c325a3db370b9e45aa6f897ff7bc8222f3
> >>> # Parent c7e103acb409f0352cb73997c053b3bdbb8dd5db
> >>> Reworked pipelined requests to use posted next events.
> >>>
> >>> This is expected to improve handling of pipelined requests in a number
> >>> of ways, including:
> >>>
> >>> 1) It will make a room for additional requests from other clients,
> >>> reducing worker monopolization by a single client.
> >>>
> >>> 2) The c->read->ready flag will be set, so nginx will either read the
> >>> additional data, or will use lingering close. This is expected to help
> >>> with clients using pipelining with some constant depth, such as apt[1][2].
> >>>
> >>> The ngx_event_move_posted_next() was modified to make it possible to
> >>> post read events on connections with kqueue. Previously, it used to
> >>> set ev->available to -1, potentially overwriting a valid positive value
> >>> provided by kqueue, so ngx_unix_recv() and ngx_readv_chain() will stop
> >>> reading from the socket before reading all the data available.
> >>>
> >>> Note that currently ngx_event_move_posted_next() will always set
> >>> the ev->ready flag. While this is expected behaviour for the ev->available
> >>> use case (where ev->ready is explicitly cleared), this is not needed for
> >>> pipelining. For pipelining, this will result in extra unneeded read()
> >>> syscall after processing of all pipelined requests, and there might be
> >>> a room for improvement here.
> >>>
> >>> [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=973861#10
> >>> [2] https://mailman.nginx.org/pipermail/nginx-devel/2023-January/ZA2SP5SJU55LHEBCJMFDB2AZVELRLTHI.html
> >>>
> >>> diff --git a/src/event/ngx_event_posted.c b/src/event/ngx_event_posted.c
> >>> --- a/src/event/ngx_event_posted.c
> >>> +++ b/src/event/ngx_event_posted.c
> >>> @@ -51,8 +51,10 @@ ngx_event_move_posted_next(ngx_cycle_t *
> >>> ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
> >>> "posted next event %p", ev);
> >>>
> >>> - ev->ready = 1;
> >>> - ev->available = -1;
> >>> + if (!ev->ready) {
> >>> + ev->ready = 1;
> >>> + ev->available = -1;
> >>> + }
> >>> }
> >>>
> >>> ngx_queue_add(&ngx_posted_events, &ngx_posted_next_events);
> >>> diff --git a/src/http/ngx_http_request.c b/src/http/ngx_http_request.c
> >>> --- a/src/http/ngx_http_request.c
> >>> +++ b/src/http/ngx_http_request.c
> >>> @@ -3129,7 +3129,7 @@ ngx_http_set_keepalive(ngx_http_request_
> >>> }
> >>>
> >>> rev->handler = ngx_http_process_request_line;
> >>> - ngx_post_event(rev, &ngx_posted_events);
> >>> + ngx_post_event(rev, &ngx_posted_next_events);
> >>> return;
> >>> }
> >>>
> >>
> >> I can confirm that the symptom disappears after applying this patch
> >
> > Thanks for testing, and thanks for the logs and traffic dumps
> > provided.
> >
> > From the logs you've provided it looks like the patch should help
> > with 1 out of 5 errors: the last request in the connection with
> > this error was pipelined, and therefore with the patch it is
> > expected to use lingering close.
> >
> > In 4 other cases the last requests are handled without pipelining:
> > once the request arrives, ngx_http_keepalive_handler() is called,
> > and nginx reads the request and responds to it, closing the
> > connection. The next request arrives at some time later, and the
> > OS responds with RST.
> >
> > E.g., the request to bzip2_1.0.8-4_amd64.deb (which corresponds to
> > the second error as seen by apt, while downloading
> > libpython3.9-stdlib_3.9.2-1_amd64.deb; connection *26, with client
> > port 40110) arrives at 14:08:36.495858, nginx responds to it, as
> > seen in nginx logs, and then closes the connection. The next
> > request arrives at 14:08:36.496977 and immediately responded with
> > RST.
> >
> > The patch does not change handling of these 4 cases, except may
> > be some mostly unrelated event handling changes, so the timing
> > might be slightly different. As such, I would expect the errors
> > to be still reproducible with the patch, but probably somewhat
> > less often.
> >
> > Could you please re-check to see if the symptoms still happen with
> > the patch, at least occasionally?
>
> You are totally right about that. I tested the previous patch
> for more times and can see the RST occasionally with less frequency.
Thanks for confirming this.
> > Overall, after looking into logs and tcpdump you've provided I
> > tend to think that the only working fix would be to introduce
> > c->pipeline flag, and force lingering close if there were any
> > pipelined requests on the connection.
> >
> > Below is the patch which implements this approach. Review and
> > testing appreciated. It can be used either separately or with the
> > previously provided patch to use posted next events.
>
> I've tested the new patch for several times and can confirm the
> problem is solved. Also, I added a new variable exposing the
> r->connection->pipeline and can confirm that the flag works
> as intended. The flag won't be set for normal keep-alive requests,
> and will only be set for pipelined requests.
>
> Thanks again for looking into this issue.
Thanks for testing.
--
Maxim Dounin
http://mdounin.ru/
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