[PATCH] HTTP: trigger lingering close when keepalive connection will be closed
Miao Wang
shankerwangmiao at gmail.com
Fri Jan 27 12:11:34 UTC 2023
Hi,
> 2023年1月27日 12:01，Maxim Dounin <mdounin at mdounin.ru> 写道：
>
> Hello!
>
> 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.
>
> 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.
>
> # HG changeset patch
> # User Maxim Dounin <mdounin at mdounin.ru>
> # Date 1674790916 -10800
> # Fri Jan 27 06:41:56 2023 +0300
> # Node ID 784d0fa0b5a0796561642a5a64dc4e9e07592852
> # Parent 4eb1383f6432b034630e6de18739b817f6565c8c
> Lingering close for connections with pipelined requests.
>
> This is expected to help with clients using pipelining with some constant
> depth, such as apt[1][2].
>
> When downloading many resources, apt uses pipelining with some constant
> depth, a number of requests in flight[1][2]. This essentially means that
> after receiving a response it sends an additional request to the server,
> and this can result in requests arriving to the server at any time. Further,
> additional requests are sent one-by-one, and can be easily seen as such
> (neither as pipelined, nor followed by pipelined requests).
>
> The only safe approach to close such connections (for example, when
> keepalive_requests is reached) is with lingering. To do so, now nginx
> monitors if pipelining was used on the connection, and if it was, closes
> the connection with lingering.
>
> [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/core/ngx_connection.h b/src/core/ngx_connection.h
> --- a/src/core/ngx_connection.h
> +++ b/src/core/ngx_connection.h
> @@ -172,6 +172,7 @@ struct ngx_connection_s {
> unsigned timedout:1;
> unsigned error:1;
> unsigned destroyed:1;
> + unsigned pipeline:1;
>
> unsigned idle:1;
> unsigned reusable:1;
> 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
> @@ -2753,7 +2753,8 @@ ngx_http_finalize_connection(ngx_http_re
> || (clcf->lingering_close == NGX_HTTP_LINGERING_ON
> && (r->lingering_close
> || r->header_in->pos < r->header_in->last
> - || r->connection->read->ready)))
> + || r->connection->read->ready
> + || r->connection->pipeline)))
> {
> ngx_http_set_lingering_close(r->connection);
> return;
> @@ -3123,6 +3124,7 @@ ngx_http_set_keepalive(ngx_http_request_
>
> c->sent = 0;
> c->destroyed = 0;
> + c->pipeline = 1;
>
> if (rev->timer_set) {
> ngx_del_timer(rev);
>
> --
> Maxim Dounin
> http://mdounin.ru/
Cheers,
Miao Wang
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