From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by passt.top (Postfix) with ESMTP id B2DE65A026F for ; Wed, 27 Sep 2023 19:05:56 +0200 (CEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1695834355; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9PcHeNz0nRDTGmsXQn3U6XQJ0se0PDccrdnXdEHMMwE=; b=HegYGl5CTx38ONRlDrdvXLQ9VK6jp4Ud05UWsDDp5IEecRHcGcvhOhynSXUzCPupBHRBcq D9apzL8DvlNsGMawfMe0T9tvldPETPRjLHs8+uKKzep0Rcg8zSod4WkFl7Dj2ftL+mZPqS Fi14mlGeD2Tnu6gRlTIXUFz10p9AARA= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-657-Jys5GBhbP6W3-ej_favsbw-1; Wed, 27 Sep 2023 13:05:54 -0400 X-MC-Unique: Jys5GBhbP6W3-ej_favsbw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 997C21C06E07; Wed, 27 Sep 2023 17:05:53 +0000 (UTC) Received: from elisabeth (unknown [10.39.208.37]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8211E170EC; Wed, 27 Sep 2023 17:05:52 +0000 (UTC) Date: Wed, 27 Sep 2023 19:05:50 +0200 From: Stefano Brivio To: David Gibson Subject: Re: [PATCH RFT 3/5] tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag Message-ID: <20230927190550.65cebdce@elisabeth> In-Reply-To: References: <20230922220610.58767-1-sbrivio@redhat.com> <20230922220610.58767-4-sbrivio@redhat.com> Organization: Red Hat MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Message-ID-Hash: 5VC53YNTJYECVY5FVGHAYALQTAHKAZ27 X-Message-ID-Hash: 5VC53YNTJYECVY5FVGHAYALQTAHKAZ27 X-MailFrom: sbrivio@redhat.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: Matej Hrica , passt-dev@passt.top X-Mailman-Version: 3.3.8 Precedence: list List-Id: Development discussion and patches for passt Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On Mon, 25 Sep 2023 14:21:47 +1000 David Gibson wrote: > On Mon, Sep 25, 2023 at 02:09:41PM +1000, David Gibson wrote: > > On Sat, Sep 23, 2023 at 12:06:08AM +0200, Stefano Brivio wrote: > > > It looks like we need it as workaround for this situation, readily > > > reproducible at least with a 6.5 Linux kernel, with default rmem_max > > > and wmem_max values: > > > > > > - an iperf3 client on the host sends about 160 KiB, typically > > > segmented into five frames by passt. We read this data using > > > MSG_PEEK > > > > > > - the iperf3 server on the guest starts receiving > > > > > > - meanwhile, the host kernel advertised a zero-sized window to the > > > receiver, as expected > > > > > > - eventually, the guest acknowledges all the data sent so far, and > > > we drop it from the buffer, courtesy of tcp_sock_consume(), using > > > recv() with MSG_TRUNC > > > > > > - the client, however, doesn't get an updated window value, and > > > even keepalive packets are answered with zero-window segments, > > > until the connection is closed > > > > > > It looks like dropping data from a socket using MSG_TRUNC doesn't > > > cause a recalculation of the window, which would be expected as a > > > result of any receiving operation that invalidates data on a buffer > > > (that is, not with MSG_PEEK). > > > > > > Strangely enough, setting TCP_WINDOW_CLAMP via setsockopt(), even to > > > the previous value we clamped to, forces a recalculation of the > > > window which is advertised to the guest. > > > > > > I couldn't quite confirm this issue by following all the possible > > > code paths in the kernel, yet. If confirmed, this should be fixed in > > > the kernel, but meanwhile this workaround looks robust to me (and it > > > will be needed for backward compatibility anyway). > > > > So, I tested this, and things got a bit complicated. > > > > First, I reproduced the "read side" problem by setting > > net.core.rmem_max to 256kiB while setting net.core.wmem_max to 16MiB. > > The "160kiB" stall happened almost every time. Applying this patch > > appears to fix it completely, getting GiB/s throughput consistently. > > So, yah. > > > > Then I tried reproducing it differently: by setting both > > net.core.rmem_max and net.core.wmem_max to 16MiB, but setting > > SO_RCVBUF to 128kiB explicitly in tcp_sock_set_bufsize() (which > > actually results in a 256kiB buffer, because of the kernel's weird > > interpretation). > > > > With the SO_RCVBUF clamp and without this patch, I don't get the > > 160kiB stall consistently any more. What I *do* get is nearly every > > time - but not *every* time - is slow transfers, ~40Mbps vs. ~12Gbps. > > Sometimes it stalls after several seconds. The stall is slightly > > different from the 160kiB stall though: the 160kiB stall seems 0 bytes > > transferred on both sides. With the RCVBUF stall I get a trickle of > > bytes (620 bytes/s) on the receiver/guest side, with mostly 0 bytes > > per interval on the sender but occasionally an interval with several > > hundred KB. > > > > That is it seems like there's a buffer somewhere that's very slowly > > draining into the receiver, then getting topped up in an instant once > > it gets low enough. > > > > When I have both this patch and the RCVBUF clamp, I don't seem to be > > able to reproduce the trickle-stall anymore, but I still get the slow > > transfer speeds most, but not every time. Sometimes, but only rarely, > > I do seem to still get a complete stall (0 bytes on both sides). > > I noted another oddity. With this patch, _no_ RCVBUF clamp and 16MiB > wmem_max fixed, things seem to behave much better with a small > rmem_max than large. With rmem_max=256KiB I get pretty consistent > 37Gbps throughput and iperf3 -c reports 0 retransmits. > > With rmem_max=16MiB, the throughput fluctuates from second to second > between ~3Gbps and ~30Gbps. The client reports retransmits in some > intervals, which is pretty weird over lo. > > Urgh... so many variables. This is probably due to the receive buffer getting bigger than TCP_FRAMES_MEM * MSS4 (or MSS6), so the amount of data we can read in one shot from the sockets isn't optimally sized anymore. We should have a look at the difference between not clamping at all (and if that yields the same throughput, great), and clamping to, I guess, TCP_FRAMES_MEM * MIN(MSS4, MSS6). -- Stefano