Messages in this thread |  | | From | Ankit Jain <> | | Date | Tue, 28 Apr 2026 14:17:31 +0530 | | Subject | Re: [PATCH net] tcp: do not shrink window clamp when SO_RCVBUF is locked |
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On Mon, Apr 27, 2026 at 9:08 PM Eric Dumazet <edumazet@google.com> wrote: > > On Mon, Apr 27, 2026 at 8:32 AM Ankit Jain <ankit-aj.jain@broadcom.com> wrote: > > > > When an application explicitly sets SO_RCVBUF, the window clamp should > > not be dynamically recalculated based on the memory scaling_ratio. > > > > Currently, tcp_measure_rcv_mss() aggressively crushes the window clamp > > down when it sees a poor skb->len to skb->truesize ratio. If the > > application explicitly locked the buffer via SO_RCVBUF, this > > recalculation causes the advertised window to drop severely. > > > > If the window drops below the interface MSS, it triggers Silly Window > > Syndrome (SWS) avoidance on the sender. The sender defers transmission > > and drops the connection into a perpetual 200ms PROBE0 timer loop, > > drastically reducing throughput. > > > > This is highly reproducible on loopback interfaces (MTU 65536) using > > Java-based workloads (like Tomcat/GemFire) where the JVM sets SO_RCVBUF > > to 32K or 64K. The bloated loopback truesize forces the scaling ratio > > to drop, crushing the window clamp to ~26K, instantly triggering SWS > > stalls and causing gigabyte transfers to take minutes instead of > > milliseconds. > > > > Since the application locked the buffer, the kernel should respect the > > clamp boundary and not dynamically crush it based on runtime ratios. > > > > Fixes: a2cbb1603943 ("tcp: Update window clamping condition") > > Cc: stable@vger.kernel.org > > Reported-by: Karen Badiryan <karen.badiryan@broadcom.com> > > Signed-off-by: Ankit Jain <ankit-aj.jain@broadcom.com> > > Make sure to add a selftests (in ./tools/testing/selftests/net/packetdrill/ ) > > Thanks.
Hi Eric,
Thank you for the review. I will formalize the packetdrill sequence from the commit notes into a proper runnable .pkt selftest. I will include it as the second patch in the upcoming v2 series.
Thanks, Ankit [unhandled content-type:application/pkcs7-signature] |  |