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McKenney" , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Minsu Jeon , Jonghyeon Kim , Siwan Kim , Gyutae Bae Subject: Backport request: Revert "softirq: Let ksoftirqd do its job" (d15121be7485) Date: Thu, 29 Oct 2026 14:42:00 +0900 Message-Id: <20261029054200.97234-1-gyutae.opensource@navercorp.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, Please backport the following upstream commit to 6.1.y and 5.15.y: d15121be7485655129101f3960ae6add40204463 Revert "softirq: Let ksoftirqd do its job" On a production 5.15 PREEMPT_NONE node, long-running overlayfs getdents64() calls prevented ksoftirqd from running. RPS IPIs continued to arrive, but NET_RX processing on that CPU stalled for up to about 3 seconds. Over 15 days, the node logged about 1,000 bonding ARP monitor failovers, with no link-state changes recorded during that period. The failovers stopped after the workload was removed. On a 5.15.27 PREEMPT_NONE test node, we used a livepatch to make ksoftirqd_running() always return false. This removes the same softirq deferral checks as the revert on this build. Under the same load, we measured three 10-minute windows: before patching, with the livepatch active, and after removing it. The number of HTTP requests taking more than 200 ms was 17, 0 and 11, respectively. TCPTimeouts increased by 25, 1 and 21. We verified with patch --dry-run that the revert applies cleanly to v6.1.190 and v5.15.223. Both already contain the related RCU fix 1dd1eff161bd. 6.6.y and later already contain the revert. The original commit was intended to prevent user-task starvation under a softirq flood. The revert's changelog points to threaded NAPI as an alternative, but it is disabled by default. The revert was merged in v6.5 without a Cc: stable tag. Syzbot has also reported bisections to this revert [1]. Paolo, Thomas, Eric: I would appreciate your thoughts on whether this revert is suitable for 6.1.y and 5.15.y. [1] https://lore.kernel.org/all/000000000000a249880603717c15@google.com/ Thanks, Gyutae Bae