From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 39CDC111AF for ; Tue, 15 Aug 2023 12:08:47 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EDA08C433C7; Tue, 15 Aug 2023 12:08:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1692101327; bh=2I9zTsaTUj2cvyrEYEqTG4+Hu3ZlC/26ibNOomQI9tc=; h=Date:Cc:Subject:To:References:From:In-Reply-To:From; b=hYSx9CNl3kWclsqirsoWwQj7isJ+V3pCYqY17Cu0xyW/8/HNiwbnIAMHpdoAhnhf1 9Pn5qNCUHgHdAF1R7caEJ7tI0j/FUrw/zZqQKBjBd4lNo/ZUKR286hD1pMjP+jNsni hBdTWOqMtlZVsW6tG4HGxlQyXrEJQ4kmuabjzvEgKIdcCpwjpGDGoUhvsd6yKKxIqt 0RosjB5x6bnbbEBCIKu8RcE73qVUw7RqvER/4IApS75iK8dyMs6D/GcaltF8uSxtJ3 aw0VvEJlD/4EI3OcwO9yLhtl+NldoZY/KlDO63ibud3Fne14YoIbKk8KDmLKMWBv4r wiLxGF+k86Hzw== Message-ID: <25de7655-6084-e6b9-1af6-c47b3d3b7dc1@kernel.org> Date: Tue, 15 Aug 2023 14:08:42 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.13.0 Cc: hawk@kernel.org, "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Peter Zijlstra , Thomas Gleixner , Wander Lairson Costa , kernel-team , Yan Zhai Subject: Re: [RFC PATCH 2/2] softirq: Drop the warning from do_softirq_post_smp_call_flush(). Content-Language: en-US To: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, netdev@vger.kernel.org References: <20230814093528.117342-1-bigeasy@linutronix.de> <20230814093528.117342-3-bigeasy@linutronix.de> From: Jesper Dangaard Brouer In-Reply-To: <20230814093528.117342-3-bigeasy@linutronix.de> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 14/08/2023 11.35, Sebastian Andrzej Siewior wrote: > This is an undesired situation and it has been attempted to avoid the > situation in which ksoftirqd becomes scheduled. This changed since > commit d15121be74856 ("Revert "softirq: Let ksoftirqd do its job"") > and now a threaded interrupt handler will handle soft interrupts at its > end even if ksoftirqd is pending. That means that they will be processed > in the context in which they were raised. $ git describe --contains d15121be74856 v6.5-rc1~232^2~4 That revert basically removes the "overload" protection that was added to cope with DDoS situations in Aug 2016 (Cc. Cloudflare). As described in https://git.kernel.org/torvalds/c/4cd13c21b207 ("softirq: Let ksoftirqd do its job") in UDP overload situations when UDP socket receiver runs on same CPU as ksoftirqd it "falls-off-an-edge" and almost doesn't process packets (because softirq steals CPU/sched time from UDP pid). Warning Cloudflare (Cc) as this might affect their production use-cases, and I recommend getting involved to evaluate the effect of these changes. I do realize/acknowledge that the reverted patch caused other latency issues, given it was a "big-hammer" approach affecting other softirq processing (as can be seen by e.g. the watchdog fixes patches). Thus, the revert makes sense, but how to regain the "overload" protection such that RX networking cannot starve processes reading from the socket? (is this what Sebastian's patchset does?) --Jesper Thread link for people Cc'ed: https://lore.kernel.org/all/20230814093528.117342-1-bigeasy@linutronix.de/#r