From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753477AbeARCzq (ORCPT ); Wed, 17 Jan 2018 21:55:46 -0500 Received: from mail.kernel.org ([198.145.29.99]:58724 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752403AbeARCzp (ORCPT ); Wed, 17 Jan 2018 21:55:45 -0500 DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 8B74B2175B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=frederic@kernel.org Date: Thu, 18 Jan 2018 03:55:41 +0100 From: Frederic Weisbecker To: Linus Torvalds Cc: Mauro Carvalho Chehab , LKML , Levin Alexander , Peter Zijlstra , Hannes Frederic Sowa , "Paul E . McKenney" , Wanpeng Li , Dmitry Safonov , Thomas Gleixner , Eric Dumazet , Radu Rendec , Ingo Molnar , Stanislaw Gruszka , Paolo Abeni , Rik van Riel , Andrew Morton , David Miller Subject: Re: [RFC PATCH 0/5] softirq: Per vector threading v2 Message-ID: <20180118025539.GA20310@lerouge> References: <1516077640-19718-1-git-send-email-frederic@kernel.org> <20180117145620.213cb5ad@vento.lan> <20180117180713.GA17735@lerouge> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 17, 2018 at 03:56:43PM -0800, Linus Torvalds wrote: > On Wed, Jan 17, 2018 at 10:07 AM, Frederic Weisbecker > wrote: > > > > I see, so you may want to test (possibly much) higher values of MAX_SOFTIRQ_RESTART, > > such as 50 or 100. > > I suspect the "number of softiqs per jiffy" is hardly interesting at all. > > We used to allow up to 2mS or ten iterations per _invocation_, never > mind per timer tick. > > I thought you were going to actally account for time, but I don't > think you ended up doing that. I did in the first version but then I thought you suggested that count per jiffy. I probably misunderstood :) > > Maybe time isn't necessarily the thing to do, but just pure "count per > jiffy" seems very bad. Indeed, the more I think about it, the more doubts I have too. At least I started to think that this metric alone is not enough. > > What I might suggest using instead: > > - do it by time. This may be too expensive, though. Keeping track of > ns-level timing per invocation can be nasty. Yeah I would like to avoid that if we can. I guess it's ok if it sums up to rdtsc but I fear it's common to have a heavier version. > > - do it by "we got a new softirq event while handling another softirq > event". That was our old count per invocation, except you could do it > per softirq, and just allow *one* (ie keep a bitmask of "I've already > handled this softirq", and if the restart results in it being > triggered *again* you say "ok, I'll just move this to a workqueue" That one is very tempting. > > - .. something else? > > I'd suggest trying the "if we get a new softirq event that we've > already seen while we were already handling softirq events" thing. > That should really take care of the networking case of "90% time spend > in softirq handling during packet storms" thing. If we spend that much > time on softirqs, we *will* get a new softirq while handling an old > one occasionally. Ok I'm going to try that for the v3. Thanks.