Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753710AbaLARPo (ORCPT ); Mon, 1 Dec 2014 12:15:44 -0500 Received: from www.linutronix.de ([62.245.132.108]:44184 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753015AbaLARPm (ORCPT ); Mon, 1 Dec 2014 12:15:42 -0500 Date: Mon, 1 Dec 2014 18:15:36 +0100 (CET) From: Thomas Gleixner To: Martin Schwidefsky cc: Frederic Weisbecker , LKML , Tony Luck , Peter Zijlstra , Heiko Carstens , Benjamin Herrenschmidt , Oleg Nesterov , Paul Mackerras , Wu Fengguang , Ingo Molnar , Rik van Riel Subject: Re: [RFC PATCH 07/30] cputime: Convert kcpustat to nsecs In-Reply-To: <20141201174842.648dfe06@mschwide> Message-ID: References: <1417199040-21044-1-git-send-email-fweisbec@gmail.com> <1417199040-21044-8-git-send-email-fweisbec@gmail.com> <20141201151402.31a6cc9a@mschwide> <20141201161031.GA27302@lerouge> <20141201174842.648dfe06@mschwide> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 1 Dec 2014, Martin Schwidefsky wrote: > On Mon, 1 Dec 2014 17:10:34 +0100 > Frederic Weisbecker wrote: > > > Speaking about the degradation in s390: > > > > s390 is really a special case. And it would be a shame if we prevent from a > > real core cleanup just for this special case especially as it's fairly possible > > to keep a specific treatment for s390 in order not to impact its performances > > and time precision. We could simply accumulate the cputime in per-cpu values: > > > > struct s390_cputime { > > cputime_t user, sys, softirq, hardirq, steal; > > } > > > > DEFINE_PER_CPU(struct s390_cputime, s390_cputime); > > > > Then on irq entry/exit, just add the accumulated time to the relevant buffer > > and account for real (through any account_...time() functions) only on tick > > and task switch. There the costly operations (unit conversion and call to > > account_...._time() functions) are deferred to a rarer yet periodic enough > > event. This is what s390 does already for user/system time and kernel > > boundaries. > > > > This way we should even improve the situation compared to what we have > > upstream. It's going to be faster because calling the accounting functions > > can be costlier than simple per-cpu ops. And also we keep the cputime_t > > granularity. For archs like s390 which have a granularity higher than nsecs, > > we can have: > > > > u64 cputime_to_nsecs(cputime_t time, u64 *rem); > > > > And to avoid remainder losses, we can do that from the tick: > > > > delta_cputime = this_cpu_read(s390_cputime.hardirq); > > delta_nsec = cputime_to_nsecs(delta_cputime, &rem); > > account_system_time(delta_nsec, HARDIRQ_OFFSET); > > this_cpu_write(s390_cputime.hardirq, rem); > > > > Although I doubt that remainders below one nsec lost each tick matter that much. > > But if it does, it's fairly possible to handle like above. > > To make that work we would have to move some of the logic from account_system_time > to the architecture code. The decision if a system time delta is guest time, > irq time, softirq time or simply system time is currently done in > kernel/sched/cputime.c. > > As the conversion + the accounting is delayed to a regular tick we would have > to split the accounting code into decision functions which bucket a system time > delta should go to and introduce new function to account to the different buckets. > > Instead of a single account_system_time we would have account_guest_time, > account_system_time, account_system_time_irq and account_system_time_softirq. > > In principle not a bad idea, that would make the interrupt path for s390 faster > as we would not have to call account_system_time, only the decision function > which could be an inline function. Why make this s390 specific? We can decouple the accounting from the time accumulation for all architectures. struct cputime_record { u64 user, sys, softirq, hardirq, steal; }; DEFINE_PER_CPU(struct cputime_record, cputime_record); Now let account_xxx_time() just work on that per cpu data structures. That would just accumulate the deltas based on whatever the architecture uses as a cputime source with whatever resolution it provides. Then we collect that accumulated results for the various buckets on a regular base and convert them to nano seconds. This is not even required to be at the tick, it could be done by some async worker and on idle enter/exit. Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/