Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753961AbaLAR1t (ORCPT ); Mon, 1 Dec 2014 12:27:49 -0500 Received: from e06smtp15.uk.ibm.com ([195.75.94.111]:55187 "EHLO e06smtp15.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753390AbaLAR1q (ORCPT ); Mon, 1 Dec 2014 12:27:46 -0500 Date: Mon, 1 Dec 2014 18:27:38 +0100 From: Martin Schwidefsky To: Thomas Gleixner 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 Message-ID: <20141201182738.2b344a18@mschwide> In-Reply-To: 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> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14120117-0021-0000-0000-000001FA6F27 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 1 Dec 2014 18:15:36 +0100 (CET) Thomas Gleixner wrote: > 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. And leave the decision making in kernel/sched/cputime.c. Yes, that is good. This would make the arch and the account_xxx_time() function care about cputime_t and all other common code would use nano-seconds. With the added benefit that I do not have to change the low level code too much ;-) -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- 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/