Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934161AbbFPJdk (ORCPT ); Tue, 16 Jun 2015 05:33:40 -0400 Received: from smtprelay4.synopsys.com ([198.182.47.9]:60803 "EHLO smtprelay.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934144AbbFPJdX convert rfc822-to-8bit (ORCPT ); Tue, 16 Jun 2015 05:33:23 -0400 From: Vineet Gupta To: Peter Zijlstra CC: "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "arnd@arndb.de" , Alexey Brodkin , "arc-linux-dev@synopsys.com" , Arnaldo Carvalho de Melo Subject: Re: [PATCH 4/8] ARCv2: perf: Support sampling events using overflow interrupts Thread-Topic: [PATCH 4/8] ARCv2: perf: Support sampling events using overflow interrupts Thread-Index: AQHQp4fiDnM4TPTb7EWe9OZ6KRgZfg== Date: Tue, 16 Jun 2015 09:33:06 +0000 Message-ID: References: <1433852372-29494-1-git-send-email-vgupta@synopsys.com> <1433852372-29494-5-git-send-email-vgupta@synopsys.com> <20150615162525.GH3644@twins.programming.kicks-ass.net> <20150616090713.GJ3644@twins.programming.kicks-ass.net> Accept-Language: en-US, en-IN Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.12.197.3] Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3239 Lines: 74 On Tuesday 16 June 2015 02:37 PM, Peter Zijlstra wrote: On Tue, Jun 16, 2015 at 05:37:40AM +0000, Vineet Gupta wrote: > On Monday 15 June 2015 09:55 PM, Peter Zijlstra wrote: > > On Tue, Jun 09, 2015 at 05:49:28PM +0530, Vineet Gupta wrote: > >> + if (arc_pmu->has_interrupts) { > >> + int irq = platform_get_irq(pdev, 0); > > Hmm, so you're requesting a regular interrupt. > > > > I see your architecture has IRQ priorities, could you play games and > > create NMIs using those? > > > > For example, never mask L1 (assuming that's the highest priority) and > > treat that as an NMI. > > I've had this idea before, however, while ARCv2 provides hardware interrupt > priorities, we really can't implement true NMI, because CLRI / SETI used at > backend of loal_irq_save() / restore() impact all priorities (statsu32 register > has a global enable interrupt bit which these wiggle). So e.g. a > spin_lock_irqsave() will lock out even the perf interrupt. Hmm, bugger. I (of course) only looked at the kernel source, since that is all I have, and the current arch/arc/ frobs with those two En bits in status32. Yeah the new intc and handling is series #B currently floated on lkml - we are discussing #C here - perf sub-series. So arcv2 changed all that, shame. Not really - arcv2 intc priorities (0-15, kind of excessive for linux usecase) is a logical extension of fixed 2 levels provided by arcompact (L2 high, L1 normal). Instead of 2 bits each of enable/active we have multiple now - spread over a couple of registers (statsu32.EN, irq_priority....). arcv2 core interrupt handling is loads better than what we had before - specially with hardware stack switching / auto reg file save etc. Plus now hardware enforces strict priority. In arcompact, the prio enable is semi-loosely enforced. L2 (high prio) can obviously interrupt L1 (low prio). But the other way around is possible too - if E1 is set - hence the beautiful code in arch/arc/kernel/irq.c : arch_local_irq_enable() Technically we could also do something here, not use SETI / CLRI for disabling / enabling interrupts, but play with allowed intr level etc - but I think the cost of writing to interrupt priority/enable registers that often will be prohibitive at best. > OTOH, we can improve the perf isr path a bit - by not routing it thru regular > interrupt return path (song and dance of CONFIG_PREEMPT_IRQ and possible > preemption). Plus there's a bit more we can do in the isr itself - not looping > thru 32 counters etc using ffs() etc - but I'd rather do that as separate series, > once we have the core support in. Yeah, borderline useful though, the reason the NMI thing is so useful is that you can profile _inside_ IRQ-disabled regions. Now your local_irq_enable() et al will be the hottest functions ever :-) Regular interrupts are really only useful for userspace profiling, which if plenty useful on its own of course. But kernel profiling is very handy too :-) Totally agree ! -- 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/