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[2001:1c00:c0c:fe00:7e79:4dac:39d0:9c14]) by smtp.gmail.com with ESMTPSA id j12sm62744701wrw.54.2020.01.22.08.57.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 22 Jan 2020 08:57:55 -0800 (PST) Subject: Re: [PATCH v1 1/3] rtc: cmos: Stop using shared IRQ To: Andy Shevchenko , Alessandro Zummo , Alexandre Belloni , linux-rtc@vger.kernel.org, "Guilherme G. Piccoli" References: <20200122144529.30307-1-andriy.shevchenko@linux.intel.com> From: Hans de Goede Message-ID: Date: Wed, 22 Jan 2020 17:57:53 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200122144529.30307-1-andriy.shevchenko@linux.intel.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-rtc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rtc@vger.kernel.org Hi, On 22-01-2020 15:45, Andy Shevchenko wrote: > As reported by Guilherme G. Piccoli: > > --- 8< --- 8< --- > > The rtc-cmos interrupt setting was changed in the commit 079062b28fb4 > ("rtc: cmos: prevent kernel warning on IRQ flags mismatch") in order > to allow shared interrupts; according to that commit's description, > some machine got kernel warnings due to the interrupt line being shared > between rtc-cmos and other hardware, and rtc-cmos didn't allow IRQ sharing > that time. > > After the aforementioned commit though it was observed a huge increase > in lost HPET interrupts in some systems, observed through the following > kernel message: > > [...] hpet1: lost 35 rtc interrupts > > After investigation, it was narrowed down to the shared interrupts > usage when having the kernel option "irqpoll" enabled. In this case, > all IRQ handlers are called for non-timer interrupts, if such handlers > are setup in shared IRQ lines. The rtc-cmos IRQ handler could be set to > hpet_rtc_interrupt(), which will produce the kernel "lost interrupts" > message after doing work - lots of readl/writel to HPET registers, which > are known to be slow. > > Although "irqpoll" is not a default kernel option, it's used in some contexts, > one being the kdump kernel (which is an already "impaired" kernel usually > running with 1 CPU available), so the performance burden could be considerable. > Also, the same issue would happen (in a shorter extent though) when using > "irqfixup" kernel option. > > In a quick experiment, a virtual machine with uptime of 2 minutes produced >> 300 calls to hpet_rtc_interrupt() when "irqpoll" was set, whereas without > sharing interrupts this number reduced to 1 interrupt. Machines with more > hardware than a VM should generate even more unnecessary HPET interrupts > in this scenario. > > --- 8< --- 8< --- > > After looking into the rtc-cmos driver history and DSDT table from > the Microsoft Surface 3, we may notice that Hans de Goede submitted > a correct fix (see dependency below). Thus, we simply revert > the culprit commit. > > Fixes: 079062b28fb4 ("rtc: cmos: prevent kernel warning on IRQ flags mismatch") > Depends-on: a1e23a42f1bd ("rtc: cmos: Do not assume irq 8 for rtc when there are no legacy irqs") > Reported-by: Guilherme G. Piccoli > Cc: Hans de Goede > Signed-off-by: Andy Shevchenko Entire series looks good to me: Signed-off-by: Hans de Goede Regards, Hans > --- > drivers/rtc/rtc-cmos.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c > index 033303708c8b..cb28bbdc9e17 100644 > --- a/drivers/rtc/rtc-cmos.c > +++ b/drivers/rtc/rtc-cmos.c > @@ -850,7 +850,7 @@ cmos_do_probe(struct device *dev, struct resource *ports, int rtc_irq) > rtc_cmos_int_handler = cmos_interrupt; > > retval = request_irq(rtc_irq, rtc_cmos_int_handler, > - IRQF_SHARED, dev_name(&cmos_rtc.rtc->dev), > + 0, dev_name(&cmos_rtc.rtc->dev), > cmos_rtc.rtc); > if (retval < 0) { > dev_dbg(dev, "IRQ %d is already in use\n", rtc_irq); >