From patchwork Wed Oct 25 09:38:47 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Zeng Heng X-Patchwork-Id: 13435815 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 469B7C25B6B for ; Wed, 25 Oct 2023 09:34:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-ID:Date:Subject:CC:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=B2rov0zTiqZXW+TUm9kVH4In7Hy27HblWmsrnjq0gU4=; b=PFmEyV0rGHf3gq eHSkMhhLYVhA1fR1KBPLE4CDz3m3eeCR3LEKOA2P75+Ytv1IFnNbTTJ8XvFNvZ8xz2gfFNdFKDTnY UitKHtKO3E+jUoo21UFymfLbo71GiIGQvdFFA9ft9hA9lcxg+gOqM5tqE5LTjofite9ksnN2BuP/S EcTZ1N2q4aLE+qU4xAtRY5oGCMMcOzQ1fSlBMfKs2rum1ujDKTyD4quo763Ig1JG9XGBLCXp1Tri5 fNAo5rWUE0YFSgB0P0aq1qAbZBxRm5sHSwOSk8VXRbUi0Te1gVILohavFP174gPXtoIuFEOw7rpvF tLpwMW9+ldmrNPt5gfow==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1qvaHF-00BpiE-2w; Wed, 25 Oct 2023 09:34:13 +0000 Received: from szxga08-in.huawei.com ([45.249.212.255]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1qvaGs-00BpWk-28 for linux-arm-kernel@lists.infradead.org; Wed, 25 Oct 2023 09:33:52 +0000 Received: from kwepemi500024.china.huawei.com (unknown [172.30.72.57]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4SFkF60pgzz15NlK; Wed, 25 Oct 2023 17:30:50 +0800 (CST) Received: from huawei.com (10.175.103.91) by kwepemi500024.china.huawei.com (7.221.188.100) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.31; Wed, 25 Oct 2023 17:33:40 +0800 From: Zeng Heng To: , , , , , , , , , , , CC: , , , , Subject: [PATCH 3/3] cpufreq: CPPC: Eliminate the impact of cpc_read() latency error Date: Wed, 25 Oct 2023 17:38:47 +0800 Message-ID: <20231025093847.3740104-4-zengheng4@huawei.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20231025093847.3740104-1-zengheng4@huawei.com> References: <20231025093847.3740104-1-zengheng4@huawei.com> MIME-Version: 1.0 X-Originating-IP: [10.175.103.91] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To kwepemi500024.china.huawei.com (7.221.188.100) X-CFilter-Loop: Reflected X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231025_023351_074001_4C02A0F7 X-CRM114-Status: GOOD ( 14.83 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org We have found significant differences in the latency of cpc_read() between regular scenarios and scenarios with high memory access pressure. Ignoring this error can result in getting rate interface occasionally returning absurd values. Here provides a high memory access sample test by stress-ng. My local testing platform includes 160 CPUs, the CPC registers is accessed by mmio method, and the cpuidle feature is disabled (the AMU always works online): ~~~ ./stress-ng --memrate 160 --timeout 180 ~~~ The following data is sourced from ftrace statistics towards cppc_get_perf_ctrs(): Regular scenarios || High memory access pressure scenarios 104) | cppc_get_perf_ctrs() { || 133) | cppc_get_perf_ctrs() { 104) 0.800 us | cpc_read.isra.0(); || 133) 4.580 us | cpc_read.isra.0(); 104) 0.640 us | cpc_read.isra.0(); || 133) 7.780 us | cpc_read.isra.0(); 104) 0.450 us | cpc_read.isra.0(); || 133) 2.550 us | cpc_read.isra.0(); 104) 0.430 us | cpc_read.isra.0(); || 133) 0.570 us | cpc_read.isra.0(); 104) 4.610 us | } || 133) ! 157.610 us | } 104) | cppc_get_perf_ctrs() { || 133) | cppc_get_perf_ctrs() { 104) 0.720 us | cpc_read.isra.0(); || 133) 0.760 us | cpc_read.isra.0(); 104) 0.720 us | cpc_read.isra.0(); || 133) 4.480 us | cpc_read.isra.0(); 104) 0.510 us | cpc_read.isra.0(); || 133) 0.520 us | cpc_read.isra.0(); 104) 0.500 us | cpc_read.isra.0(); || 133) + 10.100 us | cpc_read.isra.0(); 104) 3.460 us | } || 133) ! 120.850 us | } 108) | cppc_get_perf_ctrs() { || 87) | cppc_get_perf_ctrs() { 108) 0.820 us | cpc_read.isra.0(); || 87) ! 255.200 us | cpc_read.isra.0(); 108) 0.850 us | cpc_read.isra.0(); || 87) 2.910 us | cpc_read.isra.0(); 108) 0.590 us | cpc_read.isra.0(); || 87) 5.160 us | cpc_read.isra.0(); 108) 0.610 us | cpc_read.isra.0(); || 87) 4.340 us | cpc_read.isra.0(); 108) 5.080 us | } || 87) ! 315.790 us | } 108) | cppc_get_perf_ctrs() { || 87) | cppc_get_perf_ctrs() { 108) 0.630 us | cpc_read.isra.0(); || 87) 0.800 us | cpc_read.isra.0(); 108) 0.630 us | cpc_read.isra.0(); || 87) 6.310 us | cpc_read.isra.0(); 108) 0.420 us | cpc_read.isra.0(); || 87) 1.190 us | cpc_read.isra.0(); 108) 0.430 us | cpc_read.isra.0(); || 87) + 11.620 us | cpc_read.isra.0(); 108) 3.780 us | } || 87) ! 207.010 us | } My local testing platform works under 3000000hz, but the cpuinfo_cur_freq interface returns values that are not even close to the actual frequency: [root@localhost ~]# cd /sys/devices/system/cpu [root@localhost cpu]# for i in {0..159}; do cat cpu$i/cpufreq/cpuinfo_cur_freq; done 5127812 2952127 3069001 3496183 922989768 2419194 3427042 2331869 3594611 8238499 ... The reason is when under heavy memory access pressure, the execution of cpc_read() delay has increased from sub-microsecond to several hundred microseconds. Moving the cpc_read function into a critical section by irq disable/enable has minimal impact on the result. cppc_get_perf_ctrs()[0] cppc_get_perf_ctrs()[1] / \ / \ cpc_read cpc_read cpc_read cpc_read ref[0] delivered[0] ref[1] delivered[1] | | | | v v v v -----------------------------------------------------------------------> time <--delta[0]--> <------sample_period------> <-----delta[1]-----> Since that, freq = ref_freq * (delivered[1] - delivered[0]) / (ref[1] - ref[0]) and delivered[1] - delivered[0] = freq * (delta[1] + sample_period), ref[1] - ref[0] = ref_freq * (delta[0] + sample_period) To eliminate the impact of system memory access latency, setting a sampling period of 2us is far from sufficient. Consequently, we suggest cppc_cpufreq_get_rate() only can be called in the process context, and adopt a longer sampling period to neutralize the impact of random latency. Here we call the cond_resched() function instead of sleep-like functions to ensure that `taskset -c $i cat cpu$i/cpufreq/cpuinfo_cur_freq` could work when cpuidle feature is enabled. Reported-by: Yang Shi Link: https://lore.kernel.org/all/20230328193846.8757-1-yang@os.amperecomputing.com/ Signed-off-by: Zeng Heng --- drivers/cpufreq/cppc_cpufreq.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c index 321a9dc9484d..a7c5418bcda7 100644 --- a/drivers/cpufreq/cppc_cpufreq.c +++ b/drivers/cpufreq/cppc_cpufreq.c @@ -851,12 +851,26 @@ static int cppc_get_perf_ctrs_pair(void *val) struct fb_ctr_pair *fb_ctrs = val; int cpu = fb_ctrs->cpu; int ret; + unsigned long timeout; ret = cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t0); if (ret) return ret; - udelay(2); /* 2usec delay between sampling */ + if (likely(!irqs_disabled())) { + /* + * Set 1ms as sampling interval, but never schedule + * to the idle task to prevent the AMU counters from + * stopping working. + */ + timeout = jiffies + msecs_to_jiffies(1); + while (!time_after(jiffies, timeout)) + cond_resched(); + + } else { + pr_warn_once("CPU%d: Get rate in atomic context", cpu); + udelay(2); /* 2usec delay between sampling */ + } return cppc_get_perf_ctrs(cpu, &fb_ctrs->fb_ctrs_t1); }