From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:53826 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727246AbfAGQHt (ORCPT ); Mon, 7 Jan 2019 11:07:49 -0500 Subject: Re: [PATCH 2/2] /proc/stat: Add sysctl parameter to control irq counts latency To: Matthew Wilcox Cc: Andrew Morton , Alexey Dobriyan , Luis Chamberlain , Kees Cook , Jonathan Corbet , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, Davidlohr Bueso , Miklos Szeredi , Daniel Colascione , Dave Chinner , Randy Dunlap References: <1546873978-27797-1-git-send-email-longman@redhat.com> <1546873978-27797-3-git-send-email-longman@redhat.com> <20190107155840.GY6310@bombadil.infradead.org> From: Waiman Long Message-ID: Date: Mon, 7 Jan 2019 11:07:47 -0500 MIME-Version: 1.0 In-Reply-To: <20190107155840.GY6310@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On 01/07/2019 10:58 AM, Matthew Wilcox wrote: > On Mon, Jan 07, 2019 at 10:12:58AM -0500, Waiman Long wrote: >> Reading /proc/stat can be slow especially if there are many irqs and on >> systems with many CPUs as summation of all the percpu counts for each >> of the irqs is required. On some newer systems, there can be more than >> 1000 irqs per socket. >> >> Applications that need to read /proc/stat many times per seconds will >> easily hit a bottleneck. In reality, the irq counts are seldom looked >> at. Even those applications that read them don't really need up-to-date >> information. One way to reduce the performance impact of irq counts >> computation is to do it less frequently. >> >> A new "fs/proc-stat-irqs-latency-ms" sysctl parameter is now added to > No. No, no, no, no, no. No. > > Stop adding new sysctls for this kind of thing. It's just a way to shift > blame from us (who allegedly know what we're doing) to the sysadmin > (who probably has better things to be doing than keeping up with the > intricacies of development of every single piece of software running on > their system). > > Let's figure out what this _should_ be. As a strawman, I propose we > update these stats once a second. That's easy to document and understand. I am fine with having a fixed value (like 1s) for reporting purpose. It is just people may have many different opinions on what the right value should be. That is why I opt for flexibility in the initial patch. > >> @@ -98,7 +105,48 @@ static u64 compute_stat_irqs_sum(void) >> static void show_stat_irqs(struct seq_file *p) >> { >> int i; >> +#ifdef CONFIG_PROC_SYSCTL >> + static char *irqs_buf; /* Buffer for irqs values */ >> + static int buflen; >> + static unsigned long last_jiffies; /* Last buffer update jiffies */ >> + static DEFINE_MUTEX(irqs_mutex); >> + unsigned int latency = proc_stat_irqs_latency_ms; >> + >> + if (latency) { >> + char *ptr; >> + >> + latency = _msecs_to_jiffies(latency); >> + >> + mutex_lock(&irqs_mutex); >> + if (irqs_buf && time_before(jiffies, last_jiffies + latency)) >> + goto print_out; >> + >> + /* >> + * Each irq value may require up to 11 bytes. >> + */ >> + if (!irqs_buf) { >> + irqs_buf = kmalloc(nr_irqs * 11 + 32, >> + GFP_KERNEL | __GFP_ZERO); > Why are you caching the _output_ of calling sprintf(), rather than caching the > values of each interrupt? > It is just faster to dump the whole string buffer than redoing the number formatting each time when the values don't change. I can cache the individual sums instead if it is the preferred by most. Cheers, Longman