From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F3146C282C4 for ; Mon, 4 Feb 2019 20:01:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C1E5E2081B for ; Mon, 4 Feb 2019 20:01:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727617AbfBDUBW (ORCPT ); Mon, 4 Feb 2019 15:01:22 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38292 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725854AbfBDUBW (ORCPT ); Mon, 4 Feb 2019 15:01:22 -0500 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 9EBACC070E1A; Mon, 4 Feb 2019 20:01:21 +0000 (UTC) Received: from t460s.bristot.redhat.com (ovpn-117-55.phx2.redhat.com [10.3.117.55]) by smtp.corp.redhat.com (Postfix) with ESMTP id C94999DA8C; Mon, 4 Feb 2019 20:01:11 +0000 (UTC) From: Daniel Bristot de Oliveira To: linux-kernel@vger.kernel.org Cc: bristot@redhat.com, Thomas Gleixner , Ingo Molnar , Borislav Petkov , "H. Peter Anvin" , Greg Kroah-Hartman , Masami Hiramatsu , "Steven Rostedt (VMware)" , Jiri Kosina , Josh Poimboeuf , "Peter Zijlstra (Intel)" , Chris von Recklinghausen , Jason Baron , Scott Wood , Marcelo Tosatti , Clark Williams , x86@kernel.org Subject: [PATCH V4 8/9] jump_label: Batch updates if arch supports it Date: Mon, 4 Feb 2019 20:59:01 +0100 Message-Id: In-Reply-To: References: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Mon, 04 Feb 2019 20:01:21 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org If the architecture supports the batching of jump label updates, use it! An easy way to see the benefits of this patch is switching the schedstats on and off. For instance: -------------------------- %< ---------------------------- #!/bin/bash while [ true ]; do sysctl -w kernel.sched_schedstats=1 sleep 2 sysctl -w kernel.sched_schedstats=0 sleep 2 done -------------------------- >% ---------------------------- while watching the IPI count: -------------------------- %< ---------------------------- # watch -n1 "cat /proc/interrupts | grep Function" -------------------------- >% ---------------------------- With the current mode, it is possible to see +- 168 IPIs each 2 seconds, while with this patch the number of IPIs goes to 3 each 2 seconds. Regarding the performance impact of this patch set, I made two measurements: The time to update a key (the task that is causing the change) The time to run the int3 handler (the side effect on a thread that hits the code being changed) The schedstats static key was chosen as the key to being switched on and off. The reason being is that it is used in more than 56 places, in a hot path. The change in the schedstats static key will be done with the following command: while [ true ]; do sysctl -w kernel.sched_schedstats=1 usleep 500000 sysctl -w kernel.sched_schedstats=0 usleep 500000 done In this way, they key will be updated twice per second. To force the hit of the int3 handler, the system will also run a kernel compilation with two jobs per CPU. The test machine is a two nodes/24 CPUs box with an Intel Xeon processor @2.27GHz. Regarding the update part, on average, the regular kernel takes 57 ms to update the schedstats key, while the kernel with the batch updates takes just 1.4 ms on average. Although it seems to be too good to be true, it makes sense: the schedstats key is used in 56 places, so it was expected that it would take around 56 times to update the keys with the current implementation, as the IPIs are the most expensive part of the update. Regarding the int3 handler, the non-batch handler takes 45 ns on average, while the batch version takes around 180 ns. At first glance, it seems to be a high value. But it is not, considering that it is doing 56 updates, rather than one! It is taking four times more, only. This gain is possible because the patch uses a binary search in the vector: log2(56)=5.8. So, it was expected to have an overhead within four times. (voice of tv propaganda) But, that is not all! As the int3 handler keeps on for a shorter period (because the update part is on for a shorter time), the number of hits in the int3 handler decreased by 10%. The question then is: Is it worth paying the price of "135 ns" more in the int3 handler? Considering that, in this test case, we are saving the handling of 53 IPIs, that takes more than these 135 ns, it seems to be a meager price to be paid. Moreover, the test case was forcing the hit of the int3, in practice, it does not take that often. While the IPI takes place on all CPUs, hitting the int3 handler or not! For instance, in an isolated CPU with a process running in user-space (nohz_full use-case), the chances of hitting the int3 handler is barely zero, while there is no way to avoid the IPIs. By bounding the IPIs, we are improving a lot this scenario. Signed-off-by: Daniel Bristot de Oliveira Cc: Thomas Gleixner Cc: Ingo Molnar Cc: Borislav Petkov Cc: "H. Peter Anvin" Cc: Greg Kroah-Hartman Cc: Masami Hiramatsu Cc: "Steven Rostedt (VMware)" Cc: Jiri Kosina Cc: Josh Poimboeuf Cc: "Peter Zijlstra (Intel)" Cc: Chris von Recklinghausen Cc: Jason Baron Cc: Scott Wood Cc: Marcelo Tosatti Cc: Clark Williams Cc: x86@kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Daniel Bristot de Oliveira Signed-off-by: Daniel Bristot de Oliveira Signed-off-by: Daniel Bristot de Oliveira Signed-off-by: Daniel Bristot de Oliveira --- include/linux/jump_label.h | 3 +++ kernel/jump_label.c | 29 +++++++++++++++++++++++++++++ 2 files changed, 32 insertions(+) diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index 7e91af98bbb1..b3dfce98edb7 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -215,6 +215,9 @@ extern void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type); extern void arch_jump_label_transform_static(struct jump_entry *entry, enum jump_label_type type); +extern int arch_jump_label_transform_queue(struct jump_entry *entry, + enum jump_label_type type); +extern void arch_jump_label_transform_apply(void); extern int jump_label_text_reserved(void *start, void *end); extern void static_key_slow_inc(struct static_key *key); extern void static_key_slow_dec(struct static_key *key); diff --git a/kernel/jump_label.c b/kernel/jump_label.c index 53b7c85c0b09..944c75a0b09b 100644 --- a/kernel/jump_label.c +++ b/kernel/jump_label.c @@ -407,6 +407,7 @@ bool jump_label_can_update_check(struct jump_entry *entry, bool init) return 0; } +#ifndef HAVE_JUMP_LABEL_BATCH static void __jump_label_update(struct static_key *key, struct jump_entry *entry, struct jump_entry *stop, @@ -419,6 +420,34 @@ static void __jump_label_update(struct static_key *key, } } } +#else +static void __jump_label_update(struct static_key *key, + struct jump_entry *entry, + struct jump_entry *stop, + bool init) +{ + for_each_label_entry(key, entry, stop) { + + if (!jump_label_can_update_check(entry, init)) + continue; + + if (arch_jump_label_transform_queue(entry, + jump_label_type(entry))) + continue; + + /* + * Queue's overflow: Apply the current queue, and then + * queue again. If it stills not possible to queue, BUG! + */ + arch_jump_label_transform_apply(); + if (!arch_jump_label_transform_queue(entry, + jump_label_type(entry))) { + BUG(); + } + } + arch_jump_label_transform_apply(); +} +#endif void __init jump_label_init(void) { -- 2.17.1