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 Received: from lists1p.gnu.org (lists1p.gnu.org [209.51.188.17]) (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 2CE1FCD4F52 for ; Mon, 18 May 2026 11:02:48 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wOvj1-0006at-AX; Mon, 18 May 2026 07:01:31 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wOviw-0006YK-OU for qemu-devel@nongnu.org; Mon, 18 May 2026 07:01:26 -0400 Received: from out30-97.freemail.mail.aliyun.com ([115.124.30.97]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wOvit-0005iY-Ds for qemu-devel@nongnu.org; Mon, 18 May 2026 07:01:26 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1779102079; h=From:To:Subject:Date:Message-ID:MIME-Version; bh=6zpWYZsxVFc8HgF8nKU7F6t6On57u4ibzgThAGNU7gQ=; b=HaU9Lpd+hVndttqzpvaSdniV98Nrh0ax64CqHRITEYTYqgEQZ5TZANnKSS7fruULLEIzardLC1/boRdk8BjPWVkCOVu7Lo60yBDpDo2zLl+MmEWQaAISg1mkg18Mtc836mTvqQeRMe8S0I5pR64JTt/yX6kP37c4I5vrXC/fUGM= X-Alimail-AntiSpam: AC=PASS; BC=-1|-1; BR=01201311R581e4; CH=green; DM=||false|; DS=||; FP=0|-1|-1|-1|0|-1|-1|-1; HT=maildocker-contentspam033037009110; MF=guobin@linux.alibaba.com; NM=1; PH=DS; RN=3; SR=0; TI=SMTPD_---0X382z02_1779102078; Received: from localhost(mailfrom:guobin@linux.alibaba.com fp:SMTPD_---0X382z02_1779102078 cluster:ay36) by smtp.aliyun-inc.com; Mon, 18 May 2026 19:01:18 +0800 From: Bin Guo To: qemu-devel@nongnu.org Cc: peterx@redhat.com, farosas@suse.de Subject: [PATCH 7/8] migration/multifd: cache migrate_multifd_channels() in send/recv hot paths Date: Mon, 18 May 2026 19:01:11 +0800 Message-ID: <20260518110112.21395-8-guobin@linux.alibaba.com> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260518110112.21395-1-guobin@linux.alibaba.com> References: <20260518110112.21395-1-guobin@linux.alibaba.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=115.124.30.97; envelope-from=guobin@linux.alibaba.com; helo=out30-97.freemail.mail.aliyun.com X-Spam_score_int: -174 X-Spam_score: -17.5 X-Spam_bar: ----------------- X-Spam_report: (-17.5 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, ENV_AND_HDR_SPF_MATCH=-0.5, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, UNPARSEABLE_RELAY=0.001, USER_IN_DEF_DKIM_WL=-7.5, USER_IN_DEF_SPF_WL=-7.5 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org multifd_send() and multifd_recv() are on the per-page-batch hot path of live migration. Both functions call migrate_multifd_channels() multiple times (3-4 calls each) for modulo arithmetic in the round-robin channel selection loop. Each call goes through migrate_get_current() -> dereference MigrationState -> read parameters.multifd_channels. While each individual call is cheap, these functions execute for every page batch during the entire migration, easily millions of times. Cache the return value in a local variable at function entry. The channel count is fixed for the duration of a migration and cannot change mid-flight. For multifd_send(): 3 calls reduced to 1. For multifd_recv(): 4 calls reduced to 1. Signed-off-by: Bin Guo --- migration/multifd.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/migration/multifd.c b/migration/multifd.c index 67ee9bdf5e..cc2fa90204 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -362,13 +362,15 @@ bool multifd_send(MultiFDSendData **send_data) /* We wait here, until at least one channel is ready */ qemu_sem_wait(&multifd_send_state->channels_ready); + int thread_count = migrate_multifd_channels(); + /* * next_channel can remain from a previous migration that was * using more channels, so ensure it doesn't overflow if the * limit is lower now. */ - next_channel %= migrate_multifd_channels(); - for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) { + next_channel %= thread_count; + for (i = next_channel;; i = (i + 1) % thread_count) { if (multifd_send_should_exit()) { return false; } @@ -378,7 +380,7 @@ bool multifd_send(MultiFDSendData **send_data) * sender thread can clear it. */ if (qatomic_read(&p->pending_job) == false) { - next_channel = (i + 1) % migrate_multifd_channels(); + next_channel = (i + 1) % thread_count; break; } } @@ -998,6 +1000,7 @@ bool multifd_recv(void) int i; static int next_recv_channel; MultiFDRecvParams *p = NULL; + int thread_count = migrate_multifd_channels(); MultiFDRecvData *data = multifd_recv_state->data; /* @@ -1005,8 +1008,8 @@ bool multifd_recv(void) * using more channels, so ensure it doesn't overflow if the * limit is lower now. */ - next_recv_channel %= migrate_multifd_channels(); - for (i = next_recv_channel;; i = (i + 1) % migrate_multifd_channels()) { + next_recv_channel %= thread_count; + for (i = next_recv_channel;; i = (i + 1) % thread_count) { if (multifd_recv_should_exit()) { return false; } @@ -1014,7 +1017,7 @@ bool multifd_recv(void) p = &multifd_recv_state->params[i]; if (qatomic_read(&p->pending_job) == false) { - next_recv_channel = (i + 1) % migrate_multifd_channels(); + next_recv_channel = (i + 1) % thread_count; break; } } -- 2.50.1 (Apple Git-155)