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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 8257BC433FE for ; Wed, 8 Sep 2021 03:01:32 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 675C160EE6 for ; Wed, 8 Sep 2021 03:01:32 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234389AbhIHDCh (ORCPT ); Tue, 7 Sep 2021 23:02:37 -0400 Received: from mail.kernel.org ([198.145.29.99]:60298 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1347450AbhIHDCB (ORCPT ); Tue, 7 Sep 2021 23:02:01 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id D9D1B6115B; Wed, 8 Sep 2021 03:00:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1631070054; bh=c+u+HUlxPzxAbHVXw1b4TlISD7daj5+/hKtpepjKSNU=; h=Date:From:To:Subject:In-Reply-To:From; b=ahzLOZIPU60TwkHYAko1SoPOLhSSAm0Hr/ye/y7qSBO6R6ukJ8wax8VwStC7fcpEa nnze2kstvVk6ipHi3tO4th4QHksUGhjkeBXj7r2l3XXnK4Ie985sUuNVvwLOUBdevO 1naEYVbNn8u/PrXG54ZmBKTjBDxJuI0v4Jpd5noo= Date: Tue, 07 Sep 2021 20:00:53 -0700 From: Andrew Morton To: akpm@linux-foundation.org, aquini@redhat.com, dbueso@suse.de, linux-mm@kvack.org, llong@redhat.com, manfred@colorfullife.com, mm-commits@vger.kernel.org, torvalds@linux-foundation.org Subject: [patch 145/147] ipc: replace costly bailout check in sysvipc_find_ipc() Message-ID: <20210908030053.bzt189DZK%akpm@linux-foundation.org> In-Reply-To: <20210907195226.14b1d22a07c085b22968b933@linux-foundation.org> User-Agent: s-nail v14.8.16 Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org From: Rafael Aquini Subject: ipc: replace costly bailout check in sysvipc_find_ipc() sysvipc_find_ipc() was left with a costly way to check if the offset position fed to it is bigger than the total number of IPC IDs in use. So much so that the time it takes to iterate over /proc/sysvipc/* files grows exponentially for a custom benchmark that creates "N" SYSV shm segments and then times the read of /proc/sysvipc/shm (milliseconds): 12 msecs to read 1024 segs from /proc/sysvipc/shm 18 msecs to read 2048 segs from /proc/sysvipc/shm 65 msecs to read 4096 segs from /proc/sysvipc/shm 325 msecs to read 8192 segs from /proc/sysvipc/shm 1303 msecs to read 16384 segs from /proc/sysvipc/shm 5182 msecs to read 32768 segs from /proc/sysvipc/shm The root problem lies with the loop that computes the total amount of ids in use to check if the "pos" feeded to sysvipc_find_ipc() grew bigger than "ids->in_use". That is a quite inneficient way to get to the maximum index in the id lookup table, specially when that value is already provided by struct ipc_ids.max_idx. This patch follows up on the optimization introduced via commit 15df03c879836 ("sysvipc: make get_maxid O(1) again") and gets rid of the aforementioned costly loop replacing it by a simpler checkpoint based on ipc_get_maxidx() returned value, which allows for a smooth linear increase in time complexity for the same custom benchmark: 2 msecs to read 1024 segs from /proc/sysvipc/shm 2 msecs to read 2048 segs from /proc/sysvipc/shm 4 msecs to read 4096 segs from /proc/sysvipc/shm 9 msecs to read 8192 segs from /proc/sysvipc/shm 19 msecs to read 16384 segs from /proc/sysvipc/shm 39 msecs to read 32768 segs from /proc/sysvipc/shm Link: https://lkml.kernel.org/r/20210809203554.1562989-1-aquini@redhat.com Signed-off-by: Rafael Aquini Acked-by: Davidlohr Bueso Acked-by: Manfred Spraul Cc: Waiman Long Signed-off-by: Andrew Morton --- ipc/util.c | 16 ++++------------ 1 file changed, 4 insertions(+), 12 deletions(-) --- a/ipc/util.c~ipc-replace-costly-bailout-check-in-sysvipc_find_ipc +++ a/ipc/util.c @@ -788,21 +788,13 @@ struct pid_namespace *ipc_seq_pid_ns(str static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos, loff_t *new_pos) { - struct kern_ipc_perm *ipc; - int total, id; + struct kern_ipc_perm *ipc = NULL; + int max_idx = ipc_get_maxidx(ids); - total = 0; - for (id = 0; id < pos && total < ids->in_use; id++) { - ipc = idr_find(&ids->ipcs_idr, id); - if (ipc != NULL) - total++; - } - - ipc = NULL; - if (total >= ids->in_use) + if (max_idx == -1 || pos > max_idx) goto out; - for (; pos < ipc_mni; pos++) { + for (; pos <= max_idx; pos++) { ipc = idr_find(&ids->ipcs_idr, pos); if (ipc != NULL) { rcu_read_lock(); _