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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 D0FF9C4360C for ; Fri, 4 Oct 2019 12:24:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7A3A2215EA for ; Fri, 4 Oct 2019 12:24:49 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=yandex-team.ru header.i=@yandex-team.ru header.b="nlyf245f" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730428AbfJDMYs (ORCPT ); Fri, 4 Oct 2019 08:24:48 -0400 Received: from forwardcorp1j.mail.yandex.net ([5.45.199.163]:55982 "EHLO forwardcorp1j.mail.yandex.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727451AbfJDMYs (ORCPT ); Fri, 4 Oct 2019 08:24:48 -0400 Received: from mxbackcorp1j.mail.yandex.net (mxbackcorp1j.mail.yandex.net [IPv6:2a02:6b8:0:1619::162]) by forwardcorp1j.mail.yandex.net (Yandex) with ESMTP id 8509C2E149B; Fri, 4 Oct 2019 15:24:44 +0300 (MSK) Received: from myt5-6212ef07a9ec.qloud-c.yandex.net (myt5-6212ef07a9ec.qloud-c.yandex.net [2a02:6b8:c12:3b2d:0:640:6212:ef07]) by mxbackcorp1j.mail.yandex.net (nwsmtp/Yandex) with ESMTP id H22NnS2IBM-OidWTpac; Fri, 04 Oct 2019 15:24:44 +0300 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=yandex-team.ru; s=default; t=1570191884; bh=4sHSfmkLu8cAKROoqUfLI1PfxK6+ZhfZTCKtlJiHxAo=; h=In-Reply-To:Message-ID:From:Date:References:To:Subject:Cc; b=nlyf245fK5EjgxGlzppCdVO3UNoKEUI+tjU6aABptVesQq0cvZwwFwvINHiLaJqWb OltyxM74KARCVR3hv5FuPuaHGiFI4W4hTviMyaQ9ItMXi+eZa8sptRZpw4W+fqNBNd Qb82ECXJgyjV+AeNvkDrbQd+T29Yx/E9Ydna1lIo= Authentication-Results: mxbackcorp1j.mail.yandex.net; dkim=pass header.i=@yandex-team.ru Received: from dynamic-red.dhcp.yndx.net (dynamic-red.dhcp.yndx.net [2a02:6b8:0:40c:3d4d:a9cb:ef29:4bb1]) by myt5-6212ef07a9ec.qloud-c.yandex.net (nwsmtp/Yandex) with ESMTPSA id O4GECBlzwv-OiHC0PEu; Fri, 04 Oct 2019 15:24:44 +0300 (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client certificate not present) Subject: Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls To: Matthew Wilcox Cc: linux-mm@kvack.org, Andrew Morton , linux-kernel@vger.kernel.org References: <157018386639.6110.3058050375244904201.stgit@buzz> <20191004121017.GG32665@bombadil.infradead.org> From: Konstantin Khlebnikov Message-ID: Date: Fri, 4 Oct 2019 15:24:43 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <20191004121017.GG32665@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-CA Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/10/2019 15.10, Matthew Wilcox wrote: > On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote: >> This is very slow operation. There is no reason to do it again if somebody >> else already drained all per-cpu vectors after we waited for lock. >> + seq = raw_read_seqcount_latch(&seqcount); >> + >> mutex_lock(&lock); >> + >> + /* Piggyback on drain done by somebody else. */ >> + if (__read_seqcount_retry(&seqcount, seq)) >> + goto done; >> + >> + raw_write_seqcount_latch(&seqcount); >> + > > Do we really need the seqcount to do this? Wouldn't a mutex_trylock() > have the same effect? > No, this is completely different semantics. Operation could be safely skipped only if somebody else started and finished drain after current task called this function.