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=-18.8 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,USER_AGENT_GIT 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 247E7C2B9F7 for ; Mon, 24 May 2021 23:41:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 000386141F for ; Mon, 24 May 2021 23:41:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230018AbhEXXm2 (ORCPT ); Mon, 24 May 2021 19:42:28 -0400 Received: from mx2.suse.de ([195.135.220.15]:50488 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229503AbhEXXmT (ORCPT ); Mon, 24 May 2021 19:42:19 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1621899648; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=nVHhxBz5PMne0XZT3zO+ovDd4dDVDxEhwNUCfCb/aKc=; b=qoI9jq/pgA4tT5xbBvqsBbzL1uW1cfejSksYK4wZUN4NHWRg6f/5iWed6armkEXjfF7Hbz U1eykhAOl4Fd1aY2Ew+cgkHPxjVzz3nB4gYoT+sCs3AM8vSJM5rOav8fEYsxcagmPy1Va7 mK0Uq0T2tY8JS2UoaasFzCBRNnM92e4= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1621899648; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=nVHhxBz5PMne0XZT3zO+ovDd4dDVDxEhwNUCfCb/aKc=; b=DrPHqBVEIWTBP0i/LR5+v/LlPzI0dlXCommjortvhrcSAZt45B+ATF6MEEC7Fgpts5xDBJ NaPiLnxvyWJp+PDw== Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id E714CAF1E; Mon, 24 May 2021 23:40:47 +0000 (UTC) From: Vlastimil Babka To: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim Cc: Sebastian Andrzej Siewior , Thomas Gleixner , Mel Gorman , Jesper Dangaard Brouer , Peter Zijlstra , Jann Horn , Vlastimil Babka Subject: [RFC 04/26] mm, slub: simplify kmem_cache_cpu and tid setup Date: Tue, 25 May 2021 01:39:24 +0200 Message-Id: <20210524233946.20352-5-vbabka@suse.cz> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210524233946.20352-1-vbabka@suse.cz> References: <20210524233946.20352-1-vbabka@suse.cz> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org In slab_alloc_node() and do_slab_free() fastpaths we need to guarantee that our kmem_cache_cpu pointer is from the same cpu as the tid value. Currently that's done by reading the tid first using this_cpu_read(), then the kmem_cache_cpu pointer and verifying we read the same tid using the pointer and plain READ_ONCE(). This can be simplified to just fetching kmem_cache_cpu pointer and then reading tid using the pointer. That guarantees they are from the same cpu. We don't need to read the tid using this_cpu_read() because the value will be validated by this_cpu_cmpxchg_double(), making sure we are on the correct cpu and the freelist didn't change by anyone preempting us since reading the tid. Signed-off-by: Vlastimil Babka --- mm/slub.c | 22 +++++++++------------- 1 file changed, 9 insertions(+), 13 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 83ad64c1d9da..7b4cdc59b9ff 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2840,15 +2840,14 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, * reading from one cpu area. That does not matter as long * as we end up on the original cpu again when doing the cmpxchg. * - * We should guarantee that tid and kmem_cache are retrieved on - * the same cpu. It could be different if CONFIG_PREEMPTION so we need - * to check if it is matched or not. + * We must guarantee that tid and kmem_cache_cpu are retrieved on the + * same cpu. We read first the kmem_cache_cpu pointer and use it to read + * the tid. If we are preempted and switched to another cpu between the + * two reads, it's OK as the two are still associated with the same cpu + * and cmpxchg later will validate the cpu. */ - do { - tid = this_cpu_read(s->cpu_slab->tid); - c = raw_cpu_ptr(s->cpu_slab); - } while (IS_ENABLED(CONFIG_PREEMPTION) && - unlikely(tid != READ_ONCE(c->tid))); + c = raw_cpu_ptr(s->cpu_slab); + tid = READ_ONCE(c->tid); /* * Irqless object alloc/free algorithm used here depends on sequence @@ -3122,11 +3121,8 @@ static __always_inline void do_slab_free(struct kmem_cache *s, * data is retrieved via this pointer. If we are on the same cpu * during the cmpxchg then the free will succeed. */ - do { - tid = this_cpu_read(s->cpu_slab->tid); - c = raw_cpu_ptr(s->cpu_slab); - } while (IS_ENABLED(CONFIG_PREEMPTION) && - unlikely(tid != READ_ONCE(c->tid))); + c = raw_cpu_ptr(s->cpu_slab); + tid = READ_ONCE(c->tid); /* Same with comment on barrier() in slab_alloc_node() */ barrier(); -- 2.31.1