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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0FCECC001DC for ; Wed, 26 Jul 2023 14:32:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234550AbjGZOci (ORCPT ); Wed, 26 Jul 2023 10:32:38 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54748 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232984AbjGZOcg (ORCPT ); Wed, 26 Jul 2023 10:32:36 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 03145CE for ; Wed, 26 Jul 2023 07:31:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1690381910; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=fE1OR9ByWNiD6jQOB//eqVZjFQki6qBjJm4P5JTOE44=; b=S7JX2hAV8kgkxgDlIlMLC+P4/xercgctG0yxC5nrQgG/Rn3TdfJ9erCkjh9UOsVwULz+jQ H2+jAefcrHfYXEmoJIaK3ltoWHzo1E/snb2FJGP2aOy2kmkeowtAJgwx87qlcdBd2zOu4v fSVuDwKXX3Gnmin3rHClgZLNU0LvW/g= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-265-caxb8pjXOJOTCo-3pjhtKw-1; Wed, 26 Jul 2023 10:31:47 -0400 X-MC-Unique: caxb8pjXOJOTCo-3pjhtKw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 4FDCD800888; Wed, 26 Jul 2023 14:31:46 +0000 (UTC) Received: from tpad.localdomain (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EF18F40C2063; Wed, 26 Jul 2023 14:31:45 +0000 (UTC) Received: by tpad.localdomain (Postfix, from userid 1000) id 1CBDC40B16BD4; Wed, 26 Jul 2023 11:31:26 -0300 (-03) Date: Wed, 26 Jul 2023 11:31:26 -0300 From: Marcelo Tosatti To: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Christian Brauner Cc: Alexander Viro , Christian Brauner , Matthew Wilcox , Christoph Hellwig , Jens Axboe , Frederic Weisbecker , Dave Chinner , Valentin Schneider , Leonardo Bras , Yair Podemsky , P J P Subject: Re: [PATCH] fs/buffer.c: disable per-CPU buffer_head cache for isolated CPUs Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ping, apparently there is no objection to this patch... Christian, what is the preferred tree for integration? On Tue, Jun 27, 2023 at 05:08:15PM -0300, Marcelo Tosatti wrote: > > For certain types of applications (for example PLC software or > RAN processing), upon occurrence of an event, it is necessary to > complete a certain task in a maximum amount of time (deadline). > > One way to express this requirement is with a pair of numbers, > deadline time and execution time, where: > > * deadline time: length of time between event and deadline. > * execution time: length of time it takes for processing of event > to occur on a particular hardware platform > (uninterrupted). > > The particular values depend on use-case. For the case > where the realtime application executes in a virtualized > guest, an IPI which must be serviced in the host will cause > the following sequence of events: > > 1) VM-exit > 2) execution of IPI (and function call) > 3) VM-entry > > Which causes an excess of 50us latency as observed by cyclictest > (this violates the latency requirement of vRAN application with 1ms TTI, > for example). > > invalidate_bh_lrus calls an IPI on each CPU that has non empty > per-CPU cache: > > on_each_cpu_cond(has_bh_in_lru, invalidate_bh_lru, NULL, 1); > > The performance when using the per-CPU LRU cache is as follows: > > 42 ns per __find_get_block > 68 ns per __find_get_block_slow > > Given that the main use cases for latency sensitive applications > do not involve block I/O (data necessary for program operation is > locked in RAM), disable per-CPU buffer_head caches for isolated CPUs. > > Signed-off-by: Marcelo Tosatti > > diff --git a/fs/buffer.c b/fs/buffer.c > index a7fc561758b1..49e9160ce100 100644 > --- a/fs/buffer.c > +++ b/fs/buffer.c > @@ -49,6 +49,7 @@ > #include > #include > #include > +#include > > #include "internal.h" > > @@ -1289,7 +1290,7 @@ static void bh_lru_install(struct buffer_head *bh) > * failing page migration. > * Skip putting upcoming bh into bh_lru until migration is done. > */ > - if (lru_cache_disabled()) { > + if (lru_cache_disabled() || cpu_is_isolated(smp_processor_id())) { > bh_lru_unlock(); > return; > } > @@ -1319,6 +1320,10 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size) > > check_irqs_on(); > bh_lru_lock(); > + if (cpu_is_isolated(smp_processor_id())) { > + bh_lru_unlock(); > + return NULL; > + } > for (i = 0; i < BH_LRU_SIZE; i++) { > struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]); >