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 F07CDC433F5 for ; Wed, 13 Apr 2022 19:12:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238162AbiDMTO3 (ORCPT ); Wed, 13 Apr 2022 15:14:29 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52866 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238543AbiDMTNP (ORCPT ); Wed, 13 Apr 2022 15:13:15 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5C29176E1F; Wed, 13 Apr 2022 12:08:52 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id F0C9A218EF; Wed, 13 Apr 2022 19:08:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1649876930; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=9EyFhZgfXVFqDOsGJ8GGdwNni7OZbvbe647ljn8l0Kg=; b=crSSLVb7nuaM5pMQ9PmWBc+Yv5OTLyBrGq/yvmHYRW6kBmQpl9rH14sTEvCsnuU2T6OwUD cF6lo/Dcn/QQ04wcLreovJ8K6Pw90vXEQmSYKGJ2BWelryhwjnrvbBAWXsCtGuhrhj0lV7 MVY22fkx0No8qug34GDhqIwqPtUEx/k= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 78A6413A91; Wed, 13 Apr 2022 19:08:50 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id X2JiGsIfV2J8AgAAMHmgww (envelope-from ); Wed, 13 Apr 2022 19:08:50 +0000 Message-ID: Date: Wed, 13 Apr 2022 22:08:49 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.7.0 Subject: Re: [PATCH v2] btrfs: zstd: use spin_lock in timer callback Content-Language: en-US To: Schspa Shi Cc: dsterba@suse.cz, clm@fb.com, dsterba@suse.com, josef@toxicpanda.com, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, terrelln@fb.com References: <20220411135136.GG15609@suse.cz> <20220411155540.36853-1-schspa@gmail.com> <09c2a9ce-3b04-ed94-1d62-0e5a072b9dac@suse.com> From: Nikolay Borisov In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 13.04.22 г. 19:03 ч., Schspa Shi wrote: > Nikolay Borisov writes: > >> On 11.04.22 г. 18:55 ч., Schspa Shi wrote: >>> This is an optimization for fix fee13fe96529 ("btrfs: >>> correct zstd workspace manager lock to use spin_lock_bh()") >>> The critical region for wsm.lock is only accessed by the process context and >>> the softirq context. >>> Because in the soft interrupt, the critical section will not be preempted by >>> the >>> soft interrupt again, there is no need to call spin_lock_bh(&wsm.lock) to turn >>> off the soft interrupt, spin_lock(&wsm.lock) is enough for this situation. >>> Changelog: >>> v1 -> v2: >>> - Change the commit message to make it more readable. >>> [1] https://lore.kernel.org/all/20220408181523.92322-1-schspa@gmail.com/ >>> Signed-off-by: Schspa Shi >> >> Has there been any measurable impact by this change? While it's correct it does mean that >> someone looking at the code would see that in one call site we use plain spinlock and in >> another a _bh version and this is somewhat inconsistent. >> > Yes, it may seem a little confused. but it's allowed to save some > little peace of CPU times. > and "static inline void red_adaptative_timer(struct timer_list *t) in > net/sched/sch_red.c" > have similar usage. > >> What's more I believe this is a noop since when softirqs are executing preemptible() would >> be false due to preempt_count() being non-0 and in the bh-disabling code >> in the spinlock we have: >> >> /* First entry of a task into a BH disabled section? */ >> 1 if (!current->softirq_disable_cnt) { >> 167 if (preemptible()) { >> 1 local_lock(&softirq_ctrl.lock); >> 2 /* Required to meet the RCU bottomhalf requirements. */ >> 3 rcu_read_lock(); >> 4 } else { >> 5 DEBUG_LOCKS_WARN_ON(this_cpu_read(softirq_ctrl.cnt)); >> 6 } >> 7 } >> >> >> In this case we'd hit the else branch. > > We won't hit the else branch. because current->softirq_disable_cnt > won't be zero in the origin case. > > __do_softirq(void) > softirq_handle_begin(void) > __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET); > current->softirq_disable_cnt will be > 0 at this time. That's only relevant on CONFIG_PREEMPT_RT though, on usual kernels softirq_handle_being is empty. Furthermore, in case of the non-preempt rt if preemptible() always returns false this means that even in the __do_softirq path we'll never increment softirq_disable_cnt. So if anything this change is only beneficial (theoretically at that in preempt_rt scenarios). > ...... > zstd_reclaim_timer_fn(struct timer_list *timer) > spin_lock_bh(&wsm.lock); > __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET); > if (!current->softirq_disable_cnt) { > // this if branch won't hit > } > > softirq_handle_end(); > > In this case, the "__local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);" > won't do anything useful it only > increase softirq disable depth and decrease it in > "__local_bh_enable_ip(_RET_IP_, SOFTIRQ_LOCK_OFFSET);". > > So it's safe to replace spin_lock_bh with spin_lock in a timer > callback function. > > > For the ksoftirqd, it's all the same. >