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=-15.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 18017C63777 for ; Tue, 24 Nov 2020 05:14:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id AE2F5206FB for ; Tue, 24 Nov 2020 05:14:39 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=mg.codeaurora.org header.i=@mg.codeaurora.org header.b="pwPgDruS" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727482AbgKXFOj (ORCPT ); Tue, 24 Nov 2020 00:14:39 -0500 Received: from m42-4.mailgun.net ([69.72.42.4]:38452 "EHLO m42-4.mailgun.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727245AbgKXFOi (ORCPT ); Tue, 24 Nov 2020 00:14:38 -0500 DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.codeaurora.org; q=dns/txt; s=smtp; t=1606194877; h=Content-Transfer-Encoding: Content-Type: In-Reply-To: MIME-Version: Date: Message-ID: From: References: Cc: To: Subject: Sender; bh=N6Nn5yLXl84d59znuUBbglg1ygavpuXeQ7QoHit9LYY=; b=pwPgDruSQVSyDHAPu2FlKa6OD6gq3bkOsAcWpFxSC+qms92hBgCdBCC+eMLNqG1Dmw9Iu+Oq KygR2tdmrYUYYSN0Qt2YAVj1+pKlU5oNfxFWochhcBEKtG+HtNeZHxuOqfxaCIzbSpMzzU8m 1MjrTIsMcwbY7T/jJk//eMRKn78= X-Mailgun-Sending-Ip: 69.72.42.4 X-Mailgun-Sid: WyI0MWYwYSIsICJsaW51eC1rZXJuZWxAdmdlci5rZXJuZWwub3JnIiwgImJlOWU0YSJd Received: from smtp.codeaurora.org (ec2-35-166-182-171.us-west-2.compute.amazonaws.com [35.166.182.171]) by smtp-out-n05.prod.us-east-1.postgun.com with SMTP id 5fbc96bc9e87e163522acd32 (version=TLS1.2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256); Tue, 24 Nov 2020 05:14:36 GMT Sender: neeraju=codeaurora.org@mg.codeaurora.org Received: by smtp.codeaurora.org (Postfix, from userid 1001) id 78D63C4346E; Tue, 24 Nov 2020 05:14:35 +0000 (UTC) Received: from [192.168.0.101] (unknown [49.206.49.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) (Authenticated sender: neeraju) by smtp.codeaurora.org (Postfix) with ESMTPSA id 27EFFC433ED; Tue, 24 Nov 2020 05:14:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 27EFFC433ED Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: aws-us-west-2-caf-mail-1.web.codeaurora.org; spf=fail smtp.mailfrom=neeraju@codeaurora.org Subject: Re: [PATCH v2 tip/core/rcu 4/6] srcu: Provide polling interfaces for Tiny SRCU grace periods To: paulmck@kernel.org Cc: rcu@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, kent.overstreet@gmail.com References: <@@@> <20201121005919.17152-4-paulmck@kernel.org> <2d210fa7-3484-9cee-862b-a8f15fee8c8e@codeaurora.org> <20201123211250.GF1437@paulmck-ThinkPad-P72> From: Neeraj Upadhyay Message-ID: <9ebe7c82-6fc2-a759-dde8-eec90b483735@codeaurora.org> Date: Tue, 24 Nov 2020 10:44:24 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201123211250.GF1437@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/24/2020 2:42 AM, Paul E. McKenney wrote: > On Mon, Nov 23, 2020 at 10:13:13AM +0530, Neeraj Upadhyay wrote: >> >> >> On 11/21/2020 6:29 AM, paulmck@kernel.org wrote: >>> From: "Paul E. McKenney" >>> >>> There is a need for a polling interface for SRCU grace >>> periods, so this commit supplies get_state_synchronize_srcu(), >>> start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this >>> purpose. The first can be used if future grace periods are inevitable >>> (perhaps due to a later call_srcu() invocation), the second if future >>> grace periods might not otherwise happen, and the third to check if a >>> grace period has elapsed since the corresponding call to either of the >>> first two. >>> >>> As with get_state_synchronize_rcu() and cond_synchronize_rcu(), >>> the return value from either get_state_synchronize_srcu() or >>> start_poll_synchronize_srcu() must be passed in to a later call to >>> poll_state_synchronize_srcu(). >>> >>> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/ >>> Reported-by: Kent Overstreet >>> [ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ] >>> [ paulmck: Apply feedback from Neeraj Upadhyay. ] >>> Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/ >>> Signed-off-by: Paul E. McKenney >>> --- >>> include/linux/rcupdate.h | 2 ++ >>> include/linux/srcu.h | 3 +++ >>> include/linux/srcutiny.h | 1 + >>> kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++-- >>> 4 files changed, 56 insertions(+), 2 deletions(-) >>> >>> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h >>> index de08264..e09c0d8 100644 >>> --- a/include/linux/rcupdate.h >>> +++ b/include/linux/rcupdate.h >>> @@ -33,6 +33,8 @@ >>> #define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b)) >>> #define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b)) >>> #define ulong2long(a) (*(long *)(&(a))) >>> +#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b))) >>> +#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b))) >>> /* Exported common interfaces */ >>> void call_rcu(struct rcu_head *head, rcu_callback_t func); >>> diff --git a/include/linux/srcu.h b/include/linux/srcu.h >>> index e432cc9..a0895bb 100644 >>> --- a/include/linux/srcu.h >>> +++ b/include/linux/srcu.h >>> @@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp); >>> int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp); >>> void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp); >>> void synchronize_srcu(struct srcu_struct *ssp); >>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp); >>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp); >>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie); >>> #ifdef CONFIG_DEBUG_LOCK_ALLOC >>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h >>> index d9edb67..c7f0c1f 100644 >>> --- a/include/linux/srcutiny.h >>> +++ b/include/linux/srcutiny.h >>> @@ -16,6 +16,7 @@ >>> struct srcu_struct { >>> short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */ >>> unsigned short srcu_idx; /* Current reader array element in bit 0x2. */ >>> + unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */ >>> u8 srcu_gp_running; /* GP workqueue running? */ >>> u8 srcu_gp_waiting; /* GP waiting for readers? */ >>> struct swait_queue_head srcu_wq; >>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c >>> index 3bac1db..b073175 100644 >>> --- a/kernel/rcu/srcutiny.c >>> +++ b/kernel/rcu/srcutiny.c >>> @@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp) >>> ssp->srcu_gp_running = false; >>> ssp->srcu_gp_waiting = false; >>> ssp->srcu_idx = 0; >>> + ssp->srcu_idx_max = 0; >>> INIT_WORK(&ssp->srcu_work, srcu_drive_gp); >>> INIT_LIST_HEAD(&ssp->srcu_work.entry); >>> return 0; >> >> Minor: cleanup_srcu_struct() can probably have 2 new sanity checks? >> >> WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max); >> WARN_ON(ssp->srcu_idx & 1); > > Good point, added and under test. > >> Thanks >> Neeraj >> >>> @@ -114,7 +115,7 @@ void srcu_drive_gp(struct work_struct *wp) >>> struct srcu_struct *ssp; >>> ssp = container_of(wp, struct srcu_struct, srcu_work); >>> - if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head)) >>> + if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) >>> return; /* Already running or nothing to do. */ >>> /* Remove recently arrived callbacks and wait for readers. */ >>> @@ -147,13 +148,18 @@ void srcu_drive_gp(struct work_struct *wp) >>> * straighten that out. >>> */ >>> WRITE_ONCE(ssp->srcu_gp_running, false); >>> - if (READ_ONCE(ssp->srcu_cb_head)) >>> + if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max))) >>> schedule_work(&ssp->srcu_work); >>> } >>> EXPORT_SYMBOL_GPL(srcu_drive_gp); >>> static void srcu_gp_start_if_needed(struct srcu_struct *ssp) >>> { >>> + unsigned short cookie; >>> + >>> + cookie = get_state_synchronize_srcu(ssp); >>> + if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie)) >>> + WRITE_ONCE(ssp->srcu_idx_max, cookie); >> >> Minor: Maybe we can return in the else part of USHORT_CMP_LT check, to avoid >> scheduling work? > > How about like this? Looks good! Thanks Neeraj > > static void srcu_gp_start_if_needed(struct srcu_struct *ssp) > { > unsigned short cookie; > > cookie = get_state_synchronize_srcu(ssp); > if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie)) > return; > WRITE_ONCE(ssp->srcu_idx_max, cookie); > if (!READ_ONCE(ssp->srcu_gp_running)) { > if (likely(srcu_init_done)) > schedule_work(&ssp->srcu_work); > else if (list_empty(&ssp->srcu_work.entry)) > list_add(&ssp->srcu_work.entry, &srcu_boot_list); > } > } > > Testing this next. ;-) > > Thanx, Paul > >> Thanks >> Neeraj >> >>> if (!READ_ONCE(ssp->srcu_gp_running)) { >>> if (likely(srcu_init_done)) >>> schedule_work(&ssp->srcu_work); >>> @@ -196,6 +202,48 @@ void synchronize_srcu(struct srcu_struct *ssp) >>> } >>> EXPORT_SYMBOL_GPL(synchronize_srcu); >>> +/* >>> + * get_state_synchronize_srcu - Provide an end-of-grace-period cookie >>> + */ >>> +unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp) >>> +{ >>> + unsigned long ret; >>> + >>> + barrier(); >>> + ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1; >>> + barrier(); >>> + return ret & USHRT_MAX; >>> +} >>> +EXPORT_SYMBOL_GPL(get_state_synchronize_srcu); >>> + >>> +/* >>> + * start_poll_synchronize_srcu - Provide cookie and start grace period >>> + * >>> + * The difference between this and get_state_synchronize_srcu() is that >>> + * this function ensures that the poll_state_synchronize_srcu() will >>> + * eventually return the value true. >>> + */ >>> +unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp) >>> +{ >>> + unsigned long ret = get_state_synchronize_srcu(ssp); >>> + >>> + srcu_gp_start_if_needed(ssp); >>> + return ret; >>> +} >>> +EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu); >>> + >>> +/* >>> + * poll_state_synchronize_srcu - Has cookie's grace period ended? >>> + */ >>> +bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie) >>> +{ >>> + bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie); >>> + >>> + barrier(); >>> + return ret; >>> +} >>> +EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu); >>> + >>> /* Lockdep diagnostics. */ >>> void __init rcu_scheduler_starting(void) >>> { >>> >> >> -- >> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of >> the Code Aurora Forum, hosted by The Linux Foundation -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation