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=-15.2 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,URIBL_BLOCKED, 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 1926BC63777 for ; Sat, 28 Nov 2020 17:56:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C8577246E4 for ; Sat, 28 Nov 2020 17:56:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=mg.codeaurora.org header.i=@mg.codeaurora.org header.b="ZbjJcSvG" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729459AbgK1EOF (ORCPT ); Fri, 27 Nov 2020 23:14:05 -0500 Received: from m42-4.mailgun.net ([69.72.42.4]:44321 "EHLO m42-4.mailgun.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731037AbgK1ENL (ORCPT ); Fri, 27 Nov 2020 23:13:11 -0500 DKIM-Signature: a=rsa-sha256; v=1; c=relaxed/relaxed; d=mg.codeaurora.org; q=dns/txt; s=smtp; t=1606536789; h=Content-Transfer-Encoding: Content-Type: In-Reply-To: MIME-Version: Date: Message-ID: From: References: Cc: To: Subject: Sender; bh=CKQ67obRSTE5auRFN3Iv3F18LhaYz1abTj3Ju4cK5ug=; b=ZbjJcSvG7IbgY8CnEeXw/1p3JXGVpPBRcfX0VfiKuvdEjlwsxNG8ZGtV+KOi11ydSNu/6/8D u+4u38Z06ZvkQ15r9rBsoripcPSyLvu3wLhm4O+9rkCHMjIqjeiEn2x86IINCXZ5+UKmMP6H E+KQoDvv7zca5c7infxzInx5zw8= X-Mailgun-Sending-Ip: 69.72.42.4 X-Mailgun-Sid: WyJkZDlkNSIsICJyY3VAdmdlci5rZXJuZWwub3JnIiwgImJlOWU0YSJd Received: from smtp.codeaurora.org (ec2-35-166-182-171.us-west-2.compute.amazonaws.com [35.166.182.171]) by smtp-out-n07.prod.us-east-1.postgun.com with SMTP id 5fc1ce524146c5eefdcb0a9b (version=TLS1.2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256); Sat, 28 Nov 2020 04:13:06 GMT Sender: neeraju=codeaurora.org@mg.codeaurora.org Received: by smtp.codeaurora.org (Postfix, from userid 1001) id D5135C433C6; Sat, 28 Nov 2020 04:13:05 +0000 (UTC) Received: from [192.168.0.105] (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 B6348C433C6; Sat, 28 Nov 2020 04:12:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org B6348C433C6 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 1/6] srcu: Make Tiny SRCU use multi-bit grace-period counter 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: <@@@> <20201121005919.17152-1-paulmck@kernel.org> <20201123195543.GC1437@paulmck-ThinkPad-P72> <46f5137b-2450-2478-6274-157367264ce4@codeaurora.org> <28a22477-9fbd-603a-404c-1cf6e7cc18b5@codeaurora.org> <20201128021609.GX1437@paulmck-ThinkPad-P72> From: Neeraj Upadhyay Message-ID: <225e5fba-f763-3a1e-062b-c74cdc5f83af@codeaurora.org> Date: Sat, 28 Nov 2020 09:42:55 +0530 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20201128021609.GX1437@paulmck-ThinkPad-P72> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On 11/28/2020 7:46 AM, Paul E. McKenney wrote: > On Wed, Nov 25, 2020 at 10:03:26AM +0530, Neeraj Upadhyay wrote: >> >> >> On 11/24/2020 10:48 AM, Neeraj Upadhyay wrote: >>> >>> >>> On 11/24/2020 1:25 AM, Paul E. McKenney wrote: >>>> On Mon, Nov 23, 2020 at 10:01:13AM +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.  This >>>>>> polling needs to distinguish between an SRCU instance being idle on the >>>>>> one hand or in the middle of a grace period on the other.  This commit >>>>>> therefore converts the Tiny SRCU srcu_struct structure's srcu_idx from >>>>>> a defacto boolean to a free-running counter, using the bottom bit to >>>>>> indicate that a grace period is in progress.  The second-from-bottom >>>>>> bit is thus used as the index returned by srcu_read_lock(). >>>>>> >>>>>> Link: >>>>>> https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/ >>>>>> Reported-by: Kent Overstreet >>>>>> [ paulmck: Fix __srcu_read_lock() idx computation Neeraj per >>>>>> Upadhyay. ] >>>>>> Signed-off-by: Paul E. McKenney >>>>>> --- >>>>>>    include/linux/srcutiny.h | 4 ++-- >>>>>>    kernel/rcu/srcutiny.c    | 5 +++-- >>>>>>    2 files changed, 5 insertions(+), 4 deletions(-) >>>>>> >>>>>> diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h >>>>>> index 5a5a194..d9edb67 100644 >>>>>> --- a/include/linux/srcutiny.h >>>>>> +++ b/include/linux/srcutiny.h >>>>>> @@ -15,7 +15,7 @@ >>>>>>    struct srcu_struct { >>>>>>        short srcu_lock_nesting[2];    /* srcu_read_lock() >>>>>> nesting depth. */ >>>>>> -    short srcu_idx;            /* Current reader array element. */ >>>>>> +    unsigned short srcu_idx;    /* Current reader array >>>>>> element in bit 0x2. */ >>>>>>        u8 srcu_gp_running;        /* GP workqueue running? */ >>>>>>        u8 srcu_gp_waiting;        /* GP waiting for readers? */ >>>>>>        struct swait_queue_head srcu_wq; >>>>>> @@ -59,7 +59,7 @@ static inline int __srcu_read_lock(struct >>>>>> srcu_struct *ssp) >>>>>>    { >>>>>>        int idx; >>>>>> -    idx = READ_ONCE(ssp->srcu_idx); >>>>>> +    idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1; >>>>>>        WRITE_ONCE(ssp->srcu_lock_nesting[idx], >>>>>> ssp->srcu_lock_nesting[idx] + 1); >>>>>>        return idx; >>>>>>    } >>>>> >>>>> Need change in idx calcultion in srcu_torture_stats_print() ? >>>>> >>>>> static inline void srcu_torture_stats_print(struct srcu_struct *ssp, >>>>>    idx = READ_ONCE(ssp->srcu_idx) & 0x1; >>>> >>>> Excellent point!  It should match the calculation in __srcu_read_lock(), >>>> shouldn't it?  I have updated this, thank you! >>>> >>>>                             Thanx, Paul >>>> >>> >>> Updated version looks good! >>> >>> >>> Thanks >>> Neeraj >>> >> >> For the version in rcu -dev: >> >> Reviewed-by: Neeraj Upadhyay > > I applied all of these, thank you very much! > Welcome :) >> Only minor point which I have is, the idx calculation can be made an inline >> func (though srcu_drive_gp() does not require a READ_ONCE for ->srcu_idx): >> >> __srcu_read_lock() and srcu_torture_stats_print() are using >> >> idx = ((READ_ONCE(ssp->srcu_idx) + 1) & 0x2) >> 1; >> >> whereas srcu_drive_gp() uses: >> >> idx = (ssp->srcu_idx & 0x2) / 2; > > They do work on different elements of the various arrays. Or do you > believe that the srcu_drive_gp() use needs adjusting? My bad, I missed that they are using different elements of array. Please ignore this comment. Thanks Neeraj > > Either way, the overhead of READ_ONCE() is absolutely not at all > a problem. Would you like to put together a patch so that I can see > exactly what you are suggesting? > > Thanx, Paul > >> Thanks >> Neeraj >> >>>>> Thanks >>>>> Neeraj >>>>> >>>>>> diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c >>>>>> index 6208c1d..5598cf6 100644 >>>>>> --- a/kernel/rcu/srcutiny.c >>>>>> +++ b/kernel/rcu/srcutiny.c >>>>>> @@ -124,11 +124,12 @@ void srcu_drive_gp(struct work_struct *wp) >>>>>>        ssp->srcu_cb_head = NULL; >>>>>>        ssp->srcu_cb_tail = &ssp->srcu_cb_head; >>>>>>        local_irq_enable(); >>>>>> -    idx = ssp->srcu_idx; >>>>>> -    WRITE_ONCE(ssp->srcu_idx, !ssp->srcu_idx); >>>>>> +    idx = (ssp->srcu_idx & 0x2) / 2; >>>>>> +    WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); >>>>>>        WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* >>>>>> srcu_read_unlock() wakes! */ >>>>>>        swait_event_exclusive(ssp->srcu_wq, >>>>>> !READ_ONCE(ssp->srcu_lock_nesting[idx])); >>>>>>        WRITE_ONCE(ssp->srcu_gp_waiting, false); /* >>>>>> srcu_read_unlock() cheap. */ >>>>>> +    WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); >>>>>>        /* Invoke the callbacks we removed above. */ >>>>>>        while (lh) { >>>>>> >>>>> >>>>> -- >>>>> 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 -- QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, hosted by The Linux Foundation