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 64414C07E9D for ; Mon, 26 Sep 2022 18:04:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229815AbiIZSED (ORCPT ); Mon, 26 Sep 2022 14:04:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54678 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231158AbiIZSDi (ORCPT ); Mon, 26 Sep 2022 14:03:38 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [145.40.68.75]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id DBB82207; Mon, 26 Sep 2022 10:45:28 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 81CCEB80B9C; Mon, 26 Sep 2022 17:45:27 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0972BC433D6; Mon, 26 Sep 2022 17:45:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1664214326; bh=Bn1HSf7II+l0C0Thc4GW4IZtTClrYEYo9ov431I8puA=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=HfX/APbfw1n0RbnqwlbUysPcsqzAaX6+uKWg4yCqy2VHR1bSrldnKgonuy5nXVJr1 qVouHIH5whFMsZ32r/UbCs+YDpQ8OgMcC64jZJucGOAFQTW4i/QZGgHNzF5poL8Wk7 01WVl8VCqeAxBFuAnqGhlwmrJOBmDccU42uduJZIBQ7exf4mi5Ly3iitQ5iMcLuYuk ls7Qouv1H8sQtAWfUzrWyHZdR8vTUmNLD4kGoDPCiCmsOxxZAR+JvWgRyuINKDDp0m m81doRUwQ3BbFz/t2neGoyzp48G6QzDWDkeFL6PQVpW9tFz/eoGx8PeShXw7OeM5TY /xH2TIzrBjItA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 9F3F45C05B1; Mon, 26 Sep 2022 10:45:25 -0700 (PDT) Date: Mon, 26 Sep 2022 10:45:25 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Joel Fernandes , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, rushikesh.s.kadam@intel.com, urezki@gmail.com, neeraj.iitr10@gmail.com, rostedt@goodmis.org Subject: Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power Message-ID: <20220926174525.GP4196@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20220924224637.GA161871@lothringen> <19217A4C-7183-4D78-A714-FBFE7BB20742@joelfernandes.org> <20220925220936.GA182999@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20220925220936.GA182999@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Sep 26, 2022 at 12:09:36AM +0200, Frederic Weisbecker wrote: > On Sat, Sep 24, 2022 at 07:28:16PM -0400, Joel Fernandes wrote: > > > And then FLUSH_BP_WAKE is probably not needed anymore. > > > > It is needed as the API is in tree_nocb.h and we > > have to have that handle the details of laziness > > there rather than tree.c. We could add new apis > > to get rid of flag but it’s cleaner (and Paul seemed > > to be ok with it). > > If the wake up is handled outside the flush function, as in the > diff I just posted, there is no more user left of FLUSH_BP_WAKE, IIRC... To get rid of FLUSH_BP_WAKE, we might need to pull some rcu_data fields out from under #ifdef in order to allow them to be accessed by common code. Which might be a good tradeoff, as the size of rcu_data has not been a concern. Plus the increase in size would be quite small. Thanx, Paul > > >> @@ -512,9 +598,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, > > >> } > > >> // Need to actually to a wakeup. > > >> len = rcu_segcblist_n_cbs(&rdp->cblist); > > >> + bypass_len = rcu_cblist_n_cbs(&rdp->nocb_bypass); > > >> + lazy_len = READ_ONCE(rdp->lazy_len); > > >> if (was_alldone) { > > >> rdp->qlen_last_fqs_check = len; > > >> - if (!irqs_disabled_flags(flags)) { > > >> + // Only lazy CBs in bypass list > > >> + if (lazy_len && bypass_len == lazy_len) { > > >> + rcu_nocb_unlock_irqrestore(rdp, flags); > > >> + wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, > > >> + TPS("WakeLazy")); > > > > > > I'm trying to think of a case where rcu_nocb_try_bypass() returns false > > > (queue to regular list) but then call_rcu() -> __call_rcu_nocb_wake() ends up > > > seeing a lazy bypass queue even though we are queueing a non-lazy callback > > > (should have flushed in this case). > > > > > > Looks like it shouldn't happen, even with concurrent (de-offloading) but just > > > in case, can we add: > > > > Yes I also feel this couldn’t happen because irq is > > off and nocb lock is held throughout the calls to > > the above 2 functions. Unless I missed the race > > you’re describing? > > At least I can't find any either... > > > > > > > > > WARN_ON_ONCE(lazy_len != len) > > > > But this condition can be true even in normal > > circumstances? len also contains DONE CBs > > which are ready to be invoked. Or did I miss > > something? > > Duh, good point, nevermind then :-) > > Thanks. > > > > > Thanks, > > > > - Joel > > > > > > > >> + } else if (!irqs_disabled_flags(flags)) { > > >> /* ... if queue was empty ... */ > > >> rcu_nocb_unlock_irqrestore(rdp, flags); > > >> wake_nocb_gp(rdp, false); > > > > > > Thanks.