From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794709877; b=kV5Kepdda8+cdT2jm11ulWnM2AJXf26eFMOw12+pBusHz3OH6wj1txRZEXwpwiIQJJSw bFnICKqtkI1H6nGXDsr53cnKJP+ISFPZH2W6q99XlY0eTB26dcQcZmsARlZclqsBTK+sI jrDY5RxZ2OJATPsj/Qt0J3WX2QrBIVduzELaQqR5vMbc64bFVMWo983xN/OK1P/mk/T+8 J1L1BEHec6RV4b5t3XuoLeS3fPmDM6J+vOziGGTIGyYtFFRSsnlEGVnVxaVr5+aagS4t1 czC5dzRENJLfBTC/hHeJi1t/wTa08hSpHoAzLUb7bSkAnsxTb5Cuib3+fSmG14cTu1Q== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794709877; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=stSLXlobOeNYQpr82D2/1dF4bsjCQsp5v/wivCmGpZE=; b=1JyukNEblv8IiwZ9qobDsjIlHZHwth3SmeDmXeB7NBzxv2Imd69AQHUcn/Dgzeqgv2Uf A39mPYQrk5VZwtcxGFGvyWEnVj/LVVeqbhd4LqBc7E4600xwKTLB8qAnowckH1KQb+Iqn Pjv/fqj63MYIOl4DZlNuW/Lgt74sZjYYgH2w7Gef0LKxdUE0zP/zM/5UguCCpA1vmetz9 D7JaR4niweoOmRGf+RgBt0jHbt7qzKgMknOb4CtUyvAPRNOmC7nzA/Xlkfh0IQaIKEC+C 5jmOzY2zwL1UzWKmbifByLB2JavxCziJYgb9MfwtGDepMUlOcLGmMzfDsKeyXLgJ7BQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=GClbfwYG; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with UTF8SMTPS id 41016606A327; Sun, 15 Nov 2026 02:31:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=GClbfwYG Received: by smtp.kernel.org (Postfix) with UTF8SMTPSA id 597D61F000C4; Sun, 15 Nov 2026 02:31:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794709876; bh=stSLXlobOeNYQpr82D2/1dF4bsjCQsp5v/wivCmGpZE=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=GClbfwYGCafBU0ZDAyu0rjj2UH0PxwIhVG0H/EuvQuKw4OuKaPzy9PGEKI5SmA46B KpXpc7BLrP4Mt46AXlOs5XWGX1RQ0yE2xaCockLko979jDpgg/e/1bSadwTvWjcqBQ j8zxX6pS3qkEE5srinmB9pkYHp6T5JiSoAKkZ4nWH3RSPZLxIdYxGCeSVoIlbinJeT nOrrvC1lRKsDmsxx5fZyA8pqFSp3V8rqbe45/Z46sD0C3ImX6npxAcIbU9iAr8HH4D ZGbG3KfkzSihYShrRMW72iopMM6oUupIYVjCUh0sBjXYZ1UrU4qTIviuydQEyFgLF0 mRNhi1UTDoUvw== Date: Sat, 14 Nov 2026 21:31:15 -0500 From: jfern@kernel.org To: Chris Wiseman Cc: Frederic Weisbecker , "Paul E. McKenney" , Joel Fernandes , Joel Fernandes , Neeraj Upadhyay , Uladzislau Rezki , Boqun Feng , Josh Triplett , Tze-nan Wu , Cheng-jui Wang , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , rcu@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] rcu/nocb: Keep lazy timer intact on second lazy callback Message-ID: <20261115023115.GA3752240@joelbox> References: <18e815d3ba49a353.3137322e32332e3235332e313736.192c60.0000sendgmr@google.com> X-Mailing-List: rcu@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <18e815d3ba49a353.3137322e32332e3235332e313736.192c60.0000sendgmr@google.com> On chriswiseman@google.com wrote: > wake_nocb_gp_defer() checks waketype == RCU_NOCB_WAKE_LAZY and > rdp_gp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT in one if-statement. > > When the first lazy call_rcu() in a NOCB group arms rdp_gp->nocb_timer > for the 10-second lazy flush delay and sets rdp_gp->nocb_defer_wakeup to > RCU_NOCB_WAKE_LAZY, the next CPU in the group that queues its first lazy > callback calls wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, ...). Because > rdp_gp->nocb_defer_wakeup is no longer RCU_NOCB_WAKE_NOT, it misses the > first branch and falls into the else block for RCU_NOCB_WAKE, calling > mod_timer(&rdp_gp->nocb_timer, jiffies + 1) and resetting the 10-second > lazy timer down to 1 jiffy. The same fallthrough also shortens a pending > 2-jiffy RCU_NOCB_WAKE_BYPASS timer to 1 jiffy. > > Commit 3cb278e73be5 ("rcu: Make call_rcu() lazy to save power") wrote > this check against rdp->nocb_defer_wakeup, and commit 463d46044f04 > ("rcu: Fix delayed execution of hurry callbacks") switched it to > rdp_gp->nocb_defer_wakeup, which exposed the fallthrough across all CPUs > sharing rdp_gp. > > Give waketype == RCU_NOCB_WAKE_LAZY its own branch and only arm > nocb_timer when rdp_gp->nocb_defer_wakeup is RCU_NOCB_WAKE_NOT. > > Fixes: 3cb278e73be5 ("rcu: Make call_rcu() lazy to save power") > Fixes: 463d46044f04 ("rcu: Fix delayed execution of hurry callbacks") > Cc: stable@vger.kernel.org > Signed-off-by: Chris Wiseman Looks reasonable to me. Is there a test case that you added to find this, or any power savings that you found with this added patch? thanks, Joel Fernandes > --- > kernel/rcu/tree_nocb.h | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index b6cfee4e5039..347d718befd0 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -335,10 +335,12 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype, > * Bypass wakeup overrides previous deferments. In case of > * callback storms, no need to wake up too early. > */ > - if (waketype == RCU_NOCB_WAKE_LAZY && > - rdp_gp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) { > - mod_timer(&rdp_gp->nocb_timer, jiffies + rcu_get_jiffies_lazy_flush()); > - WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype); > + if (waketype == RCU_NOCB_WAKE_LAZY) { > + if (rdp_gp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) { > + mod_timer(&rdp_gp->nocb_timer, > + jiffies + rcu_get_jiffies_lazy_flush()); > + WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype); > + } > } else if (waketype == RCU_NOCB_WAKE_BYPASS) { > mod_timer(&rdp_gp->nocb_timer, jiffies + 2); > WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype); > -- > 2.52.0.rc1