From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 95BB51CAB5 for ; Fri, 7 Jun 2024 22:46:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717800384; cv=none; b=R57KQK8GEBR6nruNsED4NRL3QQgOoIDKwYS6e2hO5s6qG5Wb1CTfvqudfY4/mA0dyTmoPCmPZ/VXg7UEgoWFQdUEq7p6toZFiTADk5INq/Eh4pa7H5umDb5/jvX79Fwfh5S+hduB9jiiVjN1ypesMZzr/RjZ5PCqaMAjcwTCvVA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1717800384; c=relaxed/simple; bh=GjJb35OraHi0SzFdMQYu8ta0vOZOofLfX73CdwH1hRg=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=QHpl89pPzAjBOZIZMqNp1fiDeM9ltdUCHLmX5Gh5157PGBPPLn4pzG5CEEa8Y0EcRFdFyN0s6dIYNlcS6uLVenpe6LFhamIKWX2T2CzzR3wyKQEGwAyjSRwv1OORm6CPhUiKuJKtAVkbh3/EYygCOE/L+vgvmc8KOTJhqDb42I4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=CN1sNR/h; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="CN1sNR/h" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C39B2C2BBFC; Fri, 7 Jun 2024 22:46:23 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1717800384; bh=GjJb35OraHi0SzFdMQYu8ta0vOZOofLfX73CdwH1hRg=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=CN1sNR/hK2tkp1kTtzSZFm4lO60PkEXYuV/Z+2Fn3XGcJXpeznutuCCoR/8K+1hm3 0aZo0qUVG1FvY6xwNSfLA19KtLc4ZbBC0lD4c6E4bTWfxj4D6S4ynwYm3GRqALQUXS zXpRLx0HvAcA75f0OtYeDBGPCPBNtqxyqMB8Bg8mz9HahqyLPWQGq7KtsZUedTYtsY 0wVQrrwjH2qCZFWluTe+r1MTVq5g0wT8W2FQ7FJH8gZtGo5wP/Ymh863GLI18Q9uwR gFQlhbsPRkc9xxjvW6t7sSH4JmpLbWLth4Hd94PnziORQurGfFQV4mvyK1uJZIx9gH ESqnBBLa3Na3w== Date: Sat, 8 Jun 2024 00:46:21 +0200 From: Frederic Weisbecker To: Ze Gao Cc: Joel Fernandes , Thomas Gleixner , Peter Zijlstra , Ingo Molnar , Cruz Zhao , linux-kernel@vger.kernel.org, Ze Gao Subject: Re: [RFC PATCH 1/2] timer: Use is_idle_task() check instead of idle_cpu() on irq exit Message-ID: References: <20240530122401.3826022-1-zegao@tencent.com> <20240530122401.3826022-2-zegao@tencent.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20240530122401.3826022-2-zegao@tencent.com> Le Thu, May 30, 2024 at 08:24:00AM -0400, Ze Gao a écrit : > idle_cpu() was initially introduced in irq_enter()/exit() to check > whether an irq interrupts an idle cpu or not since commit > 79bf2bb335b8 ("[PATCH] tick-management: dyntick / highres functionality") > and at that time, it's implemented via a simple check if the curr > of task of that rq is idle or not. And then commit 6378ddb59215 ("time: > track accurate idle time with tick_sched.idle_sleeptime") uses the same > check to do accurate idle time accounting. > > But since commit 908a3283728d ("sched: Fix idle_cpu()"), idle_cpu() > takes scheduler stats into consideration and becomes more constrained, > and therefore it tells more than if we have interrupted an idle > process but also whether a cpu is going to be idle or not since it > takes queued tasks and queued to be woken tasks into account. > > However for tick user, it is too much as now we only rely on this check > to do nohz idle time accounting in tick_nohz_start_idle() just in case > that tick_nohz_stop_idle() is called upon irq_enter() if we actually > rupture an idle cpu(process). The use of idle_cpu() simply complicates > things here, and the introduction of sched_core_idle_cpu() in > commit 548796e2e70b ("sched/core: introduce sched_core_idle_cpu()") > proves this. > > The use of is_idle_task() just like in commit 0a8a2e78b7ee ("timer: Fix > bad idle check on irq entry") helps to save one unnecessary fix for idle > time accounting for the newly force idle state. Note this also preps for > the remove of sched_core_idle_cpu() in the following patch. > > Signed-off-by: Ze Gao > --- > kernel/softirq.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/softirq.c b/kernel/softirq.c > index 02582017759a..24c7bf3c3f6c 100644 > --- a/kernel/softirq.c > +++ b/kernel/softirq.c > @@ -617,7 +617,7 @@ static inline void tick_irq_exit(void) > int cpu = smp_processor_id(); > > /* Make sure that timer wheel updates are propagated */ > - if ((sched_core_idle_cpu(cpu) && !need_resched()) || tick_nohz_full_cpu(cpu)) { > + if ((is_idle_task(current) && !need_resched()) || tick_nohz_full_cpu(cpu)) { The reason why there is a check here for idle_cpu() (or sched_core_idle_cpu()) is to avoid calling again tick_nohz_start_idle() and then again tick_nohz_stop_idle() later from tick_nohz_idle_exit(). This can save two costly calls to ktime_get() when a real task is waiting for the CPU. So any quick clue to know if a task is going to be scheduled is good to get. And idle_cpu() gives them all: int idle_cpu(int cpu) { struct rq *rq = cpu_rq(cpu); if (rq->curr != rq->idle) return 0; // This is the necessary is_idle_task() check if (rq->nr_running) return 0; // This tells if there is a real task pending. Ok that check // is perhaps a bit redundant with need_resched()... #ifdef CONFIG_SMP if (rq->ttwu_pending) return 0; #endif // This one tells if there is a remote wakeup pending for this CPU. // And need_resched() doesn't tell about that yet... return 1; } So it looks to me that idle_cpu() is still a good fit at this place. And sched_core_idle_cpu() doesn't bring more overhead since the static key in sched_core_enabled() is rarely active (I guess...). And if it is, then the check is even more simple. Thanks. > if (!in_hardirq()) > tick_nohz_irq_exit(); > } > -- > 2.41.0 >