From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 7D54B39A7E9; Tue, 24 Feb 2026 12:35:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771936550; cv=none; b=FRL8zzet8RVVRv7aERNLYwWNeVtD+70aYN5RsbugBJfFcLZLZYx+85iEDw7sPsBOifyB+8IRRsSjLd+gzYOlqDk6WiJmj7i40dw6kUATsW+h/MmfYkxYN+lABRRz1LMNLR365SbbUaT49z9f2qMEVGfSIjhEdyi4o6Wv4A1mYZk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771936550; c=relaxed/simple; bh=lAQHMqJMqxvKFWnZMONOUGqsYIRDRmdvn21slxsCB2E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=u6IgKQWlgQQLgbvUMZrt23qS8R11ZXaE3WJmKjbFiRKV3lMWh1ZON6LIS37wtfw/CYjE4bk+bIciOzRfNIPtaAXYz6voIRPZzj56IMB+r1zer151tEcA2hmSNubKChZMul3NJlaYoCxdhrz4GELw6EqsbhyTLnrJU7OxlCPeraM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=Mb1Y/bzC; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="Mb1Y/bzC" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=NmoZ7yGJ/UNmDeoZv4NVg492ij4hS7di67j0tIjaI44=; b=Mb1Y/bzCW8J3tp0nmMGZqbjymr 5EX5/+a+CBFyQdjkfeY0I9l1y7F/QqiAxPTkIcX0JRls+skfPSD3p05DpDataRXPFzz1ayrY3Q0vl TDNDXUHKzKSIA5m/fAfzZTSyIi6zA1cLvKC7OkL4alEbpG2bBz6QgAdhusVTxHL25/fCmoclIduXk zTIlExN02ALquUMvlrmTX7zdOszPzuie/d413jqURBzfo2JC37LPixPwAo3Zqh+I9bZ0EknT4HevW Ncy5VAeL7zTMmC95PJNeT0ONdIO8tbbj/Fl/lEwp+gYgTsdNssWNZoklmqPJHKJJlUQFmQUXNQY4g zlLOVZoA==; Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vurdg-0000000761b-0wfK; Tue, 24 Feb 2026 12:35:44 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 8E54830095A; Tue, 24 Feb 2026 13:35:38 +0100 (CET) Date: Tue, 24 Feb 2026 13:35:38 +0100 From: Peter Zijlstra To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, dave.hansen@intel.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails Message-ID: <20260224123538.GW1282955@noisy.programming.kicks-ass.net> References: <20260224030700.35857-1-lance.yang@linux.dev> <20260224114152.GX1395266@noisy.programming.kicks-ass.net> <5cc90e7a-ae32-4352-8e0b-2eee5a5ee122@linux.dev> 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=us-ascii Content-Disposition: inline In-Reply-To: <5cc90e7a-ae32-4352-8e0b-2eee5a5ee122@linux.dev> On Tue, Feb 24, 2026 at 08:18:46PM +0800, Lance Yang wrote: > > > On 2026/2/24 19:41, Peter Zijlstra wrote: > > On Tue, Feb 24, 2026 at 11:07:00AM +0800, Lance Yang wrote: > > > From: Lance Yang > > > > > > When freeing page tables, we try to batch them. If batch allocation fails > > > (GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without > > > batching. > > > > > > On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via > > > tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single > > > process is unmapping memory. IPI broadcast was reported to hurt RT > > > workloads[1]. > > > > > > tlb_remove_table_sync_one() synchronizes with lockless page-table walkers > > > (e.g. GUP-fast) that rely on IRQ disabling. These walkers use > > > local_irq_disable(), which is also an RCU read-side critical section. > > > > > > This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace > > > period (synchronize_rcu()) instead of IPI broadcast. This provides the > > > same guarantee as IPI but without disrupting all CPUs. Since batch > > > allocation already failed, we are in a way slow path where sleeping is > > > acceptable - we are in process context (unmap_region, exit_mmap) with only > > > mmap_lock held. might_sleep() will catch any invalid context. > > > > So sending the IPIs also requires non-atomic context, so change there. > > Yeah, you're right! > > > What isn't explained, and very much not clear to me, is why > > tlb_remove_table_sync_one() is retained? > > Good point. tlb_remove_table_sync_one() is still needed in: > > 1) khugepaged (mm/khugepaged.c) - after pmdp_collapse_flush() > 2) tlb_finish_mmu() (tlb.h) - when tlb->fully_unshared_tables > 3) ... > > These are not slow paths like batch allocation failure. This patch only > converts this obvious slow path first. > > I'm working on converting the remaining callers as well, but not with > RCU, looking at other options (e.g. targeted IPI). OK, so with that addition to the Changelog, Acked-by: Peter Zijlstra (Intel)