From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id DFE5D1401F for ; Wed, 27 Mar 2024 10:01:05 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711533667; cv=none; b=qi35GpVGhFmmTvG4W+UcULRPDmQpa2nGkEbHl7XbUH0VFPooM3K11Vt081LJQAa8+76Z3MjqIBoDCy5jB8RC+aSJko43CwL3FFNHCve4dGofTZZR9sQ4x6+CxvoLAdrW/zGnIHdTkXrxJ2MZUhvsgjlagT1HkoibfELS+IaIppY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711533667; c=relaxed/simple; bh=DCYOGDjSl/r1Jo9son8VDKwF+TN914bBQlUbMkz+Fs4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=i/cUZysru8eFCImaI6fRXOSvuoUP8QX/W8IqZmYfBakBCt+bXpo0SjEhF7ds9f/eUKl+v2TZdQz0X0R1CcVJDFWFfJZX55qnQAABgQIBPO+wmFLOy3vhAr4rOpnm66NlljotXU+EaoZLYVHgDcofLLLbcC9Xq+Yw2IkNoEdBsew= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id E82BC2F4; Wed, 27 Mar 2024 03:01:38 -0700 (PDT) Received: from [10.57.72.121] (unknown [10.57.72.121]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id ACA2E3F694; Wed, 27 Mar 2024 03:01:02 -0700 (PDT) Message-ID: Date: Wed, 27 Mar 2024 10:01:00 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 0/4] Reduce cost of ptep_get_lockless on arm64 Content-Language: en-GB To: David Hildenbrand , Mark Rutland , Catalin Marinas , Will Deacon , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , Andrew Morton , Muchun Song Cc: linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20240215121756.2734131-1-ryan.roberts@arm.com> <0ae22147-e1a1-4bcb-8a4c-f900f3f8c39e@redhat.com> <374d8500-4625-4bff-a934-77b5f34cf2ec@arm.com> <8bd9e136-8575-4c40-bae2-9b015d823916@redhat.com> <86680856-2532-495b-951a-ea7b2b93872f@arm.com> <35236bbf-3d9a-40e9-84b5-e10e10295c0c@redhat.com> From: Ryan Roberts In-Reply-To: <35236bbf-3d9a-40e9-84b5-e10e10295c0c@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 27/03/2024 09:34, David Hildenbrand wrote: > On 26.03.24 18:51, Ryan Roberts wrote: >> On 26/03/2024 17:39, David Hildenbrand wrote: >>> On 26.03.24 18:32, Ryan Roberts wrote: >>>> On 26/03/2024 17:04, David Hildenbrand wrote: >>>>>>>>> >>>>>>>>> Likely, we just want to read "the real deal" on both sides of the >>>>>>>>> pte_same() >>>>>>>>> handling. >>>>>>>> >>>>>>>> Sorry I'm not sure I understand? You mean read the full pte including >>>>>>>> access/dirty? That's the same as dropping the patch, right? Of course if >>>>>>>> we do >>>>>>>> that, we still have to keep pte_get_lockless() around for this case. In an >>>>>>>> ideal >>>>>>>> world we would convert everything over to ptep_get_lockless_norecency() and >>>>>>>> delete ptep_get_lockless() to remove the ugliness from arm64. >>>>>>> >>>>>>> Yes, agreed. Patch #3 does not look too crazy and it wouldn't really affect >>>>>>> any >>>>>>> architecture. >>>>>>> >>>>>>> I do wonder if pte_same_norecency() should be defined per architecture >>>>>>> and the >>>>>>> default would be pte_same(). So we could avoid the mkold etc on all other >>>>>>> architectures. >>>>>> >>>>>> Wouldn't that break it's semantics? The "norecency" of >>>>>> ptep_get_lockless_norecency() means "recency information in the returned pte >>>>>> may >>>>>> be incorrect". But the "norecency" of pte_same_norecency() means "ignore the >>>>>> access and dirty bits when you do the comparison". >>>>> >>>>> My idea was that ptep_get_lockless_norecency() would return the actual >>>>> result on >>>>> these architectures. So e.g., on x86, there would be no actual change in >>>>> generated code. >>>> >>>> I think this is a bad plan... You'll end up with subtle differences between >>>> architectures. >>>> >>>>> >>>>> But yes, the documentation of these functions would have to be improved. >>>>> >>>>> Now I wonder if ptep_get_lockless_norecency() should actively clear >>>>> dirty/accessed bits to more easily find any actual issues where the bits still >>>>> matter ... >>>> >>>> I did a version that took that approach. Decided it was not as good as this way >>>> though. Now for the life of me, I can't remember my reasoning. >>> >>> Maybe because there are some code paths that check accessed/dirty without >>> "correctness" implications? For example, if the PTE is already dirty, no need to >>> set it dirty etc? >> >> I think I decided I was penalizing the architectures that don't care because all >> their ptep_get_norecency() and ptep_get_lockless_norecency() need to explicitly >> clear access/dirty. And I would have needed ptep_get_norecency() from day 1 so >> that I could feed its result into pte_same(). > > True. With ptep_get_norecency() you're also penalizing other architectures. > Therefore my original thought about making the behavior arch-specific, but the > arch has to make sure to get the combination of > ptep_get_lockless_norecency()+ptep_same_norecency() is right. > > So if an arch decide to ignore bits in ptep_get_lockless_norecency(), it must > make sure to also ignore them in ptep_same_norecency(), and must be able to > handle access/dirty bit changes differently. > > Maybe one could have one variant for "hw-managed access/dirty" vs. "sw managed > accessed or dirty". Only the former would end up ignoring stuff here, the latter > would not. > > But again, just some random thoughts how this affects other architectures and > how we could avoid it. The issue I describe in patch #3 would be gone if > ptep_same_norecency() would just do a ptep_same() check on other architectures > -- and would make it easier to sell :) Perhaps - let me chew on that for a bit. It doesn't feel as easy as you suggest to me. But I can't put my finger on why...