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(p200300cbc7336400ae104bb797128548.dip0.t-ipconnect.de. [2003:cb:c733:6400:ae10:4bb7:9712:8548]) by smtp.gmail.com with ESMTPSA id g8-20020a5d5408000000b0031c5b380291sm9838763wrv.110.2023.10.09.07.43.30 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 09 Oct 2023 07:43:30 -0700 (PDT) Message-ID: Date: Mon, 9 Oct 2023 16:43:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v6 6/9] mm: thp: Add "recommend" option for anon_orders Content-Language: en-US To: Ryan Roberts , Yu Zhao Cc: Andrew Morton , Matthew Wilcox , Yin Fengwei , Catalin Marinas , Anshuman Khandual , Yang Shi , "Huang, Ying" , Zi Yan , Luis Chamberlain , Itaru Kitayama , "Kirill A. Shutemov" , John Hubbard , David Rientjes , Vlastimil Babka , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20230929114421.3761121-1-ryan.roberts@arm.com> <20230929114421.3761121-7-ryan.roberts@arm.com> <2f64809e-0d0d-cc61-71ac-8d9b072efc3a@redhat.com> <25d1cdee-3da8-4728-aa0d-dc07eb28ea95@arm.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <25d1cdee-3da8-4728-aa0d-dc07eb28ea95@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09.10.23 13:45, Ryan Roberts wrote: > On 06/10/2023 23:28, Yu Zhao wrote: >> On Fri, Oct 6, 2023 at 2:08 PM David Hildenbrand wrote: >>> >>> On 29.09.23 13:44, Ryan Roberts wrote: >>>> In addition to passing a bitfield of folio orders to enable for THP, >>>> allow the string "recommend" to be written, which has the effect of >>>> causing the system to enable the orders preferred by the architecture >>>> and by the mm. The user can see what these orders are by subsequently >>>> reading back the file. >>>> >>>> Note that these recommended orders are expected to be static for a given >>>> boot of the system, and so the keyword "auto" was deliberately not used, >>>> as I want to reserve it for a possible future use where the "best" order >>>> is chosen more dynamically at runtime. >>>> >>>> Recommended orders are determined as follows: >>>> - PMD_ORDER: The traditional THP size >>>> - arch_wants_pte_order() if implemented by the arch >>>> - PAGE_ALLOC_COSTLY_ORDER: The largest order kept on per-cpu free list >>>> >>>> arch_wants_pte_order() can be overridden by the architecture if desired. >>>> Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous >>>> set of ptes map physically contigious, naturally aligned memory, so this >>>> mechanism allows the architecture to optimize as required. >>>> >>>> Here we add the default implementation of arch_wants_pte_order(), used >>>> when the architecture does not define it, which returns -1, implying >>>> that the HW has no preference. >>>> >>>> Signed-off-by: Ryan Roberts >>>> --- >>>> Documentation/admin-guide/mm/transhuge.rst | 4 ++++ >>>> include/linux/pgtable.h | 13 +++++++++++++ >>>> mm/huge_memory.c | 14 +++++++++++--- >>>> 3 files changed, 28 insertions(+), 3 deletions(-) >>>> >>>> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst >>>> index 732c3b2f4ba8..d6363d4efa3a 100644 >>>> --- a/Documentation/admin-guide/mm/transhuge.rst >>>> +++ b/Documentation/admin-guide/mm/transhuge.rst >>>> @@ -187,6 +187,10 @@ pages (=16K if the page size is 4K). The example above enables order-9 >>>> By enabling multiple orders, allocation of each order will be >>>> attempted, highest to lowest, until a successful allocation is made. >>>> If the PMD-order is unset, then no PMD-sized THPs will be allocated. >>>> +It is also possible to enable the recommended set of orders, which >>>> +will be optimized for the architecture and mm:: >>>> + >>>> + echo recommend >/sys/kernel/mm/transparent_hugepage/anon_orders >>>> >>>> The kernel will ignore any orders that it does not support so read the >>>> file back to determine which orders are enabled:: >>>> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h >>>> index af7639c3b0a3..0e110ce57cc3 100644 >>>> --- a/include/linux/pgtable.h >>>> +++ b/include/linux/pgtable.h >>>> @@ -393,6 +393,19 @@ static inline void arch_check_zapped_pmd(struct vm_area_struct *vma, >>>> } >>>> #endif >>>> >>>> +#ifndef arch_wants_pte_order >>>> +/* >>>> + * Returns preferred folio order for pte-mapped memory. Must be in range [0, >>>> + * PMD_ORDER) and must not be order-1 since THP requires large folios to be at >>>> + * least order-2. Negative value implies that the HW has no preference and mm >>>> + * will choose it's own default order. >>>> + */ >>>> +static inline int arch_wants_pte_order(void) >>>> +{ >>>> + return -1; >>>> +} >>>> +#endif >>>> + >>>> #ifndef __HAVE_ARCH_PTEP_GET_AND_CLEAR >>>> static inline pte_t ptep_get_and_clear(struct mm_struct *mm, >>>> unsigned long address, >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index bcecce769017..e2e2d3906a21 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -464,10 +464,18 @@ static ssize_t anon_orders_store(struct kobject *kobj, >>>> int err; >>>> int ret = count; >>>> unsigned int orders; >>>> + int arch; >>>> >>>> - err = kstrtouint(buf, 0, &orders); >>>> - if (err) >>>> - ret = -EINVAL; >>>> + if (sysfs_streq(buf, "recommend")) { >>>> + arch = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER); >>>> + orders = BIT(arch); >>>> + orders |= BIT(PAGE_ALLOC_COSTLY_ORDER); >>>> + orders |= BIT(PMD_ORDER); >>>> + } else { >>>> + err = kstrtouint(buf, 0, &orders); >>>> + if (err) >>>> + ret = -EINVAL; >>>> + } >>>> >>>> if (ret > 0) { >>>> orders &= THP_ORDERS_ALL_ANON; >>> >>> :/ don't really like that. Regarding my proposal, one could have >>> something like that in an "auto" setting for the "enabled" value, or a >>> "recommended" setting [not sure]. >> >> Me either. >> >> Again this is something I call random -- we only discussed "auto", >> and yes, the commit message above explained why "recommended" here but >> it has never surfaced in previous discussions, has it? > > The context in which we discussed "auto" was for a future aspiration to > automatically determine the order that should be used for a given allocation to > balance perf vs internal fragmentation. > > The case we are talking about here is completely different; I had a pre-existing > feature from previous versions of the series, which would allow the arch to > specify its preferred order (originally proposed by Yu, IIRC). In moving the > allocation size decision to user space, I felt that we still needed a mechanism > whereby the arch could express its preference. And "recommend" is what I came up > with. > > All of the friction we are currently having is around this feature, I think? > Certainly all the links you provided in the other thread all point to > conversations skirting around it. How about I just drop it for this initial > patch set? Just let user space decide what sizes it wants (per David's interface > proposal)? I can see I'm trying to get a square peg into a round hole. Dropping it for the initial patch set sounds like a very good idea. Telling people what to enable initially when they want to play with it will work out just fine. [Ideally, we plan ahead to have such "auto" settings in the future, as I expressed.] -- Cheers, David / dhildenb