From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (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 8F60B847B for ; Thu, 12 Jun 2025 01:39:31 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749692376; cv=none; b=DLyAXiweIi9Kbui6CdcD7aYrPy6/Zimw9pniPj5T6Me1dhl3kjEUdXMSf7WUKaxMm/ZPeik5SWyCr62AvldM90uv45l4UEm03r+58EvRClbwA5H+0unene+yMKKjgHg+yNXr13yT2GqGnD5ORIVa0BMckUnTSNBCdgAYgzSjO3U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749692376; c=relaxed/simple; bh=12dIK4IDDxkHymWycq4Ktg9zaxgm1LEYEl5ko1Z6VGw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=KmFotcG4g8FFVP7ZZCBbR3h+WIG4wtwoUTvBXs4B9/yHEZ3inVtx/CToOImEPJl4Lamz0k8409m22Ij/0fRt2CSfXlg37eZgkBR060GUVfAiVF5FSO8bwW2Wh7HUxawFyH+PElceV6Cmniup0Hp+LBarmYQnWnKEl8tK7ai5uLM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=ljxM02K0; arc=none smtp.client-ip=91.218.175.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="ljxM02K0" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1749692368; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fDaLoebKXYq1dhWh0U4LSCfiXioc71GWbSJENfRMRds=; b=ljxM02K01UuAJs38k0Uj76PvRaegxhg+IoFOPX7g5cdn2lv8wy7HbvFZNTYpQlHIbKTywn TfibyxnagfDXkXUhocJ3w5iUj67ZpqapaQZY6XfVcs0tP8kofRXslDVVmqudq9/zHTJR1d 7uifdhIuKTKINZXxppt+VsC3TJrnBNQ= Date: Thu, 12 Jun 2025 09:38:47 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH] mm/alloc_tag: add the ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable alloc_tag_counters. To: Suren Baghdasaryan Cc: Andrew Morton , Kent Overstreet , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Hao Ge References: <20250529073537.563107-1-hao.ge@linux.dev> <177e1f6b-50f0-4c0a-bb0b-514283e009a2@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Ge In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 2025/6/11 23:24, Suren Baghdasaryan wrote: > On Tue, Jun 10, 2025 at 10:27 PM Hao Ge wrote: >> >> On 2025/6/10 00:39, Suren Baghdasaryan wrote: >>> On Sun, Jun 8, 2025 at 11:08 PM Hao Ge wrote: >>>> On 2025/5/29 15:35, Hao Ge wrote: >>>>> From: Hao Ge >>>>> >>>>> Recently discovered this entry while checking kallsyms on ARM64: >>>>> ffff800083e509c0 D _shared_alloc_tag >>>>> >>>>> If ARCH_NEEDS_WEAK_PER_CPU is not defined,there's no need to statically >>>>> define the percpu variable alloc_tag_counters. >>>>> >>>>> Therefore,add therelevant macro guards at the appropriate location. >>>>> >>>>> Fixes: 22d407b164ff ("lib: add allocation tagging support for memory allocation profiling") >>>>> Signed-off-by: Hao Ge >>>>> --- >>>>> lib/alloc_tag.c | 2 ++ >>>>> 1 file changed, 2 insertions(+) >>>>> >>>>> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c >>>>> index c7f602fa7b23..d1dab80b70ad 100644 >>>>> --- a/lib/alloc_tag.c >>>>> +++ b/lib/alloc_tag.c >>>>> @@ -24,8 +24,10 @@ static bool mem_profiling_support; >>>>> >>>>> static struct codetag_type *alloc_tag_cttype; >>>>> >>>>> +#ifdef ARCH_NEEDS_WEAK_PER_CPU >>>>> DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag); >>>>> EXPORT_SYMBOL(_shared_alloc_tag); >>>>> +#endif /* ARCH_NEEDS_WEAK_PER_CPU */ >>>>> >>>>> DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT, >>>>> mem_alloc_profiling_key); >>>> Hi Suren >>>> >>>> >>>> I'm sorry to bother you. As mentioned in my commit message, >>>> >>>> in fact, on the ARM64 architecture, the _shared_alloc_tag percpu >>>> variable is not needed. >>>> >>>> In my understanding, it will create a copy for each CPU. >>>> >>>> The alloc_tag_counters variable will occupy 16 bytes, >>>> >>>> and as the number of CPUs increases, more and more memory will be wasted >>>> in this segment. >>>> >>>> I realized that this modification was a mistake. It resulted in a build >>>> error, and the link is as follows: >>>> >>>> https://lore.kernel.org/all/202506080448.KWN8arrX-lkp@intel.com/ >>>> >>>> After I studied the comments of DECLARE_PER_CPU_SECTION, I roughly >>>> understood why this is the case. >>>> >>>> But so far, I haven't come up with a good way to solve this problem. Do >>>> you have any suggestions? >>> Hi Hao, >>> The problem here is that ARCH_NEEDS_WEAK_PER_CPU is not a Kconfig >>> option, it gets defined only on 2 architectures and only when building >>> modules here https://elixir.bootlin.com/linux/v6.15.1/source/arch/alpha/include/asm/percpu.h#L14 >>> and here https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21. >>> A nicer way to deal with that is to make if a Kconfig option which is >>> enabled only for alpha and s390 and then do something like this: >>> >>> #if defined(MODULE) && defined(ARCH_NEEDS_WEAK_PER_CPU) >>> #define MODULE_NEEDS_WEAK_PER_CPU >>> #endif >>> >>> and change all the usages of ARCH_NEEDS_WEAK_PER_CPU with >>> MODULE_NEEDS_WEAK_PER_CPU. >>> Did I explain the idea clearly? >>> Thanks, >>> Suren. >>> >> Hi Suren > Hi Hao, > >> Thanks for your guidance. >> I understand this train of thought. >> >> I've been thinking about a problem: I only added the >> ARCH_NEEDS_WEAK_PER_CPU >> >> macro to isolate the definition of _shared_alloc_tag. >> >> Since s390 defines this macro, why did this build error occur? Hi Suren > The problem is that ARCH_NEEDS_WEAK_PER_CPU is not a Kconfig option, > it's just a definition, for s390 it's here: > https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21 > So, even for s390 if you are building core kernel code (not a module), > ARCH_NEEDS_WEAK_PER_CPU will be undefined, however if you are building > a module on s390 then it is defined. So, your change effectively > results in _shared_alloc_tag being compiled out in the core kernel > while it's used when you build a module. Therefore during linking > modules can't link to that symbol in the core kernel. Hope this > explains the issue. Thank you so, so, so much! I understand now, and thank you for such a detailed explanation. > The way I would fix this is by making ARCH_NEEDS_WEAK_PER_CPU a > Kconfig option and enable it for s390 and alpha, would replace old > definitions from > https://elixir.bootlin.com/linux/v6.15.1/source/arch/s390/include/asm/percpu.h#L21 > and https://elixir.bootlin.com/linux/v6.15.1/source/arch/alpha/include/asm/percpu.h#L14 > with: > > #if defined(MODULE) && defined(ARCH_NEEDS_WEAK_PER_CPU) > #define MODULE_NEEDS_WEAK_PER_CPU > #endif > > Then use MODULE_NEEDS_WEAK_PER_CPU instead of ARCH_NEEDS_WEAK_PER_CPU > in all the current places in the kernel code. Lastly, to compile out > _shared_alloc_tag your current patch should work fine because on s390 > and alpha ARCH_NEEDS_WEAK_PER_CPU will be defined after all these > changes. > Does that make sense? I quite agree with this approach. Thanks Best Regards Hao >> Could you please help explain it again? >> >> Thanks >> Best Regards >> Hao >> >>>> Thanks >>>> >>>> Best Regards >>>> >>>> Hao >>>> >>>> >>>> >>>>