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 9744414F113; Fri, 3 May 2024 09:14:25 +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=1714727667; cv=none; b=NyVSRZUGE9Jh6WTT7g10ya1xie/Qw+qIG/OAfcPdgJog1nyyfjf4i+PVtNAWoJr7cC4jjY4ppEYDP6RmHViSb+I/3wdkMMw97SINTIyXgORdXr+dwltIVj45SUrRzi66OwPZ/6l0ARF5giSIR7MQ2C2JTxiCWa3COj9gnrslX0g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714727667; c=relaxed/simple; bh=4ra0slbfvRchcptOPIK9YRDh4kjzt5cthlclRiu0G1g=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=rFEGrJuXKbzZYEYw5hncgn5Q3x5Kh59nnIzr+V8FxNWP5tdr6WMCk9j5jHX6Hs8m2zkG8SQADx/y93VHO/O/OgFjcRn4Of+ABSstmzMz02/poYpVMYF7fCeVERGzIT+BQY2PKTLtZmKYH/W+O8nLungBtect+GrZw02KV9UG5gM= 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 361482F4; Fri, 3 May 2024 02:14:50 -0700 (PDT) Received: from [10.163.34.188] (unknown [10.163.34.188]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BD2CE3F73F; Fri, 3 May 2024 02:14:18 -0700 (PDT) Message-ID: <312ae9b3-e328-410e-9f2f-02e6dccadff2@arm.com> Date: Fri, 3 May 2024 14:44:10 +0530 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] arm64: implement raw_smp_processor_id() using thread_info Content-Language: en-US To: Puranjay Mohan , Catalin Marinas , Will Deacon , Sumit Garg , Stephen Boyd , Douglas Anderson , "Peter Zijlstra (Intel)" , Thomas Gleixner , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, bpf@vger.kernel.org Cc: puranjay12@gmail.com References: <20240502123449.2690-1-puranjay@kernel.org> <20240502123449.2690-2-puranjay@kernel.org> From: Anshuman Khandual In-Reply-To: <20240502123449.2690-2-puranjay@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/2/24 18:04, Puranjay Mohan wrote: > Historically, arm64 implemented raw_smp_processor_id() as a read of > current_thread_info()->cpu. This changed when arm64 moved thread_info > into task struct, as at the time CONFIG_THREAD_INFO_IN_TASK made core > code use thread_struct::cpu for the cpu number, and due to header > dependencies prevented using this in raw_smp_processor_id(). As a > workaround, we moved to using a percpu variable in commit: > > commit 57c82954e77f ("arm64: make cpu number a percpu variable") > > Since then, thread_info::cpu was reintroduced, and core code was made to > use this in commits: > > commit 001430c1910d ("arm64: add CPU field to struct thread_info") > commit bcf9033e5449 ("sched: move CPU field back into thread_info if > THREAD_INFO_IN_TASK=y") > > Consequently it is possible to use current_thread_info()->cpu again. This captures Mark's earlier suggestion on the commit message. > > This decreases the number of emitted instructions like in the following > example: > > Dump of assembler code for function bpf_get_smp_processor_id: > 0xffff8000802cd608 <+0>: nop > 0xffff8000802cd60c <+4>: nop > 0xffff8000802cd610 <+8>: adrp x0, 0xffff800082138000 > 0xffff8000802cd614 <+12>: mrs x1, tpidr_el1 > 0xffff8000802cd618 <+16>: add x0, x0, #0x8 > 0xffff8000802cd61c <+20>: ldrsw x0, [x0, x1] > 0xffff8000802cd620 <+24>: ret > > After this patch: > > Dump of assembler code for function bpf_get_smp_processor_id: > 0xffff8000802c9130 <+0>: nop > 0xffff8000802c9134 <+4>: nop > 0xffff8000802c9138 <+8>: mrs x0, sp_el0 > 0xffff8000802c913c <+12>: ldr w0, [x0, #24] > 0xffff8000802c9140 <+16>: ret > > A microbenchmark[1] was built to measure the performance improvement > provided by this change. It calls the following function given number of > times and finds the runtime overhead: > > static noinline int get_cpu_id(void) > { > return smp_processor_id(); > } > > Run the benchmark like: > modprobe smp_processor_id nr_function_calls=1000000000 > > +--------------------------+------------------------+ > | | Number of Calls | Time taken | > +--------+-----------------+------------------------+ > | Before | 1000000000 | 1602888401ns | > +--------+-----------------+------------------------+ > | After | 1000000000 | 1206212658ns | > +--------+-----------------+------------------------+ > | Difference (decrease) | 396675743ns (24.74%) | > +---------------------------------------------------+ > > Remove the percpu variable cpu_number as it is used only in > set_smp_ipi_range() as a dummy variable to be passed to ipi_handler(). > Use irq_stat in place of cpu_number here. > > [1] https://github.com/puranjaymohan/linux/commit/77d3fdd > > Signed-off-by: Puranjay Mohan > --- > Changes in v1 -> v2: > v1: https://lore.kernel.org/all/20240501154236.10236-1-puranjay@kernel.org/ > - Remove the percpu variable cpu_number > - Add more information to the commit message. > --- > arch/arm64/include/asm/smp.h | 13 +------------ > arch/arm64/kernel/smp.c | 9 ++------- > 2 files changed, 3 insertions(+), 19 deletions(-) > > diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h > index efb13112b408..2510eec026f7 100644 > --- a/arch/arm64/include/asm/smp.h > +++ b/arch/arm64/include/asm/smp.h > @@ -25,22 +25,11 @@ > > #ifndef __ASSEMBLY__ > > -#include > - > #include > #include > #include > > -DECLARE_PER_CPU_READ_MOSTLY(int, cpu_number); > - > -/* > - * We don't use this_cpu_read(cpu_number) as that has implicit writes to > - * preempt_count, and associated (compiler) barriers, that we'd like to avoid > - * the expense of. If we're preemptible, the value can be stale at use anyway. > - * And we can't use this_cpu_ptr() either, as that winds up recursing back > - * here under CONFIG_DEBUG_PREEMPT=y. > - */ > -#define raw_smp_processor_id() (*raw_cpu_ptr(&cpu_number)) > +#define raw_smp_processor_id() (current_thread_info()->cpu) > > /* > * Logical CPU mapping. > diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c > index 4ced34f62dab..98d4e352c3d0 100644 > --- a/arch/arm64/kernel/smp.c > +++ b/arch/arm64/kernel/smp.c > @@ -55,9 +55,6 @@ > > #include > > -DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number); > -EXPORT_PER_CPU_SYMBOL(cpu_number); > - > /* > * as from 2.5, kernels no longer have an init_tasks structure > * so we need some other way of telling a new secondary core > @@ -742,8 +739,6 @@ void __init smp_prepare_cpus(unsigned int max_cpus) > */ > for_each_possible_cpu(cpu) { > > - per_cpu(cpu_number, cpu) = cpu; > - > if (cpu == smp_processor_id()) > continue; > > @@ -1021,12 +1016,12 @@ void __init set_smp_ipi_range(int ipi_base, int n) > > if (ipi_should_be_nmi(i)) { > err = request_percpu_nmi(ipi_base + i, ipi_handler, > - "IPI", &cpu_number); > + "IPI", &irq_stat); > WARN(err, "Could not request IPI %d as NMI, err=%d\n", > i, err); > } else { > err = request_percpu_irq(ipi_base + i, ipi_handler, > - "IPI", &cpu_number); > + "IPI", &irq_stat); > WARN(err, "Could not request IPI %d as IRQ, err=%d\n", > i, err); > } set_smp_ipi_range() now looks similar to what we have on arm32 platform. Besides the arch/arm64/include/asm/smp.h changes (not sure if is just a clean up, then might be a stand alone patch on its own) this patch LGTM.