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[109.154.46.208]) by smtp.gmail.com with ESMTPSA id rg8-20020a1709076b8800b00a3e28471fa4sm6461293ejc.59.2024.02.23.07.58.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Feb 2024 07:58:16 -0800 (PST) From: Qais Yousef To: Jens Axboe , Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, Sudeep Holla , Wei Wang , Jaegeuk Kim , Bart Van Assche , Christoph Hellwig , Qais Yousef Subject: [PATCH v2 1/2] sched: Add a new function to compare if two cpus have the same capacity Date: Fri, 23 Feb 2024 15:57:48 +0000 Message-Id: <20240223155749.2958009-2-qyousef@layalina.io> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20240223155749.2958009-1-qyousef@layalina.io> References: <20240223155749.2958009-1-qyousef@layalina.io> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The new helper function is needed to help blk-mq check if it needs to dispatch the softirq on another CPU to match the performance level the IO requester is running at. This is important on HMP systems where not all CPUs have the same compute capacity. Signed-off-by: Qais Yousef Reviewed-by: Bart Van Assche --- include/linux/sched/topology.h | 6 ++++++ kernel/sched/core.c | 11 +++++++++++ 2 files changed, 17 insertions(+) diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h index a6e04b4a21d7..11e0e00e0bb9 100644 --- a/include/linux/sched/topology.h +++ b/include/linux/sched/topology.h @@ -176,6 +176,7 @@ extern void partition_sched_domains(int ndoms_new, cpum= ask_var_t doms_new[], cpumask_var_t *alloc_sched_domains(unsigned int ndoms); void free_sched_domains(cpumask_var_t doms[], unsigned int ndoms); =20 +bool cpus_equal_capacity(int this_cpu, int that_cpu); bool cpus_share_cache(int this_cpu, int that_cpu); bool cpus_share_resources(int this_cpu, int that_cpu); =20 @@ -226,6 +227,11 @@ partition_sched_domains(int ndoms_new, cpumask_var_t d= oms_new[], { } =20 +static inline bool cpus_equal_capacity(int this_cpu, int that_cpu) +{ + return true; +} + static inline bool cpus_share_cache(int this_cpu, int that_cpu) { return true; diff --git a/kernel/sched/core.c b/kernel/sched/core.c index a76c7095f736..adbaabb23fa1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3953,6 +3953,17 @@ void wake_up_if_idle(int cpu) } } =20 +bool cpus_equal_capacity(int this_cpu, int that_cpu) +{ + if (!sched_asym_cpucap_active()) + return true; + + if (this_cpu =3D=3D that_cpu) + return true; + + return arch_scale_cpu_capacity(this_cpu) =3D=3D arch_scale_cpu_capacity(t= hat_cpu); +} + bool cpus_share_cache(int this_cpu, int that_cpu) { if (this_cpu =3D=3D that_cpu) --=20 2.34.1 From nobody Mon Feb 9 07:07:06 2026 Received: from mail-ed1-f51.google.com (mail-ed1-f51.google.com [209.85.208.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8CF9D84A37 for ; Fri, 23 Feb 2024 15:58:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.208.51 ARC-Seal: i=1; a=rsa-sha256; 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[109.154.46.208]) by smtp.gmail.com with ESMTPSA id rg8-20020a1709076b8800b00a3e28471fa4sm6461293ejc.59.2024.02.23.07.58.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Feb 2024 07:58:17 -0800 (PST) From: Qais Yousef To: Jens Axboe , Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann Cc: linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, Sudeep Holla , Wei Wang , Jaegeuk Kim , Bart Van Assche , Christoph Hellwig , Qais Yousef Subject: [PATCH v2 2/2] block/blk-mq: Don't complete locally if capacities are different Date: Fri, 23 Feb 2024 15:57:49 +0000 Message-Id: <20240223155749.2958009-3-qyousef@layalina.io> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20240223155749.2958009-1-qyousef@layalina.io> References: <20240223155749.2958009-1-qyousef@layalina.io> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" The logic in blk_mq_complete_need_ipi() assumes SMP systems where all CPUs have equal compute capacities and only LLC cache can make a different on perceived performance. But this assumption falls apart on HMP systems where LLC is shared, but the CPUs have different capacities. Staying local then can have a big performance impact if the IO request was done from a CPU with higher capacity but the interrupt is serviced on a lower capacity CPU. Use the new cpus_equal_capacity() function to check if we need to send an IPI. Without the patch I see the BLOCK softirq always running on little cores (where the hardirq is serviced). With it I can see it running on all cores. This was noticed after the topology change [1] where now on a big.LITTLE we truly get that the LLC is shared between all cores where as in the past it was being misrepresented for historical reasons. The logic exposed a missing dependency on capacities for such systems where there can be a big performance difference between the CPUs. This of course introduced a noticeable change in behavior depending on how the topology is presented. Leading to regressions in some workloads as the performance of the BLOCK softirq on littles can be noticeably worse on some platforms. Worth noting that we could have checked for capacities being greater than or equal instead for equality. This will lead to favouring higher performance always. But opted for equality instead to match the performance of the requester without making an assumption that can lead to power trade-offs which these systems tend to be sensitive about. If the requester would like to run faster, it's better to rely on the scheduler to give the IO requester via some facility to run on a faster core; and then if the interrupt triggered on a CPU with different capacity we'll make sure to match the performance the requester is supposed to run at. [1] https://lpc.events/event/16/contributions/1342/attachments/962/1883/LPC= -2022-Android-MC-Phantom-Domains.pdf Signed-off-by: Qais Yousef Reviewed-by: Bart Van Assche --- block/blk-mq.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index 2dc01551e27c..ea69047e12f7 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1167,10 +1167,11 @@ static inline bool blk_mq_complete_need_ipi(struct = request *rq) if (force_irqthreads()) return false; =20 - /* same CPU or cache domain? Complete locally */ + /* same CPU or cache domain and capacity? Complete locally */ if (cpu =3D=3D rq->mq_ctx->cpu || (!test_bit(QUEUE_FLAG_SAME_FORCE, &rq->q->queue_flags) && - cpus_share_cache(cpu, rq->mq_ctx->cpu))) + cpus_share_cache(cpu, rq->mq_ctx->cpu) && + cpus_equal_capacity(cpu, rq->mq_ctx->cpu))) return false; =20 /* don't try to IPI to an offline CPU */ --=20 2.34.1