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Tue, 08 Jul 2025 04:10:18 -0700 (PDT) Received: from localhost ([2a02:8308:a00c:e200:f873:aafc:f154:af28]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b4708d0beesm12854474f8f.36.2025.07.08.04.10.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 08 Jul 2025 04:10:18 -0700 (PDT) Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=UTF-8 Date: Tue, 08 Jul 2025 13:10:17 +0200 Message-Id: Subject: Re: [External] [PATCH] RISC-V: store percpu offset in CSR_SCRATCH Cc: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , "linux-riscv" , To: "yunhui cui" From: =?utf-8?q?Radim_Kr=C4=8Dm=C3=A1=C5=99?= References: <20250704084500.62688-1-cuiyunhui@bytedance.com> In-Reply-To: X-Stat-Signature: j6szuqb4feppejfgjbrsiua6765ydsnp X-Rspamd-Queue-Id: 746332000B X-Rspam-User: X-Rspamd-Server: rspam07 X-HE-Tag: 1751973020-861932 X-HE-Meta: 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 NiDh2IOC 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 2025-07-08T18:07:27+08:00, yunhui cui : > This patch cleverly differentiates whether an exception originates > from user mode or kernel mode. However, there's still an issue with > using CSR_SCRATCH: each time handle_exception() is called, the > following instructions must be executed: > > REG_L s0, TASK_TI_CPU(tp) > slli s0, s0, 3 > la s1, __per_cpu_offset > add s1, s1, s0 > REG_L s1, 0(s1) > csrw CSR_SCRATCH, s1 We can minimize the cost at exception entry by storing the precomputed offset in thread_info, which bloats the struct, and also incurs update cost on cpu migration, but should still be a net performance gain. The minimal code at exception entry would be: REG_L s0, TASK_TI_PERCPU_OFFSET(tp) csrw CSR_SCRATCH, s0 > Should we consider adding a dedicated CSR (e.g., CSR_SCRATCH2) to > store the percpu offset instead? > See: https://lists.riscv.org/g/tech-privileged/topic/113437553#msg2506 It would be nice to gather more data on the CSR_SCRATCH approach. Basically, the overhead of "REG_L s0, TASK_TI_PERCPU_OFFSET(tp)". (Or the longer sequence if we think it is worth it.) Can you benchmark the patch after reverting percpu.h, so we include the overhead of switching CSR_SCRATCH, but without any benefits provided by the per-cpu offset? The baseline would be the patch with reverted percpu.h, and reverted the sequence that sets the CSR_SCRATCH in handle_exception, so we roughly estimate the benefit of adding CSR_SCRATCH2. The CSR_SCRATCH2 does add overhead to hardware, and to domain context switches, and we also have to do something else for a few years anyway, because it's not even ratified... It's possible we might not benefit enough from CSR_SCRATCH2 to make a good case for it. Thanks.