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[31.30.173.61]) by smtp.gmail.com with ESMTPSA id h8-20020a17090634c800b00731582babcasm4125139ejb.71.2022.11.01.03.37.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 01 Nov 2022 03:37:12 -0700 (PDT) Date: Tue, 1 Nov 2022 11:37:11 +0100 From: Andrew Jones To: linux-riscv@lists.infradead.org, kvm-riscv@lists.infradead.org Cc: Paul Walmsley , Palmer Dabbelt , Albert Ou , Anup Patel , Heiko Stuebner , Conor Dooley , Atish Patra , Jisheng Zhang Subject: Re: [PATCH 0/9] RISC-V: Apply Zicboz to clear_page and memset Message-ID: <20221101103711.poswtnd5cehzwf2s@kamzik> References: <20221027130247.31634-1-ajones@ventanamicro.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20221027130247.31634-1-ajones@ventanamicro.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20221101_033715_036228_046CC4A6 X-CRM114-Status: GOOD ( 36.14 ) X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On Thu, Oct 27, 2022 at 03:02:38PM +0200, Andrew Jones wrote: > When the Zicboz extension is available we can more rapidly zero naturally > aligned Zicboz block sized chunks of memory. As pages are always page > aligned and are larger than any Zicboz block size will be, then > clear_page() appears to be a good candidate for the extension. While cycle > count and energy consumption should also be considered, we can be pretty > certain that implementing clear_page() with the Zicboz extension is a win > by comparing the new dynamic instruction count with its current count[1]. > Doing so we see that the new count is less than half the old count (see > patch4's commit message for more details). Another candidate for the > extension is memset(), but, since memset() isn't just used for zeroing > memory and it accepts arbitrarily aligned addresses and arbitrary sizes, > it's not as obvious if adding support for Zicboz will be an overall win. > In order to make a determination, I've done some analysis and wrote my > conclusions in the bullets below. > > * When compiling the kernel without CONFIG_RISCV_ISA_ZICBOZ, memset() > doesn't change, so that's fine. > > * The overhead added to memset() when the Zicboz extension isn't present, > but CONFIG_RISCV_ISA_ZICBOZ is selected, is 3 jumps to known targets, > which I believe is fine. > > * The overhead added to a memset() invocation which is not zeroing memory > is 7 instructions, where 3 are branches. This seems fine and, > furthermore, memset() is almost always invoked to zero memory (99% [2]). > > * When memset() is invoked to zero memory, the proposed Zicboz extended > memset() always has a lower dynamic instruction count than the current > memset() as long as the input address is Zicboz block aligned and the > length is >= the block size. > > * When memset() is invoked to zero memory, the proposed Zicboz extended > memset() is always worse for unaligned or too small inputs than the > current memset(), but it's only at most a few dozen instructions worse. > I think this is probably fine, especially considering the large majority > of zeroing invocations are 64 bytes or larger and are aligned to a > power-of-2 boundary, 64-byte or larger (77% [2]). FYI, I spotted a bug in the bpf script used for this result, so I fixed it and reran it. The new result is 67%, which is no longer a large majority, but still a solid majority. > > [1] I ported the functions under test to userspace and linked them with > a test program. Then, I ran them under gdb with a script[3] which > counted instructions by single stepping. > > [2] I wrote bpftrace scripts[4] to count memset() invocations to see the > frequency of it being used to zero memory and have block size aligned > input addresses with block size or larger lengths. The workload was > just random desktop stuff including streaming video and compiling. > While I did run this on my x86 notebook, I still expect the data to > be representative on RISC-V. Note, x86 has clear_page() so the > memset() data regarding alignment and size weren't over inflated by > page zeroing invocations. Grepping also shows the large majority of > memset() calls are to zero memory (93%). > > [3] https://gist.github.com/jones-drew/487791c956ceca8c18adc2847eec9c60 > [4] https://gist.github.com/jones-drew/1e860692cf6fc0fb2a82a04c9ce720fe > > These patches are based on the following pending series > > 1. "[PATCH v2 0/3] RISC-V: Ensure Zicbom has a valid block size" > 20221024091309.406906-1-ajones@ventanamicro.com > > 2. "[PATCH 0/8] riscv: improve boot time isa extensions handling" > 20221006070818.3616-1-jszhang@kernel.org > Also including the additional patch proposed here > 20221013162038.ehseju2neic2xu5z@kamzik > > The patches are also available here > https://github.com/jones-drew/linux/commits/riscv/zicboz > > To test over QEMU this branch may be used to enable Zicboz > https://gitlab.com/jones-drew/qemu/-/commits/riscv/zicboz > > To test running a KVM guest with Zicboz this kvmtool branch may be used > https://github.com/jones-drew/kvmtool/commits/riscv/zicboz > > Thanks, > drew > > Andrew Jones (9): > RISC-V: Factor out body of riscv_init_cbom_blocksize loop > RISC-V: Add Zicboz detection and block size parsing > RISC-V: insn-def: Define cbo.zero > RISC-V: Use Zicboz in clear_page when available > RISC-V: KVM: Provide UAPI for Zicboz block size > RISC-V: KVM: Expose Zicboz to the guest > RISC-V: lib: Improve memset assembler formatting > RISC-V: lib: Use named labels in memset > RISC-V: Use Zicboz in memset when available > > arch/riscv/Kconfig | 13 ++ > arch/riscv/include/asm/cacheflush.h | 3 +- > arch/riscv/include/asm/hwcap.h | 1 + > arch/riscv/include/asm/insn-def.h | 50 ++++++ > arch/riscv/include/asm/page.h | 6 +- > arch/riscv/include/uapi/asm/kvm.h | 2 + > arch/riscv/kernel/cpu.c | 1 + > arch/riscv/kernel/cpufeature.c | 10 ++ > arch/riscv/kernel/setup.c | 2 +- > arch/riscv/kvm/vcpu.c | 11 ++ > arch/riscv/lib/Makefile | 1 + > arch/riscv/lib/clear_page.S | 28 ++++ > arch/riscv/lib/memset.S | 241 +++++++++++++++++++--------- > arch/riscv/mm/cacheflush.c | 64 +++++--- > 14 files changed, 325 insertions(+), 108 deletions(-) > create mode 100644 arch/riscv/lib/clear_page.S > > -- > 2.37.3 > _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv