From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Jones Date: Tue, 1 Nov 2022 11:53:40 +0100 Subject: [PATCH 0/9] RISC-V: Apply Zicboz to clear_page and memset In-Reply-To: <20221101103711.poswtnd5cehzwf2s@kamzik> References: <20221027130247.31634-1-ajones@ventanamicro.com> <20221101103711.poswtnd5cehzwf2s@kamzik> Message-ID: <20221101105340.uz6o7kdg3sl5gdmo@kamzik> List-Id: To: kvm-riscv@lists.infradead.org MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Tue, Nov 01, 2022 at 11:37:11AM +0100, Andrew Jones wrote: > 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. Additionally, I've now seen on a mostly idle system that at least half the objects getting zeroed were too small (< 64 bytes). Considering the memset profile is workload dependent, it's difficult to say with certainty that we should patch it. More experiments are likely necessary. > > > > > [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 at ventanamicro.com > > > > 2. "[PATCH 0/8] riscv: improve boot time isa extensions handling" > > 20221006070818.3616-1-jszhang at kernel.org > > Also including the additional patch proposed here > > 20221013162038.ehseju2neic2xu5z at 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 > >