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([2a02:6b6a:b4d7:0:f0b8:522f:5d6e:d788]) by smtp.gmail.com with ESMTPSA id h20-20020a05600c351400b003b4935f04a4sm2479548wmq.5.2022.11.24.05.55.27 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Nov 2022 05:55:28 -0800 (PST) Message-ID: <95efd030-27f6-5668-a25e-9fbf210bfa1c@bytedance.com> Date: Thu, 24 Nov 2022 13:55:27 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [External] Re: [v2 0/6] KVM: arm64: implement vcpu_is_preempted check Content-Language: en-US To: Marc Zyngier Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-doc@vger.kernel.org, virtualization@lists.linux-foundation.org, linux@armlinux.org.uk, yezengruan@huawei.com, catalin.marinas@arm.com, will@kernel.org, steven.price@arm.com, mark.rutland@arm.com, bagasdotme@gmail.com, fam.zheng@bytedance.com, liangma@liangbit.com, punit.agrawal@bytedance.com References: <20221104062105.4119003-1-usama.arif@bytedance.com> <87k048f3cm.wl-maz@kernel.org> <180b91af-a2aa-2cfd-eb7f-b2825c4e3dbe@bytedance.com> <86r0y1nmep.wl-maz@kernel.org> From: Usama Arif In-Reply-To: <86r0y1nmep.wl-maz@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 18/11/2022 00:20, Marc Zyngier wrote: > On Mon, 07 Nov 2022 12:00:44 +0000, > Usama Arif wrote: >> >> >> >> On 06/11/2022 16:35, Marc Zyngier wrote: >>> On Fri, 04 Nov 2022 06:20:59 +0000, >>> Usama Arif wrote: >>>> >>>> This patchset adds support for vcpu_is_preempted in arm64, which >>>> allows the guest to check if a vcpu was scheduled out, which is >>>> useful to know incase it was holding a lock. vcpu_is_preempted can >>>> be used to improve performance in locking (see owner_on_cpu usage in >>>> mutex_spin_on_owner, mutex_can_spin_on_owner, rtmutex_spin_on_owner >>>> and osq_lock) and scheduling (see available_idle_cpu which is used >>>> in several places in kernel/sched/fair.c for e.g. in wake_affine to >>>> determine which CPU can run soonest): >>> >>> [...] >>> >>>> pvcy shows a smaller overall improvement (50%) compared to >>>> vcpu_is_preempted (277%). Host side flamegraph analysis shows that >>>> ~60% of the host time when using pvcy is spent in kvm_handle_wfx, >>>> compared with ~1.5% when using vcpu_is_preempted, hence >>>> vcpu_is_preempted shows a larger improvement. >>> >>> And have you worked out *why* we spend so much time handling WFE? >>> >>> M. >> >> Its from the following change in pvcy patchset: >> >> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c >> index e778eefcf214..915644816a85 100644 >> --- a/arch/arm64/kvm/handle_exit.c >> +++ b/arch/arm64/kvm/handle_exit.c >> @@ -118,7 +118,12 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu) >> } >> >> if (esr & ESR_ELx_WFx_ISS_WFE) { >> - kvm_vcpu_on_spin(vcpu, vcpu_mode_priv(vcpu)); >> + int state; >> + while ((state = kvm_pvcy_check_state(vcpu)) == 0) >> + schedule(); >> + >> + if (state == -1) >> + kvm_vcpu_on_spin(vcpu, vcpu_mode_priv(vcpu)); >> } else { >> if (esr & ESR_ELx_WFx_ISS_WFxT) >> vcpu_set_flag(vcpu, IN_WFIT); >> >> >> If my understanding is correct of the pvcy changes, whenever pvcy >> returns an unchanged vcpu state, we would schedule to another >> vcpu. And its the constant scheduling where the time is spent. I guess >> the affects are much higher when the lock contention is very >> high. This can be seem from the pvcy host side flamegraph as well with >> (~67% of the time spent in the schedule() call in kvm_handle_wfx), For >> reference, I have put the graph at: >> https://uarif1.github.io/pvlock/perf_host_pvcy_nmi.svg > > The real issue here is that we don't try to pick the right vcpu to > run, and strictly rely on schedule() to eventually pick something that > can run. > > An interesting to do would be to try and fit the directed yield > mechanism there. It would be a lot more interesting than the one-off > vcpu_is_preempted hack, as it gives us a low-level primitive on which > to construct things (pvcy is effectively a mwait-like primitive). We could use kvm_vcpu_yield_to to yield to a specific vcpu, but how would we determine which vcpu to yield to? IMO vcpu_is_preempted is very well integrated in a lot of core kernel code, i.e. mutex, rtmutex, rwsem and osq_lock. It is also used in scheduler to determine better which vCPU we can run on soonest, select idle core, etc. I am not sure if all of these cases will be optimized by pvcy? Also, with vcpu_is_preempted, some of the lock heavy benchmarks come down from spending around 50% of the time in lock to less than 1% (so not sure how much more room is there for improvement). We could also use vcpu_is_preempted to optimize IPI performance (along with directed yield to target IPI vCPU) similar to how its done in x86 (https://lore.kernel.org/all/1560255830-8656-2-git-send-email-wanpengli@tencent.com/). This case definitely wont be covered by pvcy. Considering all the above, i.e. the core kernel integration already present and possible future usecases of vcpu_is_preempted, maybe its worth making vcpu_is_preempted work on arm independently of pvcy? Thanks, Usama > > M. >