From: Ankur Arora <ankur.a.arora@oracle.com> To: "Christoph Lameter (Ampere)" <cl@linux.com> Cc: Mihai Carabas <mihai.carabas@oracle.com>, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, pbonzini@redhat.com, wanpengli@tencent.com, vkuznets@redhat.com, rafael@kernel.org, daniel.lezcano@linaro.org, akpm@linux-foundation.org, pmladek@suse.com, peterz@infradead.org, dianders@chromium.org, npiggin@gmail.com, rick.p.edgecombe@intel.com, joao.m.martins@oracle.com, juerg.haefliger@canonical.com, mic@digikod.net, arnd@arndb.de, ankur.a.arora@oracle.com Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed Date: Thu, 30 Nov 2023 22:59:45 -0800 [thread overview] Message-ID: <871qc6qufy.fsf@oracle.com> (raw) In-Reply-To: <277fbd0d-25ea-437e-2ea7-6121c4e269db@linux.com> Christoph Lameter (Ampere) <cl@linux.com> writes: > On Wed, 22 Nov 2023, Mihai Carabas wrote: > >> La 22.11.2023 22:51, Christoph Lameter a scris: >>> On Mon, 20 Nov 2023, Mihai Carabas wrote: >>> >>>> cpu_relax on ARM64 does a simple "yield". Thus we replace it with >>>> smp_cond_load_relaxed which basically does a "wfe". >>> Well it clears events first (which requires the first WFE) and then does a >>> WFE waiting for any events if no events were pending. >>> WFE does not cause a VMEXIT? Or does the inner loop of >>> smp_cond_load_relaxed now do 2x VMEXITS? >>> KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See >>> kvm_handle_wfx(). >> >> In KVM ARM64 the WFE traping is dynamic: it is enabled only if there are more >> tasks waiting on the same core (e.g. on an oversubscribed system). >> >> In arch/arm64/kvm/arm.c: >> >> 457 >-------if (single_task_running()) >> 458 >------->-------vcpu_clear_wfx_traps(vcpu); >> 459 >-------else >> 460 >------->-------vcpu_set_wfx_traps(vcpu); > > Ahh. Cool did not know about that. But still: Lots of VMEXITs once the load has > to be shared. Yeah, anytime there's more than one runnable process. Another, more critical place where we will vmexit is the qspinlock slowpath which uses smp_cond_load. >> This of course can be improved by having a knob where you can completly >> disable wfx traping by your needs, but I left this as another subject to >> tackle. Probably needs to be adaptive since we use WFE in error paths as well (for instance to park the CPU.) Ankur
WARNING: multiple messages have this Message-ID (diff)
From: Ankur Arora <ankur.a.arora@oracle.com> To: "Christoph Lameter (Ampere)" <cl@linux.com> Cc: Mihai Carabas <mihai.carabas@oracle.com>, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com, pbonzini@redhat.com, wanpengli@tencent.com, vkuznets@redhat.com, rafael@kernel.org, daniel.lezcano@linaro.org, akpm@linux-foundation.org, pmladek@suse.com, peterz@infradead.org, dianders@chromium.org, npiggin@gmail.com, rick.p.edgecombe@intel.com, joao.m.martins@oracle.com, juerg.haefliger@canonical.com, mic@digikod.net, arnd@arndb.de, ankur.a.arora@oracle.com Subject: Re: [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed Date: Thu, 30 Nov 2023 22:59:45 -0800 [thread overview] Message-ID: <871qc6qufy.fsf@oracle.com> (raw) In-Reply-To: <277fbd0d-25ea-437e-2ea7-6121c4e269db@linux.com> Christoph Lameter (Ampere) <cl@linux.com> writes: > On Wed, 22 Nov 2023, Mihai Carabas wrote: > >> La 22.11.2023 22:51, Christoph Lameter a scris: >>> On Mon, 20 Nov 2023, Mihai Carabas wrote: >>> >>>> cpu_relax on ARM64 does a simple "yield". Thus we replace it with >>>> smp_cond_load_relaxed which basically does a "wfe". >>> Well it clears events first (which requires the first WFE) and then does a >>> WFE waiting for any events if no events were pending. >>> WFE does not cause a VMEXIT? Or does the inner loop of >>> smp_cond_load_relaxed now do 2x VMEXITS? >>> KVM ARM64 code seems to indicate that WFE causes a VMEXIT. See >>> kvm_handle_wfx(). >> >> In KVM ARM64 the WFE traping is dynamic: it is enabled only if there are more >> tasks waiting on the same core (e.g. on an oversubscribed system). >> >> In arch/arm64/kvm/arm.c: >> >> 457 >-------if (single_task_running()) >> 458 >------->-------vcpu_clear_wfx_traps(vcpu); >> 459 >-------else >> 460 >------->-------vcpu_set_wfx_traps(vcpu); > > Ahh. Cool did not know about that. But still: Lots of VMEXITs once the load has > to be shared. Yeah, anytime there's more than one runnable process. Another, more critical place where we will vmexit is the qspinlock slowpath which uses smp_cond_load. >> This of course can be improved by having a knob where you can completly >> disable wfx traping by your needs, but I left this as another subject to >> tackle. Probably needs to be adaptive since we use WFE in error paths as well (for instance to park the CPU.) Ankur _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2023-12-01 7:04 UTC|newest] Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-11-20 14:01 [PATCH v2] Enable haltpoll for arm64 Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 1/7] x86: Move ARCH_HAS_CPU_RELAX to arch Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-27 14:44 ` Petr Mladek 2023-11-27 14:44 ` Petr Mladek 2023-11-28 14:09 ` Mihai Carabas 2023-11-28 14:09 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 2/7] x86/kvm: Move haltpoll_want() to be arch defined Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-29 20:55 ` Rafael J. Wysocki 2023-11-29 20:55 ` Rafael J. Wysocki 2023-11-20 14:01 ` [PATCH 3/7] governors/haltpoll: Drop kvm_para_available() check Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 4/7] arm64: Select ARCH_HAS_CPU_RELAX Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 5/7] arm64: Define TIF_POLLING_NRFLAG Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 6/7] cpuidle-haltpoll: ARM64 support Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-20 14:01 ` [PATCH 7/7] cpuidle/poll_state: replace cpu_relax with smp_cond_load_relaxed Mihai Carabas 2023-11-20 14:01 ` Mihai Carabas 2023-11-22 20:51 ` Christoph Lameter 2023-11-22 20:51 ` Christoph Lameter 2023-11-22 21:33 ` Mihai Carabas 2023-11-22 21:33 ` Mihai Carabas 2023-11-27 20:17 ` Christoph Lameter (Ampere) 2023-11-27 20:17 ` Christoph Lameter (Ampere) 2023-12-01 6:59 ` Ankur Arora [this message] 2023-12-01 6:59 ` Ankur Arora 2023-12-11 11:46 ` Will Deacon 2023-12-11 11:46 ` Will Deacon 2024-01-28 21:22 ` Mihai Carabas 2024-01-28 21:22 ` Mihai Carabas 2024-01-29 18:15 ` Will Deacon 2024-01-29 18:15 ` Will Deacon 2024-02-05 12:28 ` Mihai Carabas 2024-02-05 12:28 ` Mihai Carabas 2024-02-05 19:33 ` Ankur Arora 2024-02-05 19:33 ` Ankur Arora 2024-01-17 21:19 ` [PATCH v2] Enable haltpoll for arm64 Christoph Lameter (Ampere) 2024-01-17 21:19 ` Christoph Lameter (Ampere) 2024-01-25 14:39 ` Mihai Carabas 2024-01-25 14:39 ` Mihai Carabas 2024-01-25 15:16 ` Rafael J. Wysocki 2024-01-25 15:16 ` Rafael J. Wysocki
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