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Thu, 17 Sep 2020 04:42:40 -0700 (PDT) Date: Thu, 17 Sep 2020 19:42:31 +0800 From: Leo Yan To: Marc Zyngier Cc: Sergey Senozhatsky , Arnaldo Carvalho de Melo , Mark Rutland , Peter Zijlstra , Will Deacon , John Garry , Mathieu Poirier , Namhyung Kim , Suleiman Souhlal , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCHv3] perf kvm: add kvm-stat for arm64 Message-ID: <20200917114231.GE12548@leoy-ThinkPad-X240s> References: <20200917003645.689665-1-sergey.senozhatsky@gmail.com> <20200917100950.GC12548@leoy-ThinkPad-X240s> <20200917101219.GD12548@leoy-ThinkPad-X240s> <652f10660f09bd608b825233713f775a@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <652f10660f09bd608b825233713f775a@kernel.org> User-Agent: Mutt/1.9.4 (2018-02-28) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 17, 2020 at 11:21:15AM +0100, Marc Zyngier wrote: [...] > > > > +const char *vcpu_id_str = "id"; > > > > > > On Arm64, ftrace tracepoint "kvm_entry" doesn't contain the field "id" > > > or field "vcpu_id", thus it always reads out the "id" is 0 and it is > > > recorded into Perf's structure vcpu_event_record::vcpu_id and assigned > > > to perf thread's private data "thread::private". > > > > > > With current code, it will not mess up different vcpus' samples > > > because > > > now the samples are analyzed based on thread context, but since all > > > threads' "vcpu_id" is zero, thus all samples are accounted for > > > "vcpu_id=0" and cannot print out correct result with option "--vcpu": > > > > > > > > > $ perf kvm stat report --vcpu 4 > > > > > > Analyze events for all VMs, VCPU 4: > > > > > > VM-EXIT Samples Samples% Time% Min Time > > > Max Time Avg time > > > > > > Total Samples:0, Total events handled time:0.00us. > > > > > > > > > This is an issue I observed, if we want to support option "--vcpu", > > > seems we need to change ftrace event for "kvm_entry", but this will > > > break ABI. > > > > > > Essentially, this issue is caused by different archs using different > > > format for ftrace event "kvm_entry", on x86 it contains feild > > > "vcpu_id" but arm64 only just records "vcpu_pc". > > > > > > @Marc, @Will, do you have any suggestion for this? Do you think it's > > > feasible to add a new field "vcpu_id" into the tracepoint "kvm_entry" > > > for Arm64's version? > > The question really is: how will you handle the ABI breackage? > I don't see a good solution for it, apart from having a *separate* > tracepoint that collects all the information you need. And even that is > really ugly. I searched a bit and found in practice it's not impossible to add new parameters for existed tracepoint, e.g. [1][2] are two examples to add new parameters for existed tracepoints and have been merged into mainline kernel. IIUC, we keep the old parameters for a tracepoint so this can avoid to break ABI if any apps have used this tracepoint, and adding a new parameter for the tracepoint should be safe. If you agree with this, I'd like to suggest to apply below change. How about you think for this? diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c index 46dc3d75cf13..d9f9b8e1df77 100644 --- a/arch/arm64/kvm/arm.c +++ b/arch/arm64/kvm/arm.c @@ -736,7 +736,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) /************************************************************** * Enter the guest */ - trace_kvm_entry(*vcpu_pc(vcpu)); + trace_kvm_entry(vcpu->vcpu_id, *vcpu_pc(vcpu)); guest_enter_irqoff(); ret = kvm_call_hyp_ret(__kvm_vcpu_run, vcpu); diff --git a/arch/arm64/kvm/trace_arm.h b/arch/arm64/kvm/trace_arm.h index 4691053c5ee4..e1d3e7a67e8b 100644 --- a/arch/arm64/kvm/trace_arm.h +++ b/arch/arm64/kvm/trace_arm.h @@ -12,18 +12,20 @@ * Tracepoints for entry/exit to guest */ TRACE_EVENT(kvm_entry, - TP_PROTO(unsigned long vcpu_pc), - TP_ARGS(vcpu_pc), + TP_PROTO(unsigned int vcpu_id, unsigned long vcpu_pc), + TP_ARGS(vcpu_id, vcpu_pc), TP_STRUCT__entry( + __field( unsigned int, vcpu_id ) __field( unsigned long, vcpu_pc ) ), TP_fast_assign( + __entry->vcpu_id = vcpu_id; __entry->vcpu_pc = vcpu_pc; ), - TP_printk("PC: 0x%08lx", __entry->vcpu_pc) + TP_printk("vcpu: %u, PC: 0x%08lx", __entry->vcpu_id, __entry->vcpu_pc) ); TRACE_EVENT(kvm_exit, Thanks, Leo [1] https://lkml.org/lkml/2019/2/26/282 [2] https://lore.kernel.org/linux-mm/20191106080037.GA59367@google.com/t/