linux-kernel - Re: [PATCHv3] perf kvm: add kvm-stat for arm64

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Message-ID: <20200917114231.GE12548@leoy-ThinkPad-X240s>
Date:   Thu, 17 Sep 2020 19:42:31 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        John Garry <john.garry@...wei.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv3] perf kvm: add kvm-stat for arm64

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/